inorganic anti-corrosive pigments

7/28/2019 Inorganic Anti-Corrosive Pigments

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MNL17-EB/Jun. 1995

no rganic A nti-C orrosivePigmentsu s t i n 1

T I S C O R R O S I O N ?O S IO N I S A N E L EC T RO C H EM I CA L P R OC E SS t h a t t a k e s p l a c e

on. During deterioration, the metal may form com-or be taken into solution. The following factors mustthermody namical ly unstable metal.ytic cond uctor of ions.An electrolyte (an electrical condu cto r such as a moistpath).

If corrosion is extreme, the meta l will, in essence, ret urn tostable comp osition of its original ore, disintegrating

ic conduc tor of ions, andRates of corrosion vary, often depend ing o n the electrolyte., metallic surfaces in c ontact with acidic solutions

solutions will be some what less corrosiveode in the presence o f soil or a dissimilar metal [4]. Whiley types of corrosion exist, this discussion is limited to the

Common examples of this sort of corrosion include thee red rusting of iron, the white rusting of zinc, and

S E O F C O A T I N G S T O P R O T E C TC O R R O S I O NBefore the beginnin g of recorded time and ever afterward,

1 Vic e P r e s i d e n t a n d t e c h n i c a l d i r e c t o r , H A L O X P i g m e n t s , a d i v i-o f H a m m o n d L e a d P ro d u c t s , 1 32 6 S u m m e r S t r ee t , H a m m o n d ,4 6 3 2 0 - 2 24 0 .

and mechanisms. For our purposes, we will limit our discus-sion to examination of the following five mechan isms of pro-tecting metals by using coatings:9 Barri er effect of film or vehicle.9 Barrier pigment effect.9 Sacrificial pigments.9 Vehicle enhan cement (film formulation and development).9 Active inorganic inhibitive pigmentation.

B a r r i er E f f e c tAmong the oldest inhibitive methods known, barrier coat-ings provide a protective, physical shield between a metal andair, moisture, or chemicals. While the c omposition and thick-ness of barrier coatings vary widely, they generally have a low

permeability or moi sture va por transmission rate. As a rule,the lower the transmission rate, the less likelihood that oxy-gen and moisture will reach the substrate. Some commonbarrier coatings include organic paints and lacquers, metalliccoatings (hot dip), and heavy mastics.Barrier Pigment Effect

Pigments not only add color to coatings but can also pro-tect metal from corr osion by reinforcing the film and limitingpermeability. "Lamellar" pigments, for example, such as micaand micaceous iron oxide, form a wall of flat, thin particleswithin a paint film. These resist penetration, forcing water towend a long, tort uous path toward the subst rate (Fig. 1).Metallic flakes of aluminum, bronze, or steel produce similareffects.Since not all pigments are compatible with all resin sys-tems, care must be taken whe n adding pigments to a coating.In highly acid or alkaline environments, inert or chemicallyresistant pigments should be used.Sacrificial P igments

Zinc is a "natural" for use in protective coat ings since zinc-rich coatings offer cathodic protection when applied toferrous substrates, that is, the zinc acts as a cathode duringthe corrosion process, receiving the attack normally sufferedby the ferrous metal [5]. To some degree, the duration of thepigment's efficacy is dependen t on the thickness of the sacrifi-cial coating.Vehic le E nhancem entThe formulation of a coating itself can add greatly to corro-sion protection. The addition of pigments, for example, can

2389 1995 by ASTM In te rna t iona l www.as tm.o rg


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C H A P T E R 2 7 - - 1 N O R G A N I C A N T I - C O R R O S I V E P I G M E N T S 239Coating withMetal lic Pigment or Mica Unreinforced Coating

WaterI I

I~ 1 1 l i t..'--E ? , J l ~

I I t

I I l l : I I]

Substrate

W a ~ rO O Q,= , O o o ~ 1 7 6

11/11111111/111111SubstrateFIG . 1 -B arr ier pigmen t ef fect . W ater takes a straight path through an unreinforced f i lm,but is forced to take an extended path through lamel lar part icles of a metal l ic f lake ormica-reinforced f i lm (af ter Alan Smith, In o r g a n i c P r i m e r P i g m e n t s [ 2 ] ) .

onents in linseed oil/alkyd primers to produce lead[6]. Other additives are used to enha nce o ther coating or

The majority of this chapter will focus on the following

ce a protective coating on

passivation and is) cathodi c/anodic and (2) oxida-Cathodic and A nodic Passivat ion--Inhib i t ive inorganic pig-

ess: cathodic reactions, anodic reactions, and ionicSince the cathodic and anodic processes are the basis forrrosion process, control of these processes can play annt role in corrosion inhibition. The cathodic process

, or cathode. In the anodic process, metal ions passAs polarizing agents, inhibitive pigments work in a varietyys. Some slow the electrolytic process by retarding theons throug h resistance polarization, that is, by increas-'s electrical resistance on the sur face of the anode or

enhancing polarization by raising the concentration of posi-tively charged hydroge n ions at the cathode.Inhibitive pigments such as zinc chromate, phosphate, sili-cate, and borate suppress corrosion by enhancing anodicpassivation in acid solutions. In neutral solution, the pig-ments are adsorbed onto the metal surface, reducing anodicactivity.

Oxidat ivepassivat ion--Though oxidation is usually thoug htto contribute to the de terioration of a metal, oxidation some-times crea tes a protective layer on a metallic surface. Passiva-tion can o ccur if a metal is oxidized to a stable comp oun d thatis part of the electrolyte [9]. Consider, for example, the pro-tective layer of aluminum oxide on aluminum metal. Theoxide minimizes atmospheric corrosion despite the reactivenature of the metal.Strong oxidizing conditions are normally required for pas-sivation to occur. Take the case of iron and nitric acid.Exposed to concentrated nitric acid, iron will develop a verythin inhibitive passive film; dilute nitric acid, however, willattack the metal [10].

Neutral i za t ion--Another mechanism of inhibitive pig-ments is neutralization of corrosive substances as sulfates,acids, and chlorides. This can often be achieved throug h theuse inhibitors in a basic environment, which decrease thecorrosive effect of acids and enhance the precipitation ofcorrosive elements.

S O M E W E L L - K N O W N I N O R G A N I C A N T I-C O R R O S I V E P I G M E N T SCoatings industry suppliers are developing high-perfor-mance inorganic pigments to meet the growing demand fornontoxic inhibitive coatings that are both economical andhighly effective. The section that follows discusses the chem-istries, physical properties, functions, advantages, and disad-vantages of some of the most common inorganic inhibitivepigments.Wherever available, the specific gravity, color, pH, oil ab-sorption, and water solubility of each pigment is recorded,supplied in most cases from pigment manufacturers' pub-lished data. Large discrepancies can exist between such data


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40 PAINT AND COATING TESTING MANUAL

O x i d a t i o nreact ionoccu r r i ng . . . .a t a n o d e

C u r r e n t f l o w M e t a l li c c o n d u c t o r,nA n o d e C a t h o d e( M e t a l 1 )

Jf / M1

/ ( M e t a l 2 )

~ O x y g e n n e l e c tr o ly t eE lect ro ly te~ R e d u c t i o neact ion~ , . occu rr ing at cath od eC u r r en t f l ow t h r ou gh e l ec t r o l y t e

F IG . 2 -E lect roch emica l corrosion. A s imp le ce l l show ing the comp onents requ i red forcathod ic and anod ic corrosion processes (a f ter Dean Burger , "Fundamenta ls and Pre-vention of Metal l ic Corrosion," C o r r o s i o n a n d C o r r o s i o n P r o t e c t i o n H a n d b o o k , Phi l ip A .Schw eitzer , Ed . , Marce l Dekker , Inc ., New Y ork and Basel , 1983 ) .

mple, measured oil absorptions as high as 85 for someoil absorption of ap proximately 25.

For the following pigments, specific gravity is expressed ascolor indicates general appearance; pH is measured onoil absorption is expressed as g/100 g and nor-water solubility is expressed as percent soluble.

nt densi ty of 1200 g/L or 10 lb/U.S, gallon. If paint densi-widely from these figures, simple extrapola tions m ay

(BaO-B203.H20) [11-14]The grades shown in Table 1 indicate three grades of com-

onl y used borate inhibitor. Grade I is the primarily

Modified barium metaborates may be used in many kindsrange o f solvent and water-based

TABLE 1--Typical properties of modified barium metaborates.G r a d e I G r a d e I I G r a d e I I I

Specific gravity 3.30 3.70 3.30Color White White WhitepH 10.0 9.1 10.0Oil absorption 30 20 23Water solubility 0.3% 0.2% 0.4%Typical loading levels: 3 to 15%, based on total weight.Some heavy duty maintenancecoatings may require up to 40%

tion, and the anodic passivation from the metaborate ionadds to the pigment's anti-corrosion performance.

Barium me taborates are often considered environmentallyacceptable alternatives to more traditional, toxic pigments;however, it must be remembered that soluble barium is anacute toxicant, and its presence in high levels may causepigments to be classified as hazardous wastes.The pigment has other disadvantages. As a soluble com-pound, it may be quite reactive with many systems. To con-trol its solubility, it is often coated with a silica modification.When barium metaborate is formulated with certain acidicresins or acid-catalyzed baking systems, the resultant coatingmay exhibit stability problems. Therefore, ca re must be takenwhen formulating water-reducible and latex resin systems us-ing this pigment to minimize its solubility and reactivity.B. Zinc Borate

Zinc borate, usually used to provide flame retardancy inplastics and cellulosic fibers, is not normally thou ght o f as aninhibitive pigment. However, recent research indicates thatzinc borate, used in combination with modified barium me-taborate or zinc phosphate, exhibits synergistic performanceproperties resulting in enhanced corrosion prevention. This


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CHAPTER 27--INORGANIC ANTI-CORROSIVE PIGMENTS 241

Chromates have long been used as inhibitors since the

are less widely used than inxavalent chromi um pigments are either con-[15]. Evidence indi-g

carcinogenic forms of hexavalent chromium. Also,Hygienists

[16].(4ZnO.6K20.4CrOr3H20)

Specific gravity 3.45Color YellowpH 7.5Oil absorpti on 25Water solubility 0.1% [17]This inhibitive pigment, also known as zinc chromate or

levels. Because of their yellow color,Although zinc chromates have a threshold limit value of001 mg/m 3 and are therefore fairly toxic, these p igments are

ll make up mo re tha n 30% of the entireHow do zinc chromat es prevent corrosion? While the spe-ic mechan ism is not wholly understood, there are several

existing oxide film [18].s precipitated by zinc chromates contain aprotective mixture of ferric and chromi c oxides.(ZnCrO4-4Zn[OH]z)

Specific gravity 3.65Color YellowpH 7.5Oil abso rpti on 53Water solubility


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42 P A I N T A N D C O A T IN G T E S T I N G M A N U A LAlthough soap formation is red lead's chief protective

use of the pigment is not restricted to oil-based

acking characteristics, and fine particle size of an ideal

Specific gravity 4.1Color OrangepHOil abso rpti on 14Water solubility


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C H A P T E R 2 7 - - I N O R G A N I C A N T I - C O R R O S I V E P I G M E N T S 243resin systems, including high acid n umbe r alkyds,gs, high-pe rformance resins and acid-reactivity and

Zinc phosphate is thought to inhibit corrosion in three

tain oleoresinous vehicles.Zinc phosphate has a creditable track record of effective

rforman ce of the pigment.

The coating industry has produced several variations ofto improve the pigment's per-

ecom mend ed loading levels of these modified versions

appear to offer improved perfor mance in acceler-rcial applications.Modified versions o f zinc phosph ate include:

A l u m i n u m z i nc p h o s p h a t e - - T h e higher phosphate contentand solubility of this aluminu m/zinc coprecipitated phos-phate contribute to its enhanced corrosion inhibition.Bas ic z inc phos pha te hydr a te - - An organic surface treat-ment (claimed to be an electrochemically effective inhibi-tor) enhances the performance of this pigment.Bas ic z inc molybdenum phos pha te - - Th i s pigment makesuse of a low level of molybdate (1.5%) to enhance inhibi-tion.Zinc s ilicophosphate hy dra te--T his pigment appears to be acomplex composite of barium sulfate, silica, magnesiumoxide, and sodium dichromate. Trace amounts ofhexavalent chromium may account for the pigment's im-proved performance, but even these very low chromatelevels could possibly present waste disposal problems.A l u m i n u m T r i p h o s p h a t e [2 6]

Available commercially in three grades (Table 2), alumi-triphosphate is a relatively recent chromate-free inhibi-condensa tion product, it is often modified with zinc ionom the tripoly phosphate ion's ability toas well as higher phospha te levels re sulting

Each grade is recom mend ed for specific applications:Grade /--For use in solvent-based primers (alkyds andepoxides). Grade I is modified with zinc and silicate.Grade H- -F or use in solvent and water-based systems. Thepigment's lower water solubility apparently makes it mor e

TABLE 2--Typical properties of aluminum triphosphate.Grade I Grade II Grade III

Specific gravity 3.0 3.1 3.0Color White White WhitepH 6.5 6.5 6.5Oil absorption 37 32 30Water solubility (max) 1% 1% 1%

suitable fo r water-based coatings . Grade II is modified withzinc and silicate.Gr ade I I I - - F or solvent and water-based systems. Easierdispersion makes the pigmen t more effective in low buildcoatings. Grade III is modified with zinc.

P h o s p h i t e sA . Z i n c H y d r o x y P h o s p h i t e [[2ZnO(OH)z.ZnHPaOa].X2H2-O, wh er e X = 1 to 17] [27]

Specific gravityColorpHOil absorptionWater solubility (Max.)Typical loading levels

3.9White7.0180.04%10-25% inmaintenance applications;5-15% in generalindustrial and water-based applications

Zinc hydroxy phosphite (also called zinc phospho-oxide)results from the reaction between zinc oxide and phospho-rous acid. Anodic passivation of the phosphite ion is theprimary mechanism of inhibition, although the pigment'sability to form inhibitive zinc soaps in oleoresinous systemsalso contributes to corrosion prevention. While not recom-mended for use in high-ac id-number or water-soluble resins,the pigmen t is compatible with a variety of resin systems.S i l i c a t e s

Silicate pigments contribute to corrosion inhibition in sev-eral ways, chiefly through anodic and cathodic passivation.In addition, in oleoresinous systems, these pigments forminhibitive soaps of barium, calcium, strontium, and zinc. Theinhibitive value of silicate pigments is further enhanced bytheir alkalinity and solubility.A . C a l c i u m B o r o s i l i c a t e [ 2 8 - 3 0 ]

Available in three commercial grades (Table 3), calciumborosilicate is effective in a range of applications:a . Gr ades I and I I I - - R ecommended for use in protectivecoating systems-based on traditional alkyd technology,these pigments differ primarily in their B203 content:10.6% for Grade I and 15.6% for Grade III. They are gener-ally used for such applications as trade sales, industrialmaintenance, railroad and tank coatings, and shopprimers. While they can be used in a variety of resin sys-tems, including medium and long oil alkyds, epoxy-esters,and modified alkyds, they are not suited for use in high-acid-number resins, acid-catalyzed systems, water-based


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44 P A I N T A N D C O A T I N G T E S T I N G M A N U A LTABLE 3--Typical properties of calcium borosilicates.

Grade I Grade II Grade IIIgravity 2.65 2.71 2.65r Wh it e White White10.1 10.1 10.1absorption 36 27 41r solubility 0.35% 0.34% 0.37%(max)loading levels 10-20% 10-20% in primers, 10-20%2.5-10% intopcoats and DTMfinishes

resins, epoxides, and other h igh-performan ce resins, or forimmersion or semi-immersion service.b. Grade H-- Thi s pigment, a low-oil-absorption version ofthe other two grades, is used primarily in high solids,medium gloss topcoats, direct-to-metal (DTM) coatings,and self-priming alkyd systems. PVC levels range from 40to 45% in primers and 15 to 25% in topcoats and DTMcoatings.

Four modifications (see Table 4), each representing a dif-Calcium Bar ium Phosphosi lica te (G rad e/ ) - -T his pigmentis for use in conventional and high solids solvent-basedepoxy systems. The pigment is also used in organic zinc-rich primers as an anti-settling agent.Calcium Bariu m Phosphosilicate (Grade II )- -A low-oil-ab-sorption version of this pigment, Grade II is for use in mos twater-borne systems, including latexes and water-reduc-ible systems. When formulated in alkyds, this product, likemany other phosphate pigments, will demonstrate excel-lent real-world performance, but poor performance in ac-celerated testing.Calcium Stro nt ium Phosphosi licate (Grade I I I ) - - Th is pig-men t is for use in most water-based acrylic lacquers, as wellas water-reducible caulks and sealants. When the produ ctused with zinc phosphate in water-based lacquer systemsin a 1:1 ratio, a synergistic effect is produced which im-proves performance.Calcium Stront ium Zinc Phosphosi lica te (Grade IV ) - -T hisgrade is for use in a broad range of resin systems, includingtraditional and high-solid alkyds, latexes, epoxides, water-reducible alkyds, high-acid-n umber resins, vinylidene chlo-ride latexes, a nd catalyzed baking systems. A recen t devel-opment, Grade IV is the mos t versatile and effective of thephosphosilicate inhibitive pigments. Because of its fineparticle size and low oil absorption, this pigment is effec-

TABLE 4--Typical properties of phosphosilicate inhibitors.Grade I Grade I Grade II Grade V

gravity 2.97 2.97 2.90 3.01or White White W h i t e White7.9 8.3 7.8 7.0absorption 44.5 31.5 53.5 26.0ter solubility 0.02% 0 .0 2 % 0 . 0 3 % 0.02%loading levels 5-10% 5-10% 5-10% 2.5-15%

tive in thin film applications and in systems requiring ahigh gloss.

M i s c e l l a n e o u s I n h ib i t iv e P i g m e n t sA . I o n E x c h a n g e P i g m e n t

Specific gravi ty 1.8Color WhitepH 9.2Oil absorpti on 50 [35]Claimed to be effective in all paint systems, this pigmentuses the me chani sm of ion exchange with corrosive species toprevent corrosion. Loa ding levels should be half the levels of

typical corrosion-inhibiting pigments.B . Z i n c O x i d e

Research has suggested that zinc oxide possesses both pas-sivating and cathodic inhibitive capabilities [36,37]. Also,when used in latex metal primers and certain oleoresinoussystems (such as SSPC Paint No. 25) [38], zinc oxide exhibitsobvious anti-corros ive action. ~Often used with more conventional active inhibitors suchas zinc chromate and calcium borosilicates, zinc oxide ap-pears to raise the cross-link density of many paint films,making them harder. Zinc oxide may also absorb ultravioletlight, thus protecting the resin. Since the pigment tends tohave seeding problems and can cause film brittleness, caremust be taken when using zinc oxide in inhibitive coatingapplications.

Z i n c M e t a l i n P r i m e r sThe prime focus of this discussion is inorganic inhibitivepigments which function through the mechanism of solubleinhibitive species; however, zinc metal--which functionsthrough the mechanism of cathodic protection--deserves a

brief mention.In many applications, including the protection of steelstructures, zinc-rich coatings offer the best corrosio n inhibi-tion available, especially when topcoated with eithe r epoxy orurethane. Actual particles or flakes of zinc in the paint filmprotect the ferrous substrate by acting as the anode andcor roding sacrificially. While the correct loading level of zincmetal in zinc-rich primers is still a matter of debate, thesecoatings usually contain 80 to 85% zinc metal. Zinc-richprimers can be formulated as inorganic coating systems(ethyl silicate binders) or organ ic systems (such as epoxides).Zinc-rich primers require near-white blast surface prepara-tion and must be fully cured before topcoating. Two majorproblems associated with zinc-rich formulation are pigmentsettling and hydrog en gassing.

B a r r ie r P i g m e n t sA . A l u m i n u m F la ke

Aluminum flake, typically supplied as a slurry, is produc edby atomizing molten aluminum and ball-milling the powderinto a solvent. The resultant flake is frequently used to im-prove the appearance and anti-corrosion performance of


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CHAPTER 27- -INOR GANIC ANTI-CORROSIVE PIGMENTS 2 4 5r a i l r o a d c a r s , a u t o m o b i l e s ,o f f ic e e q u i p m e n t .

C o a t i n g s c o n t a in i n g a l u m i n u m f la k e s t a n d u p w e l l t o h e a tm o i s t u r e . T h e y a l s o r e s i s t th e d a m a g i n g e f f e c t s o f u l t r a v i-l ig h t , c o o l in g t h e s u b s t r a t e a n d p r e v e n t i n g d e t e r i o r a t i o nh e b i n d e r .

A l u m i n u m f l a k e i s a v a i l a b l e i n t w o g r a d e s , l e a f i n g a n dn g . A s t e a r a t e c o a t i n g o n t h e l e a f i n g g r a d e a l l o w s t h e

e r c o a t s , le a fi n g g r a d e a l u m i n u m f la k e is n o r m a l l y u s e dd i s a d v a n t a g e o f u s i n g a l u m i n u m f l ak e i s t h a t t h e

e n t i s v e ry r e ac t iv e w i t h w a t e r , a n d h y d r o g e n g a s s in gy o c c u r i f m o i s t u r e l ev e ls a r e n o t m a i n t a i n e d b e l o w 0 .1 5 % .

S t a i n l e s s s te e l a n d a n u m b e r o f st e e l a l lo y s a r e s o m e t i m e sa k e s t e e l f la k e , w h i c h i s u s e d i n c o a t i n g s f o r a p p l i c a -

o n s r e q u i r in g m a x i m u m r e s i s ta n c e t o a b r as i o n .

S p e c i f i c g r a v i t y 4 . 9C o l o r D a r k G r a yO i l a b s o r p t i o n 11T y p i c a l l o a d i n g s 3 0 - 5 0 %A n a t u r a ll y o c c u r r i n g f o r m o f h e m a t i t e w i t h a n F e2 O a c o n -

9 2 to 9 5 % , M I O h a s b e e n u s e d i n p r o t e c t iv eu l a t i o n s f o r o v e r 1 0 0 y e a r s . B e c a u s e o f it s la m e l -i t i s t y p i c a ll y u s e d i n p r i m e r s t o i m p r o v e t h e i rr r e s is t a n c e . U s e d i n t o p c o a t s , M I O e n h a n c e s r e s i s t-

C O N S I D E R A T I O N SH e a l t h a n d s a f e ty f a c to r s h a v e a l w a y s b e e n s e r io u s l y c o n -e d d u r i n g t h e d e v e l o p m e n t a n d e v a l u a ti o n o f n e w

i v e c o a ti n g s . U s e o f l e ad , h e x a v a l e n t c h r o m i u m c o m -a n d o t h e r t o x ic p i g m e n t s i s c o n s t a n t l y m o n i t o r e d t os a r e m e t . E n v i r o n m e n -r e g u l a t i o n s h a v e d r i v e n t h e i n h i b i t iv e c o a t i n g s i n d u s t r y t ot o d e v e l o p a n d e v a l u a t e n e w , n o n t o x i c p r o d -a n d i n g p e r f o r m a n c e r e c o r d s.T w o p r i m a r y e n v i r o n m e n t a l c o n c e r n s w i t h i n t h e i n d u s t r ye r p r o t e c t i o n a n d w a s t e d i s p o sa l . S e v e ra l s te p s h a v ee n t o e n s u r e w o r k e r s a f e ty . F o r e x a m p l e , a ll w o r k e r so k n o w t h e d a n g e r s i n v o lv e d w i t h h a n d l i n g t h e

d a y a n d t h e r i g h t t o b eb i l i t y t o h a n d l e t h e s e m a t e r i a l s s a f e ly . B e -r p t i o n o f to x i c p i g m e n t s t h r o u g h i n h a l a t io n i s aj o r c o n c e r n , w o r k e r s a r e p r o t e c te d b y e n g i n e e r i n g c o n -i p m e n t ) , a n d e x t e n s i v e s a f e ty t r a i n in g . T h e p r e -t o c o m p l e t e l y e li m i n a t e th e h a z a r d o u s m a t e -s u b s t i t u t e a le s s t o x i c p i g m e n t , i f p o s s i b l e .

W a s t e d i s p o s a l p e r t a in s t o p l a n t w a s t e p r o d u c e d b y p a i n tu f a c t u r i n g a n d w a s t e f r o m p a i n t in g o p e r a t i o n s . T h e la t -u d e s o v e r s p r a y a n d s p e n t b l a s t a b r a s iv e s c o n t a m i -

n a t e d w i t h l e a d a n d c h r o m a t e - b a s e d p i g m e n t s . U n d e r t h e s er e g u l at i o n s, c o n t a i n m e n t a n d d i s p o s a l o f t h e s e m a t e r i a l sc o u l d w e l l i n c r e a s e t h e c o s t o f a t y p i c a l m a i n t e n a n c e r e p a i n t -i n g p r o j e c t b y t e n f o ld . R e g u l a t i o n s c o n c e r n i n g t h e d i s p o s a l o fi n d u s t r i a l w a s t e s i n t h e U n i t e d S t a t e s a r e s t r i c t, d u e i n p a r t t or e v i s i o n s o f t h e D r i n k i n g W a t e r R e g u l a t i o n s o f t h e S a f eD r i n k i n g W a t e r A c t, w h i c h i s a n e f f o r t t o p r e v e n t c o n t a m i n a -t i o n o f t h e w a t e r s u p p l y .T h e c o a t i n g s i n d u s t r y i s u n d e r l e g i s l a t i v e p r e s s u r e t o d e -v e l o p e n v i r o n m e n t a l l y a c c e p t a b l e , h i g h - p e r f o r m a n c e , l o n g -l a s ti n g r e p l a c e m e n t s f o r l e a d a n d c h r o m a t e - b a s e d p i g m e n t sa n d r e d u c e l e v el s o f V O C . T h e s e a r e n o t e a s y t a s k s . S o m e o ft h e f o r m u l a t i o n a n d a p p l i c a ti o n f a c t o r s w h i c h a f f ec t t h e i n -d u s t r y ' s e f f o r t s a t c o m p l i a n c e h a v e a l r e a d y b e e n d i s c u s s e d ;t h e f o l l o w i n g i s a l o o k a t s o m e o f t h e s t r i n g e n t t e c h n i q u e s i nu s e t o e v a l u a t e t h e p e r f o r m a n c e o f n e w , l es s t o x ic i n h i b i t o r sa n d p r o t e c t i v e c o a t i n g s .

H O W F O R M U L A T I O N A F F E C T S T H EP E R F O R M A N C E O F I N H I B I T I V EP I G M E N T SW h e n f o r m u l a t i n g w i t h i n h i b it o r s, a h o s t o f f o r m u l a t i o n

f a c t o r s m u s t b e t a k e n a c c o u n t s i n c e t h e s e f a c t o r s o f t e n d e t e r -m i n e t h e s e l e c t i o n o f i n h i b i ti v e p i g m e n t s a n d g r e a t l y a f f e c tt h e d e v e l o p m e n t o f i n h i b i ti v e c o a t i n g s . T h e m o s t c r i ti c a lf o r m u l a t i o n f a c t o r s i n c lu d e P V C ( p i g m e n t v o l u m e c o n c e n t r a -t i o n ), s e l e c t in g a n e x t e n d e r p i g m e n t , s o l u b i li t y a n d r e a c t i v i t y( t h a t is , s e l e c ti n g a v e h i c l e a n d u n d e r s t a n d i n g h o w i ts c o m p o -n e n t s i n t e r a c t ), a n d t h e c o m p l e x i n t e r r e l a t io n s h i p s b e t w e e na l l t h e s e f a c t o r s .Pigment Volume Concentration

P i g m e n t v o l u m e c o n c e n t r a t i o n ( P V C ) i s t h e r a t i o o f p i g -m e n t v o l u m e t o t h e t o t a l n o n v o l a t i l e v o l u m e i n a c o a t in g .P r o b a b l y t h e m o s t i m p o r t a n t f o r m u l a t i o n f a c t o r t o c o n s i d e rw h e n e v a l u a t i n g a n t i - c o r r o s i v e p i g m e n t s , P V C i s e s p e c i a l l yc r u c ia l w h e n t w o o r m o r e i n h i b i t o rs a r e b e in g c o n s i d e r e d f o ra g i v e n a p p l i c a t i o n .E s s e n t ia l t o t h e u n d e r s t a n d i n g o f t h e i m p o r t a n c e o f P V C ist h e c o n c e p t o f a " f o r m u l a t i o n w i n d o w . " T h i s t e r m r e p r e s e n t st h e P V C r a n g e a t w h i c h a n i n h i b i t i v e p i g m e n t w i l l g i v e i t s b e s tp e r f o r m a n c e a n d i s c l e a r ly d e p i c t e d i n F ig . 3 . H e r e , t h e p e r -f o r m a n c e i n a s a lt fo g c a b i n e t o f t h r e e p o p u l a r c h r o m a t e - f r e ei n h i b it o r s i s c o m p a r e d t o th a t o f z in c c h r o m a t e i n a m e d i u mo il al k y d p r i m e r . L o a d i n g s a r e e q u a l - - a b o u t 1 0 % o f t o t a lf o r m u l a w e i g h t - - a c r o s s a PV C l a d d e r a t c o n s t a n t v o l u m es o l i d s .I n t h e f i g u r e , t h e t h r e e n o n t o x i c i n h i b i t o r s e a c h y i e l d e dp e r f o r m a n c e e q u i v a l e n t to z i n c c h r o m a t e , b u t o n l y w i t h i n as p e ci fi c PV C r a n g e , a r a t h e r n a r r o w f o r m u l a t i o n w i n d o w .C o n v e r s e l y , z i n c c h r o m a t e o f f e r e d f a r g r e a t e r f o r m u l a t i o nv e r sa t il it y , p e r f o r m i n g o v e r a m u c h b r o a d e r P V C r a n g e .T h e f i g u r e i l l u s tr a t e s b o t h t h e i m p o r t a n c e o f a d d r e s s i n g t h ev a r i a b le o f P V C a n d p o s e s a p r o b l e m f a c in g t h e f o r m u l a t o r :N o n e o f th e e n v i r o n m e n t a l l y a cc e p t a b l e i n h i b i t o r s - - i n -c l u d in g t h e t h r e e i n t h e f i g u r e - - c o m e c l os e to m a t c h i n g t h et r a d i t i o n a l l e a d a n d c h r o m a t e - b a s e d p i g m e n t s f o r o v e r a l l e ff i-c i e n c y a n d f o r m u l a t i o n f le x ib i li ty . W h i l e e a c h o f th e t h r e ec h r o m a t e - f r e e i n h i b i t o r s w a s f o r m u l a t e d t o e q u a l t h e p e r -


9/14

46 P A I N T A N D C O A T IN G T E S T I N G M A N U A L

oa.

" . . \< > < > v . . . . .~ % v~ - . ~ .\ "....-" , . 0 ".\ %'.1x ~ - . ~ o o -JI I I I ~ d

3 0 3 5 4 0 4 5 5 0 5 5P V C

I N H I B I T I V E P I G M E N Tm m Z i n c P h o s p h o - o x i d e [ ] [ ] [ ] D M o d i f i e d B a r i u m M e t a b o r a t e9 9 9 9 9 B o r o s i l i c a t e ~ Z i n c C h r o m at e

P V C L o a d i n g L e v el s ( F o r m u l a t io n W i n d o w s )o f C h r o m a t e -f r e e I n h i b i to r s a n d Z i n c C h r o m a t eF I G . 3 - M e d i u m o il a l k y d P V C l a d d e r : s a l t s p r a y p e r f o r m a n c e . F o r m u l a t i o n w i n -

d o w s o f f o u r i n h i b i t iv e p i g m e n t s ,

Many factors influence the formula tion window of an inhi-

and (2) the effect of these propert ies o n CPVCtical pig ment volume concentration).The conc ept of CPVC, introduced in the late forties, postu-m at which there is just eno ugh binder to coat the pig ment

[39,40]. Sinceabsorptio n than more traditional inhibitors, it isanti-corros ive coatings which exceed CPVC

across a PVC range to achieve optim um performance.C range; then weight blends of these paints may be used to

L o a d i n g L e ve lsIn the "old days" of inhibitive coatings formulations, before

ption : if a little was good, then a lot must be better. This

"rule" worked as long as formulators used red lead and oth erlead-based pigments whose very high specific gravities andlow oil absorptions permitted relatively high loadings. Wh enlarge amoun ts of nonlead inhibitors are used in formulations,the resulting coatings often perform well in accelerated cor-rosion tests. However, most nonlead inhibitors are muchmore reactive in vehicle systems than lead-based inhibitiveproducts, and this reactivity can lower the actual perform-ance of coatings formulated with high inhibitor loadings.

In general, inhibitor loading should be carefully balanced,keeping in mind that more is not necessarily better, at least asfar as less-toxic, more-reactive inhibitors are concerned.

Comparing inhibitive pigm ents--Ce rtainly not as straight-forward a matter as comparin g apples to oranges, compar-ing pigments may be accomplished in several differentways: on the basis of equal volume, equal weight, equalcost, or equal performance. The choice of method can dras-tically affect the results of a formulator's evaluations.Equal Volume--This is the easiest evaluation method andby far the least desirable. Replacing a given volume of oneinhibitor with the same volume of another is simple andrequires the least number of formulation changes. How-ever, the wide variations in specific gravities of inhibitors(from a low of 2.5 to a high of 9.1) and oil absorptions,which vary from 10 to 70, make the results of these evalua-tions highly questionable.Equal Weight- -This is a valid method of evaluating inhibi-tors with loading levels equal to or exceeding 120 g per liter


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C H A P T ER 2 7 - -I N O R G A N I C A N T I -C O R R O S I V E P I G M E N T S 247(1 lb/gal). Care must be taken to adjus t the levels of ex-tender pigment and vehicle solids when replacing one in-hibitor with another. This is to ensure that the inhibitorscan be compared at a constant loading level over a givenPVC range at constant volume solids.Equal Cost- -T his method is valid if the inhibito r levels areless than 120 g per liter (1 lb/gal), as long as comparisonsare conduc ted at a co nstant loading level over a PVC rang eat cons tant volume solids.Equal Performance--The most tedious approach, thismeth od ensures the most cost-effective formulation possi-ble. Basically a combi nation of simultaneous evaluations o fa PVC ladder and an inhibitor loading ladder, this ap-proach permits the formulator to evaluate each inhibitorfor its optimum formulation window.

Although a wide choice of extenders is available for inhi-

received muc h attention. This may be the result of thent perfo rmance masked any flaw contrib-by an extender. With emphasis on environmentally ac-

(and less effective) inhibitors, however, selection of

Figure 4 contains the results of work in latex maintenan ces. The data here clearly show that the per formanc e of

ting the appropriate extender, therefore, ma y be as cru-to coating pe rforman ce as selecting the vehicle system oritive pigmen t [41].

Inhibitors should be carefully selected for compatibility

problems arising from reactivity and solubility. For example,a pigment must have some solubility within a given system inorder to participate actively in passivation. Also, a highlyreactive pigment can react with binders and other ingredientsin a coating system, causing instability. Adverse chemicalreactions between the inhibitor and the vehicle in the can o rthe cured paint film can effect coating properties such asviscosity, drying time, cure, adhesion, and weatherability.Therefore, inhibitive pigments must be chosen carefully toprevent these formulation problems [42].Some chemical reactions, such as the reactions betweenbasic pigments and acid catalysts, neutralize inhibitive actionby preventing film cure. Some reactions ma y even acceleratecorrosion. Others may do good or harm, depending on wherethey occur. For example, the formation of metallic soaps on apaint film caused by a reaction between red lead an d linseedoil acids promot es c orrosion inhibition; however, if this samereaction oc curs in the paint can, viscosity may increase to thepoint that the liquid paint becomes unusable.Generally, inhibitive pigments retard or prevent blisteringcaused by corrosion in pinholes or scribes or by moisturedrawn into the paint film by electrical activity of the corro-sion cells. But use of inhibitors may lead to osmotic blister-ing, especially during humi dity a nd condensa tion testing. Os-motic blistering is caused by the presence of water andsoluble pigment at the juncture of the substrate and thecoating film [43]. To prevent this condition, low-solubilitypigments should be used.

A P P L I C A T I O N A N D P R O T E C T I V E C O A T I N GP E R F O R M A N C ECoating application (how, where, why, and for how long acoating will be used) mu st be carefully considered whe n for-mulating environmentally acceptable alternatives to tradi-tional toxic inhibitors. The length of time a protective coati ngcan effectively protect a ferrous surface has been said to

1 0

wOz 6


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4 8 P A I N T A N D C O A T IN G T E S T I N G M A N U A L

t , h o w t h e c o a t i n g i s a p p l i e d , t h e s u b s t r a t e , a n d t h eo f t h e s u b s t r a t e [44].C o n s i d e r t h e e n v i r o n m e n t w h e r e t h e in h i b i t iv e c o a t i n g w i ll

i n d u s t r i a l a t m o s p h e r e s , h e a v y t ra f f ic , h i g h h e a t , o re x t r e m e c o n d i t i o n s ? O r w i l l i t b e u s e d i n a r u r a l s e t ti n g ,

s w e r i n g t h e s e q u e s t i o n s w i l l h e l pi t h t h e c o a t i n g .

O t h e r f a c t o r s to c o n s i d e r w h e n d e c i d in g h o w t o a p p l y ac o a t i n g i n c l u d e t h e t y p e o f a p p l i c a t o r ( b r u s h ,

e r , s p r a y , o r d ip ) , a d e q u a t e a n d u n i f o r m f i lm t h i c k n e s sd r y i n g ti m e , a n d t e m p e r a t u r e . T h o u g h a p p l i c a t io n t e c h -

a r e o f t e n g i v e n li tt le a t t e n t i o n , t h e y c a n p r o f o u n d l yt h e p e r f o r m a n c e o f a p r o t e c t i v e c o a t i n g .

S u b s t r a t e s a r e a l s o i m p o r t a n t f a c t o r s i n t e s t i n g p r o t e c t i v eo l l e d s te e l g e n e r a l l y s h o w s m o r e u n d e r c u t t i n g a t s c r i b e s

s a n d b l a s t e d h o t - r o l l e d s te e l o r t r e a t e d s t e el . B e c a u s e i t i sf r e q u e n tl y u s e d t o c o m p a r e t h e p e r f o r m a n c e

d i f f e r e n t in h i b i t iv e p i g m e n t s . O n t h e o t h e r h a n d , u s e o f ah a t i z e d s u b s t r a t e s e e m s to i m p r o v e t h e p e r f o r m a n c e o f

c o a t i n g s , a p p a r e n t l y e n h a n c i n g a d h e s i o n , p a s -

A w o r d o f c au t i o n : W h e n s e l e c ti n g s u b s t r a t e s f o r t e s ti n gl e v a l u a t io n s o f a s y s t e m o n t h e s u b s t r a t e t h a t t h e c o a t i n g

u a t io n s m a y b e c o n d u c t e d o n a n y a p p r o p r i a t eP r o t e c t i v e c o a t i n g s a r e f r e q u e n t l y a p p l i e d t o t h e f o l l o w i n g

h o t - r o l l e d s t ee l; p r e - r u s t e d h o t r o l l e d s t e e l, a n d g a l v a -N o m a t t e r w h a t t h e s u b s t r a t e , t h e s u r f a c e p r e p a r a t i o n o f

s t r a t e , o r i ts c o n d i t i o n , i s v i t al t o t h e e v a l u a t i o n o fe e x p e c t a n c y o f a c o a t i n g s y s t e m . I n a d d i t i o n t o c l e a n -

f a c e , im p r o v i n g a d h e s i o n , a n d m a x i m i z i n g th e p e r -c e o f t h e i n h i b it iv e p i g m e n t , s u r f a c e p r e p a r a t i o nr e m o v e c o n t a m i n a n t s - - s u c h a s c h lo r i d es a n d s u l-

t e s - f r o m t he s u b s t ra t e 's s u rf a ce [47].T h o u g h m a n y m e t h o d s o f s u r f a c e p r e p a r a t i o n e x is t, t h e iri v e n e s s v a r i e s w i d e ly . F o r e x a m p l e , s t u d i e s s u g g e s t t h a t

t h a n o n a h a n d - c l e a n e d o r w e a t h e r e d s u b s t r a t e [48].l o n g e v it y i s a tt r i b u t e d t o b e t t e r a d h e s i o n a n d a

r e i n t i m a t e c o n t a c t b e t w e e n s u b s t r a t e a n d c o a ti n g , w h i c ho w s t h e i n h i b i t o r t o m a x i m i z e i t s p e r f o r m a n c e .S o m e o f t h e c o m m o n l y u s e d s u r f a c e c le a n i n g t ec h n i q u e s: c e n t r i f u g a l b l a st i n g ; m e t a l l i c a n d n o n m e t a l l i c a b r a -

e r t o ol s; c h e m i c a ls ; p i c kl in g . L e s s c o m m o n m e t h o d s i n-

E V A L U A T IN G I N H I B I T I V E P I G M E N T A N DC O A T I N G P E R F O R M A N C EP i g m e n t a n d c o a t i n g e v a l u a t io n t e c h n i q u e s - - e s p e c i a l l y a c -

c e le r at ed l a b o r a t o ry t e s t s - - a r e m o r e i m p o r t a n t n o w t h a ne v e r b e f o r e . T h e r e a s o n : U s e r s a n d m a n u f a c t u r e r s a l ik e d e -m a n d t h a t n e w i n h i b it iv e p r o d u c t s n o t o n l y m e e t e n v i ro n -m e n t a l r e g u l a ti o n s , b u t a l s o m e e t o r e x c e ed t h e p e r f o r m a n c es t a n d a r d s s e t b y e s t a b li s h e d, e n v i r o n m e n t a l l y u n a c c e p t a b l ep i g m e n t s . T h i s d i s c u s s i o n w i l l d e a l w i t h t h e t w o p r i n c i p l em e t h o d s o f e v a l u a t i o n u s e d b y t h e c o a t i n g s i n d u s t r y : a c c e l e r -a t e d t e s t i n g a n d " r e a l - w o r l d " ( n a t u r a l a t m o s p h e r i c ) t e s t i n g .Accelerated Corrosion Test ing

I n r e a l - w o r l d a p p l i c a t i o n s , p r o t e c t i v e c o a t i n g s a r e o f t e ne x p e c t e d t o p r o v i d e u p t o 2 0 y e a r s o f s e r v i c e . A c c e l e r a t e dt e s t i n g a t t e m p t s t o p r e d i c t t h e l if e e x p e c t a n c y o f a p r o t e c t i v ec o a t i n g i n a s h o r t s p a n o f t i m e , f r o m h a l f a y e a r t o a y e a r . T h i se v a lu a t iv e " f a st f o r w a r d i n g " m e e t s t h e n e e d s o f m a n u f a c t u r -e r s a n d u s e r s a l i k e , w h o d e m a n d p r o m p t d e v e l o p m e n t a n dc o m m e r c i a l i z a t io n o f n e w p r o d u c t s . T h o u g h s e v e r a l k i n d s o fa c c e l e r a t e d t e s t i n g p r o c e d u r e s a r e i n u s e , a l l h a v e e s s e n t i a l l yt h e s a m e p u r p o s e s : t o s i g n i fi c a n t ly s h o r t e n t h e t i m e n e e d e df o r a c o a t i n g t o f a i l a n d t o e v a l u a t e t h e c a u s e s o f i ts f a i lu r e .

A l t h o u g h a c c e l e r a t e d c o r r o s i o n t e s t i n g i s t h e m o s t c o m -m o n l y u s e d e v a l u a t io n m e t h o d f o r in h i b it iv e p i g m e n t s a n dp r o t e c t i v e c o a t i n g s , t h e r e s u l t s o f a c c e l e r a t e d t e s t s c o r r e l a t ep o o r l y w i t h t h e r e s u l t s o f r e a l - w o r l d t e s ti n g . B e c a u s e o f t h i s ,t h e v a l i d i ty o f a c c e l e r a t e d t e s t in g , p a r t i c u l a r l y t h e s a l t s p r a yt e st , h a s b e e n t h e t a r g e t o f s e ri o u s d o u b t s a n d q u e s t i o n s . I sa c c e l e r a t e d t e s t i n g r e l e v a n t t o r e a l- w o r l d c o n d i t i o n s ? D o t h et e s t s a c c u r a t e l y p r e d i c t p r o d u c t p e r f o r m a n c e ? E v i d e n c e i sg r o w i n g t o s u p p o r t t h e t h e o r y t h a t m a n y f o r m u l a t i o n v a r ia -t io n s u s e d t o e n h a n c e a c o a t in g ' s sa l t s p r a y p e r f o r m a n c e m a ya c t u a ll y d e t r a c t f r o m t h e c o a t i n g ' s re a l -w o r l d p e r f o r m a n c e .I t h a s a l r e a d y b e e n d e m o n s t r a t e d t h a t, i n m a n y c a s e s , ac -c e l e r a t e d t e s t i n g i s a n u n r e l i a b l e p r e d i c t o r o f a c o a t i n g ' s l o n g -t e r m p r o t e c t i v e p e r f o r m a n c e . F i g u r e 5 c l e a r l y i l lu s t r a t e s t h i sd e f ic i e n cy , d e p i c t i n g s a l t s p r a y r e s u l t s w h i c h t o t a l ly c o n t r a -d i c t th e r e s u l t s o f n a t u r a l e x p o s u r e t e s t i n g f o r tw o p r o t e c t i v ec o a t i n g s .D e s p i t e s u c h s t r o n g e v i d e n c e a g a i n s t t h e r e l ia b i l it y o f th es a l t s p r a y t e s t , it s p o p u l a r i t y c o n t i n u e s . F o r g o o d o r b a d r e a -s o n s , i t i s t h e d r i v i n g f o r c e b e h i n d t h e e s t a b l i s h m e n t o f p e r -f o r m a n c e e v a l u a ti o n c r i t e ri a f o r m a n y p r o t e c ti v e c o a t in g s y s-t e m s a n d i s o f t e n t h e only c r i t e r i o n u s e d t o e v a l u a t e a ni n h i b i t i v e c o a t i n g . I n s o m e c a s e s , r a w m a t e r i a l s u p p l i e r s a r ef o r c e d t o o f f e r p r o d u c t s b a s e d s o l e ly o n t h e i r s a l t s p r a y p e r -f o r m a n c e , o f t e n w i t h o u t r e g a r d t o th e c o a t i n g s ' p e r f o r m a n c ei n t h e r e a l w o r l d . A n o t h e r s e c t i o n o f t h is b o o k d i s c u s s e s t h i ss i t u a t i o n f u r t h e r a n d d e t a i l s t h e r e s e a r c h t h e i n d u s t r y i sd o i n g t o a d d r e s s t h e p r o b l e m .Rea l -Wor ld Corros ion Tes t ing (Na tura l Atm osph er icExp o s u r e T e s t i n g

C o n s i d e r e d b y m a n y r e s e a r c h e r s t o b e t h e m o s t d e p e n d a b l em e t h o d o f p re d i c t in g t h e p e r f o r m a n c e o f a p r o t e c t iv e c o a ti n g ,r e a l - w o r l d t e s t in g e x p o s e s p r o d u c t s t o r e a l - t im e , a c t u a l a t m o -s p h e r i c c o n d i t i o n s . I n e s s e n c e , t h e t e s t s a p p r o x i m a t e t h et y p e s o f e n v i r o n m e n t s f o r w h i c h t h e c o a t in g s w e r e d e s ig n e d .


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C H A P TE R 2 7 - - I N O R G A N I C A N T I -C O R R O S I V E P I G M E N T S 2 4 9t h i c k n e s s e s . N e i t h e r o f t h e s e p r o c e d u r e s a r e e f f e c t iv e ,h o w e v e r , s i n c e n e i t h e r r e p r e s e n t s t h e a p p l i c a t i o n f o rw h i c h t h e c o a t i n g w a s d e s i g n e d .

2. Lack of Controls - -Es tabl ish ing c o n t r o l s a n d m e a s u r i n g ac o a t i n g ' s c o n s i s t e n c y o f p e r f o r m a n c e i s a r e a l c h a ll e n g e i nr e a l- w o r l d t es t in g . I n o u t d o o r e x p o s u r es , e n v i r o n m e n t a lf a c t o r s s u c h a s t e m p e r a t u r e a n d h u m i d i t y c h a n g e f r o my e a r t o y e a r a t t h e s a m e s i t e , m a k i n g r e p r o d u c i b l e r e s u l t sn e x t t o i m p o s s i b l e .

3. Multiple Stres ses--B eca use a c o a t i n g e x p o s e d to a n o u t -d o o r e n v i r o n m e n t e n c o u n t e r s m a n y s t re s s es , i t i s n o t e a s yt o d e t e r m i n e t h e e f f e c t s o f e a c h i n d i v i d u a l s t r e s s .4 . Vary ing Env i r o nm en t s - - I n o u t d o o r t e s t in g , e s t a b l i s h i n g as t a n d a r d t e s t e n v i r o n m e n t i s v i r t u a l ly i m p o s s i b l e . E n v i r o n -m e n t s v a r y d r a m a t i c a l l y f r o m o n e p a r t o f t h e c o u n t r y t ot h e o t h er , f r o m d e s e r t t o s e a s h o re , f r o m m o u n t a i n s t oa l l u v i a l p l a i n s . L o c a l i n d u s t r i e s a n d f l u c t u a t i o n s i nw e a t h e r p a t t e r n s a l s o c o n t r ib u t e t o t h e p r o b l e m .

I n s p i t e o f t h e s e d r a w b a c k s , r e a l - w o r l d t e s t i n g i s e m e r g i n ga s a v i a b l e a l te r n a t i v e t o t r a d i t i o n a l a c c e l e r a t e d t e s t in g . E v e nc o m p a r a t i v e l y b r i e f e x p o s u r e s o f s ix t o e i g h t m o n t h s c a n y i e ldi m p o r t a n t d a t a . H o w e v e r , th e v a l u e o f t h e s e d a t a d e p e n d s o ns o u n d e x p e r i m e n t s a n d p r o p e r e v a l u a t io n t ec h n i q u e s . F o r -m u l a t o r s c a n n o l o n g e r d e p e n d o n " o n e p a i n t /o n e p a n e l " t e s td e s i g n s .S t a t i s t i c a l A n a l y s i s

A n o t h e r m e t h o d o f c o r r e la t in g p e r f o r m a n c e d a t a a n d p r e -d i c t in g c o a t i n g p e r f o r m a n c e i s t h e u s e o f s t a t is t i c a l a n a ly s i s ,w h i c h i s g r o w i n g i n p o p u l a r i t y . V a r i o u s t e c h n i q u e s u s e d t od e s i g n m o r e m e a n i n g f u l , a c c u r a t e e v a l u a ti o n s i n c lu d e c o r r e -l a t i o n m e t h o d s , s u r v i v a l a n a l y s i s , c u r v e f i t t i n g , t i m e s e r i e sa n a l y s i s , a n d r e l i a b i l i t y a n d l i f e a n a l y s i s .

S e v e ra l t e ch n i q u e s h a v e b e e n d e v e l o p e d f o r a ss e s s in g m a c -r o s c o p i c d a m a g e t o p r o t e c t i v e c o a t i n g s . T h o u g h c o a t i n g sh a v e t r a d i t i o n a l l y b e e n a s s e s s e d b y v i s u a l in s p e c t i o n , t h e s ep r o c e d u r e s c a n a p p a r e n t l y d e t e c t e a r l y s ta g e s o f c o r r o s io n ,g i v i n g m o r e r e l e v a n t , a c c u r a t e i n f o r m a t i o n i n a s h o r t e r e x p o -s u r e p e r i o d .

F IG . 5 -Protect ive coat ing per formance data .

D e s p i t e t h e i r e x c e l le n t a b i li t y t o m e a s u r e a n d p r e d i c t a n t i -v e c o a t in g p e r f o r m a n c e , o u t d o o r e x p o s u r e t e s ts h a v e

. Long Dur a t ion - - M o n i tor ing t h e d e g r a d a t i o n o f p r o t e c t i v ec o a t i n g s i s a m a r a t h o n t a s k l a s t i n g f r o m 5 t o 2 0 y e a r s . T oa c c e l e r a t e t h e r a t e o f f a i lu r e i n o u t d o o r e x p o s u r e s , t e s t i n gl a b s h a v e e x p o s e d th e p a i n t s t o v e r y h a r s h e n v i r o n m e n t s o ra p p l i e d t h e c o a t i n g s a t l o w e r - t h a n - r e c o m m e n d e d f i l m

R E C E N T T R E N D S A N D D E V E L O P M E N T SN e w P r o d u c t D e v e l o p m e n tT h e F e d e r a t i o n o f S o c i e t i e s f o r C o a t i n g s T e c h n o l o g y( F S C T ) m o n i t o r s t h e d e v e l o p m e n t a n d p e r f o r m a n c e e v a l u a -t io n s o f m a n y n e w p r o d u c t s , m a n y o f w h i c h r e fl e ct th e t r e n dt o w a r d l e s s to x i c i n h i b i t o r s [51]. A n o t h e r g r o w i n g t r e n d i s th ed e v e l o p m e n t o f p r o d u c t s w i t h l o w e r o i l a b s o r p t i o n , n e c e s s a r yf o r t h e f o r m u l a t i o n o f h ig h s o l i d s v e h ic l e s .B e c a u s e p r o p e r u s e o f au x i l ia r y e x te n d e r s c a n e n h a n c e i n-h i b i t iv e p r o p e r t i e s o f p r o t e c t i v e c o a t in g s ( e s p e c ia l l y th o s e i n -c o r p o r a t i n g n o n t o x i c i n h i b i t o r s ) , t h e t r e n d t o w a r d d e v e l o p -i n g a u x i l i a r y e x t e n d e r p i g m e n t s i s l i k e l y t o c o n t i n u e .A u x i l ia r y p i g m e n t s a r e s t a n d a r d e x t e n d e r s m o d i f i e d b y s u r -f a c e t r e a t m e n t s o f s il a n e s, t i t a n a te s , z i c o n a t e s , a n d o t h e rc o m p o u n d s . S o m e c o m m o n l y u s e d a u x il ia r y p i g m e n t s i n -c l u d e c a l c i u m c a r b o n a t e s , c l a y s , t a l c s , s i l i c a s , a n d w o l -l a s t o n i t e .


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5 0 P A I N T A N D C O A T I N G T E S T I N G M A N U A L

D r i v e n b y c h a n g i n g r e g u l a t o r y s t a n d a r d s a n d t h e i n d u s t r y 's [7]p t p r o d u c t d e v e l o p m e n t , a v a r i e ty o f a l t e r n a -

v e a c c e l e r a t e d t e s t in g p r o c e d u r e s a r e b e i n g r e s e a r c h e d a n d [8 ]. T h e c h a l l e n g e is t o m a k e a c c e l e r a t e d t e s t in g a c c u -

r e f l e c t r e a l - w o r l d c o n d i t i o n s a n d y i e l d m e a n i n g f u l , r e - [9 ]s t ic re s u lt s . T h e C o r r o s i o n C o m m i t t e e o f t h e F e d e r a t i o n o f

e s fo r C o a t i n g T e c h n o l o g y ( F S C T ) c o m m i s s i o n e d a [10][52] o n t h i s s u b je c t d ir e c t e d to m a n u f a c t u r e r s a n dp r o t e c t i v e c o a t i n g s . T h e r e s u l ts o f t h e s u r v e y s h o w e d [11 ]

r t i c i p a n t s w e r e e x p l o r i n g th e u s e o f th e f o l l o w i n gi f i e d a c c e l e r a t e d t e s t i n g te c h n i q u e s : ( 1 ) c y c l i c [12]

; ( 2 ) c y c li c n a t u r a l e x p o s u r e s ; ( 3) e l e c t r o c h e m i c a l t e s t i n g ; [13]( 4) p h y s i c o - c h e m i c a l , p h y s i c a l, a n d m e c h a n i c a l t e s t in g [14]

T h e S t e e l S t r u c t u r e s P a i n t i n g C o u n c i l ( S S P C ) i s e v a l u a t i n g [15]p r o m i s i n g o f t h e s e a c c e l e r a t e d c y c li c c o r r o s i o n

. T h e s u r v e y i t s e l f i s t o o e x t e n s i v e t o b e i n c l u d e d h e r e b u tr e a d i l y a v a i l a b l e f r o m t h e F S C T . [16]

B e f o r e t h e b e g i n n i n g o f r e c o r d e d h i s t o r y a n d e v e r s in c e ,t i n g f e r r o u s s u r f a c e s f r o m t h e d i s i n t e g r a t i n g e f f e c ts o fo n h a s b e e n o n e o f m a n k i n d ' s m o s t c o m m o n a n d p e r -

b a t t le s . F r o m t h e l e a d c o m p o u n d H o m e r ' s b l a c k -e a r e d o n t h e i r w a g o n w h e e l s t o t h e s o p h i s t ic a t e d

t o d a y , t h e s c i e n c e o f p r e v e n t i n g c o r -p l e x a s t h e c o r r o s i o n p r o c e s s i t se l f. O v e r th e

, m a n y f a c t o rs h a v e a r i s e n w h i c h d e m a n d c o n s i d e r -s u r f a c e p r e p a r a t i o n , v e h i c l e a n d

e n t s e l e c t i o n , a n d P V C , t o n a m e j u s t a f e w .N o w t h e c o a t in g s i n d u s t r y i s e n g a g e d i n a n e w a p p r o a c h t oa g e - o l d c o r r o s i o n - p r o t e c t i o n p r o b l e m : d e v e l o p i n g e ff e c-

v e n e w n o n t o x i c i n h i b i t o r s t o t a k e t h e p l a c e o f t r a d i t i o n a lr o m a t e - b a s e d p i g m e n t s . T h e r e s e a r c h i s w e l l u n -

t h e n e x t t e n y e a r s m a y w e l l s e e t h e w i d e s p r e a do f t h e s e s a fe , h i g h - p e r f o r m a n c e a l t e rn a t i v es . T h i s

b e a c c o m p l i s h e d n o t b y s c i e nt i fi c c u r i o s i ty o rm e r c i a l d e m a n d a l o ne , b u t a l s o b y c o n c e r n f o r th e p r o -

o u r f r a gi le h u m a n h a b i t a t .

[1 ] Chi l ton , J . P . , Principles of Metallic Corrosion, T h e R o y a lIn s t i t u t e o f Ch emi s t ry , W . He i f e r an d S o n LTD , Camb r id g e ,England , 1964 .[2] Sm ith , A. , Inorganic Primer Pigments, F e d e r a t i o n S e r i e s o nCo a t i n g s Tech n o lo g y , Fed er a t i o n o f S o c i e t i e s fo r Co a t i n g s Tech -nology , Ph i ladelph ia, PA, 1988.[3] Chi l ton , J . P . , Principles of Metallic Corrosion, Th e Ro y a l In s t i -t u t e o f Ch emis t ry , W . He i f e r an d S o n LTD , Camb r id g e , En g l an d ,1964.[4] Schwei tzer , P . A. , What Every Engineer Should Know AboutCorrosion, M a rce l D ek k er , I n c ., New Y o rk an d B ase l , 1 9 87 .[5 ] G leek m an , L . W . , " S e l ec t i n g M ate r i a l s o f Co n s t ru c t i o n , "Corrosion and Corrosion Protection Handbook, P . A . S ch wei t z e r ,Ed . , M arc e l D ek k er , I n c . , New Y o rk an d B ase l , 1 9 83 .[6 ] E ick h o f f , A . J , , " Co r ro s i o n In h ib i ti v e P ig men t s an d H o w T h eyFu n c t i o n , " Steel Structures Painting Manual, Vol. 1, Good Paint-

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