Corrosion Protection Methods

Coatings

Corrosion Protection by Coatings

Metallic Coatings

Other Inorganic Coatings

Paint Coatings

Other Forms of Organic Coatings

Pre-treatment Before Coating

Metallic Coatings

Methods of application Dipping

Electroplating

Spraying

Cementation

Diffusion

Method selection considerations Corrosion resistance that is required, expected

lifetime of component, # of parts being produced, environmental considerations

Hot Dipping

Immerse metal (usu. Steel) in molten metal bath (Zn/Al/Al-Zn)

Continuous process: eg galvanizing sheet

Batch process: galvanising fabricated parts, nuts, bolts, fastners

Electroplating

Substrate is made cathode Aqueous electrolyte of depositing metal 4-30 m thick Zn, Cd, Cr, Cu, Au, Ni, Sn, Ag and alloys Sn-Zn, Zn-Ni,

brass, bronze, gold alloys, nickel alloys Electroplated Zn more uniform thickness and other

surface characteristics vs. hot dipped- automobile body sheets

Electroless plating: chemical reduction of metal-salt solutions, precipitated metal forms a adherent layer eg. Electroless Ni

Cementation

Tumbling the work in a mix (metal powder+flux) at high T

Powder metal diffuses into base metal

Eg. Al, Zn

Thermal Spraying

A gun simultaneously melts and propels small droplets Feedstock: powder, rod, wire, liquid

Flame-powder spraying

Flame-wire spraying

Plasma spraying- 12,000C, powder feedstock

Coating characteristics: Porosity (can be filled with thermoplastic resins for

Corr Resist.) , any thickness, adherent, can be applied on fabricated structures

Classification of coatings

All coatings provide barrier protection Barrier between corrosive environ & metal Coatings can get damaged during shipment/use Galvanic action at the base of the scratch-

performance

Noble coatings Only barrier protection Ni, Ag, Cu, Pb, Cr

Sacrificial coatings Barrier protection Cathodic protection

Current flow at defects in noble/sacrificial coatings

Cr/Ni on steel -coating is undermined -coating should have minimum pores -thick

Zn/Cd on steel -base metal is cathodically protected -degree of porosity inconsequential -thick- longer protection

Area over which sacrificial protection extends

f( solution conductivity)

Cathodic current densities fall off with distance from anode

Fall off rate is high in distilled/soft water

It is low in seawater

eg. 3mm wide Zn coating defect on steel begins to show rust in the center in soft water

Several feet wide defect in seawater no rusting!

Zn coatings Barrier and Galvanic protection Corrosion products- Zn compounds (carbonate/hydroxides)

White, colorless

Rusting appears sooner with severity of atmosphere Rural (0.2-3 m/yr) < marine (0.5-8) < industrial atmosphere (2-

16)

Hot dip galvanizing Metallurgical bonding (diffusion) to steel => Zn-Fe alloys

very adherent

Continuous process thinner coatings Batch process- thick 0.1-0.2 % Al addition suppresses Fe transport into coating USE: atmospheric corrosion protection of steel- roofing,

automotive- truck and bus parts, fencing, A/C, farm implements

Electrogalvanized- electrodeposited Zn alloy coatings too- Zn-Fe, Zn-Co, Zn-Ni

Galvanizing provides good surface for organic coatings Extend life of Zn coating

Aqueous envorn at RT Corro rate is lowest in pH 7-12 Seawater: 0.13 mm thick no rust for ~ 5 yr Aerated hot water >60C

Reversal of polarity- Zn develops noble characteristics pitting is possible Waters high in carbonates & nitrates favor the reversal Waters high in chlorides & sulphates decrease reversal

Normally

porous Zn(OH)2 product

insulating

Zn is anodic

Reversal

ZnO

electrically semi-conducting

in aerated waters, works as oxygen electrode with noble potential

Ni Coatings

Usually electroplated Cu underlayer

To reduce required thickness of (expensive) nickel layer To facilitate buffing operation, Cu is soft Ni underlayer is used in automotive industry for microcracked Cr

deposition

Minimum prescribed thickness For indoor exposures: 0.008-0.013 mm Outdoor: 0.02-0.04 mm Thicker coatings for use near sea coast/ industrial environments Thinner for dry, unpolluted environs. Chemical industry : 0.025 to 0.25 mm

Fogging: In industrial atmospeheres, reflectivity from Ni sulphate film formation Coat with very thin 0.0003 mm Cr overlayer

Electroless nickel For chemical industry Ni is reduced by hypophosphite at boiling point Typical solution NiCl2.6H2O 30g/l; Sodium hypophosphite 10g/l; sodium

hydroxyacetate 50 g/l, pH: 4-6 Deposit is a Ni-(7-9 %) P alloy Corrosion resistance is comparable to electrolytic Ni Adv: Uniform coating even on intricate parts

Cadmium coatings By electrodeposition Cathodic protection to steel Lower p.d. between steel/Cd as compared to steel/Zn

it maintains potential of steel below the critical pot for SCC, and Above the pot for H cracking More reliable protection than Zn in moist environ

expensive than Zn Brighter metallic appearance Better electrical contact Better solderability, used in electronic equipment More resistant to attack by aqueous condensate/salt spray Coeff. Of friction () is lower than Zn, low torque resistance; use:

fastening hardware & connectors (where frequent dismantling is required)

Extensively used in aerospace applications

Cd coatings Resists attack (unlike Zn) by alkalies in aqueous

media Like Zn it is corroded by dilute acids and aqueous

ammonia Cd salts (corrosion products) are toxic than Zn

salts

No contact with food products Galvanised coatings are OK for drinking water only not for

food

Cd plating solutions toxic, waste disposal problem Cd replacements are being sought

Tin Coatings

Tin is active to steel in most food products

Galvanic + barrier protection

On the outside of a can, Sn is cathodic to Fe barrier protection

On the inside, Sn is always anodic cathodic protection

Potential reversal occurs from complexing of stannous ions, Sn2+ by food products => aSn2+

potential of Sn becomes more active

Tin salts are non-toxic, tasteless, colorless

Tinplate (low C-steel strip coated with tin) food/beverages containers- millions of tons of tinplate

Electrodeposited more uniform and thin than hot-dipped tinplate is heated Sn melts=> FeSn2 form, then passivated

in chromic acid/sodium dichromate Can-makers tinplate has 5 layers

Steel sheet 200-300 m FeSn2 layer 0.08 m Tin layer, 0.3 m Passive film 0.002 m Oil film 0.002 m

Galvanic protection is lost in presence of dissolved O food should not be retained in tinplated cans after

opening Sn has high hydrogen overpotential H+ reduction is

insignificant

Al coatings Steel is aluminized by hot-dipping/spraying/cementation Molten bath Al+ Si to retard formation of brittle iron

aluminide layer Oxidation resistance up to 680C

Use: automotive mufflers, oven construction, hot-dip Al coated 409 SS- auto exhaust systems: 760C

Al-44Zn-1.5Si: cathodic protection, excellent resistance to marine and industrial atmospheres, and oxidation resistance at high T

Sprayed Al coatings: sealed with laquers/paints 0.08-2mm thick, longer life than Zn in industrial atm.

Cemented Al coatings (Calorizing) Al powder+ Alumina + NH4Cl as flux in Hydrogen atm at 1000C Al-Fe surface alloy forms- resists oxidation in air/oil-refining

sulphurous atm up to 950C Protect gas turbine blades (Ni based alloys) from oxidation

Cr coatings

Cr highly resistant to atmospheric corrosion

thin bight overlay over other coatings

Retains decorative appeal for long time Extreme hardness

Low

Non-galling ( galling: wear caused by adhesion between sliding surfaces)

Corrosion resistance

used where wear & abrasion resistance is required

Protects underlying Ni layer from fogging

Cladding

Cladding dissimilar metals High-T roll bonding/ co-extrusion Pressure welded diffusion bond Composite sheet/plate/tubing has favorable

properties of both the alloys eg. SS clad on to Cu- for corrosion resistance, retaining

thermal & electrical conductivity of Cu Roll bonding thin Al alloy layer (active) on high

strength Al alloy plate Sacrificial, pitting, exfoliation & SCC of high strength alloy is

inhibited

Inorganic Coatings

Glass coatings (alkali borosilicates) Vitreous enamels (porcelain enamels)/glass linings Fused on metals Powdered glass is applied on pickled steel (Cu, brass, Al too) surface Heated in a furnace to glass softening T (750-850C)

powder melts, flows, and then hardens to a smooth, durable vitreous coating Several coats may be applied

Decorative Corrosion protection

Resist strong acids, mild alkalies excellent barrier to water/Oxygen- pore free coatings are must

Weakness: Susceptible to mechanical damage, cracking by thermal shock Crazing : network of cracks Repair- tamping Au/Ta foil into cracks

Gasoline pump casings, advertising signs, decorative building panels, plumbing fixtures, appliances etc

Aeroplane exhaust tubes- high T gases Long life in soils, too

Portland Cement coatings

Low cost CTE ~ steel Ease of application/repair

Centrifugal interior of pipings Troweling, spraying 5- 25 mm thick- may be reinforced with wire mesh 8-10 days cure is required

Use: Cast iron, steel water pipes- water & soils sides Interior of hot/cold water tanks, oil tanks, protect against seawater, mine waters

Limitations: Damage from mechanical/ thermal shocks Sulphate rich waters attack the cement

Chemical conversion coatings

In-situ chemical reaction with metal surface Phosphate coatings (steel)

Parkerizing/Bonderizing Brush/spray the cold/hot Mn or Zn acid orthophosphate (eg.

ZnH2PO4 + H3PO4) solution Sometimes accelerators (Cu2+, NO3-) are added Reaction product porous metal phosphate firmly bonded

to steel surface

Useful as a base for paints- good adherence Decreases the tendency for corrosion to undercut the

paint film at scratches Auto bodies- first & most imp layer is ~3 m

phosphate coating thinnest yet it anchors subsequent layers

Oxide coatings on steel Controlled high T oxidation in air

Or Immersion in hot conc. alkali solutions containing nitrate, chlorates

Blue/brown/black in color (mostly Fe3O4)

Corrosion protection is obtained after rubbing in inhibiting oils/waxes eg. Oxidized gun barrels

Oxide coatings on Al Room T anodic oxidation (anodizing) in suitable electrolyte (eg. Dilute H2SO4) at >100 A/m

2.

Al2O3 coat: 0.0025- 0.025 mm thick

Must be hydrated to improve protection

(sealing) to close the porous Al oxide

Hot dilute chromate solution seal provides better protection Oxide coating may be dyed for colors

hydrated aluminum oxide (boehmite) has a greater volume than the aluminum oxide Steam/hot water exposure for several minutes

Anodized coats provide good base for paints on Al

MgF2 on Mg by anodizing in (10-30% NH4HF2) at 90-120V - base for finishing treatments

Chromate coatings On Zn

Immerse in acidified sodium dichromate solution (200 g/l) for few sec at RT, rinse, dry

Zn chromate surface- slight yellow

Protects against spotting/staining by condensed moisture

Atmospheric corrosion resistance

For Zn-Al coatings and Cd coatings, too

Organic Coatings

Physical barrier between the substrate and the corrosive environment

May also serve as a reservoir for inhibiting compounds Liquid-applied organic coatings

Suitable for atmospheric corrosion protection & mild corrosive conditions

Liquid Solvent

Carries the dissolved/suspended resin & inert pigment particles Solvent evaporation cures the coating Industrial coatings are futher cured by baking below ~300C

Resin Provides the chemical & corrosion resistance

Pigment Decreases permeability, provides opacity & colour => shields the cured resin

from degrading UV exposure May possess corrosion inhibition function

Minor additives Improve coating flow, emulsification, uniformity, reduce pigment settling

Painting system (suggested)

Conditions

Unpainted Structural steel embedded in concrete, or protected by fire-proofing membrane in dry interiors

Latex or 1 coat Interior- dry, very mild

Oil base Exterior: normally dry, life >6 years

Vinyl, coal-tar epoxy, epoxy, chlorinated rubber

Frequent wetting by fresh water: condensation, splash, spray, frequent immersion

Vinyl, coal-tar epoxy, epoxy, Zn rich

Frequent wetting by salt water

Vinyl Chemical exposure: acidic pH < 5

Zn-rich Chemical exposure: pH 5-10

Epoxy Chemical exposure: mild organic solvents, aliphatic hydrocarbons

None suitable Chemical exposure: severe- oxidizing chemicals, strong organic solvents, high T exposures

Primer coats:

resins that have good adhesion to the metal substrate and readility amenable to further coating

Improve durability of top coats

Top coats

Have maximum resistance to weather & chemical envorns.

Oil based coatings (enamels) Alkyd resins

2/3 rd of the coatings used for corrosion protection Polymerize and cross-link by oxidation (air) during

drying/evaporation of the oil solvent Ease of application & low cost Suitable for steel surfaces exposed to atmosphere

Modified alkyds Additional organic groups (modifiers)

Silicon modified alkyds- durability, gloss retention, moisture & heat resistance

Marine maintenance, petroleum processing equipment Amino-resin-modified aldehydes

High humidity applications (fridge, washing machines) Phenolic modifications

For water immersion

Vinyls Do not cross-link during curing, soften with T 14% PVA (polyvinyl acetate- a copolymer) disrupts the 86%

PVC (polyvinyl chloride) chains to allow dissolution in ketones

Curing: solvent evaporation deposits uncross-linked long chain polymer- consolidates into a film (such coating are called laquers)

Pigment: rutile TiO2 Multiple coats make the lacquers durable Can act as primers for other more resistant top coats,

vinyl coatings partially dissolve in many organic solvents => good adherence

Chlorinated rubber coatings

Thermoplastic

Cure by solvent evaporation (like vinyls)

Prepared by reacting Cl with natural/synthetic rubber + add stabilizers/plasticizers

Resist water vapour penetration

Epoxies Thermosetting coatings

Highly cross-linked & unreactive ( difficult to top coat)

Mix dissolved resin (glycidyl ether + pigment) with curing agent (polyamine/polyamide) before application

Curing agent co-polymerizes with resin Polyamine is a smaller molecule => tight, more chemical resistant

coating Polyamide => flexible coatings, slightly reduced resistance to

chemicals

Modified epoxies Application-wise tailored properties Coal-tar epoxies- coal tar as filler improves moisture resistance Phenolic cross-linked epoxies: resistant to alkalies, acids, solvents

Coatings for tanks, cans, drums process equipment

Zn bearing coatings Zn powder as a filler Binder one of the organic resins

Paints with Zn pigments- 80 wt % pigment out of which 20 wt % is Zn oxide Zn/ZnO combination inhibits atmospheric rusting in primers/finish coats Excellent coverage, adhesion & abrasion resistance Less galvanic protection Good for rural & mild industrial atmosphere

Zn-rich coatings- 92-95 wt % metal Zn when dried, with no oxide Good galvanic protection Zn seals coating defects with Zn corrosion products & at defects provides galvanic protection

undercutting at paint/metal interface is suppressed- Unique characteristic!

To ensure electrical conductivity surface preparation is critical Remove all surface oxides/organics Zn is added immediately before spraying to minimise settling

Popular in aggressive atmospheres Zn rich primers are used under automotive topcoats (topcoats provide

decorative finish) Marine structures, ship hulls, ship superstructures, highway bridges, sewage &

water treatment plants, many other Not suitable at pH10 => Zn readily dissolves

Surface preparation

Must for adherence & subsequent performance

Poor coating applied to well-prepared surface is better than vice versa

Time & expenses are justified

Contaminants

Oxides, scales, loose dirt, organic matter- grease, oil

Specification Description

Solvent cleaning Oil, grease, soil, salt by cleaning with solvent, alkali, steam etc. Does not remove rust/mill scale

Hand-tool cleaning/ power tool cleaning

Loose rust & mill scale, loose paint- hand/power toolchipping, scraping, sanding, wire brushing, grinding

Blast cleaning Removal of all visible rust, mill scale, paint, foreign matter by blast (dry/wet using sand/grit/shot)

Pickling Removal of all mill scale, rust by chemical reaction or electrolysis

Exposure tests for evaluation

Accelerated lab tests field exposure tests - too long for

development programs/quality control Accelerated lab test to simulate same

mechanism of degradation as in service/field

Salt fog test Accelerated atmospheric exposure test Spray of salt-laden water in closed high

humidity chamber Conditions approach oxygen saturation,

and continuous immersion as the spray condenses on specimens

Coatings are usually scribed to create a well defined defect and attack at the scribe is observed

Alternate immersion cycles To enhance undercutting

50-75 % reduction in time required for equivalent undercutting as compared to continuous salt-fog tests

0.25 h immersion in 5% NaCl, air saturated 1.25 h drying at room temperature, ambient humidity 22.5 h exposure at 48C & 90% relative humidity

Accelerated Lab test ranking may not conform with the field tests

Final qualification of coated product is by service testing in the field

Coupons/panels exposed in atmosphere or in process streams Periodic examination

Degradation and failure of coatings

Penetration by reactant species water, oxygen, SO2, other electrolytes

Cathodic disbondment (salt fog test):

Attributed to formation of OH- from reduction rxn

O2 + 2H2O + 4e 4OH- (1)

Coatings: macroscopic & microscopic defects

Pinholes, voids, mechanical scapes, scratches

Allow access of the environ to base metal

Anodic rxn Fe Fe2+ + 2e at the defect

Coupled to nereby cathod beneath the coating

O & H2O must migrate through the coating for (1) to occur

Alkalinity reacts with organic polymer to disbond the coating

Oxide lifting Anodic corrosion products (oxides) accumulate under the

coating Lifting action and resultant undercutting

Occurs only during alternate wetting & drying (not continuous immersion)

Possible Mechanism -flocculant oxide corrosion products in water are compacted by drying -rust/scale forms an inner layer of Fe2+ & Fe(OH)2 ppt -these oxidize to Fe3O4 -Lamellar compact oxide corrosion product layers are formed from alternate wetting/drying cycles.

Cathodic rxns occur at the exposed metal surface/ or on magnetite (conducting) Electrochemical corrosion during wet exposure Deposition of colloidal corrosion products during dry periods- buildup of compact products - Conversion coatings inhibit disbondment

Electrochemical testing

EIS (Electrochemical impedance spectroscopy)

Advanced research tool for coating development & evaluation

Development of an electrical equivalent circuit to simulate the electrochemical behavior of the metal-electrolyte interface

Generally suitable circuit for modeling the coated metal systems

Rp= polarization resistance R = solution resistance Rcp = Coating pore resistance Cd = double-layer capacitance Cc= coating capacitance

Areas for Using EIS

fast evaluation of coatings

inhibitor performance

extremely low CR (

Ideal Resistor

follows Ohms law at all V and I

R is independent of frequency.

AC I and V signals though a resistor are in phase.

In reality, ideal resistors rarely exist

some other complex circuit elements that do not fulfill all of the above conditions => Impedance

Like resistor an impedance is a measure of resistance to the flow of current in a circuit

I

ER

Electrochemical Impedance

apply a sinusoidal voltage purturbation (AC potential) to a cell and measure the current through the cell => get impedance

Electrochemical Impedance is normally measured using a small excitation signal. This is done so that the cell's response is

pseudo-linear. In a linear (or pseudolinear) system, the

current response to a sinusoidal potential will also be sinusoidal at the same frequency but shifted in phase.

Pseudo-linear response

Impedance analysis of linear circuits is much easier than analysis of non-linear ones

Electrochemical cells are non-linear, doubling the voltage does not necessarily double the current response

But it can be pseudo-linear if very small perturbation pulse is applied 1-10mV AC

Purturbation (excitation) signal

v is potential at time t

Vm is the amplitude

is the radial frequency, rad/s

f is in Hz

In (pseudo) linear system, response signal i(t) is shifted in phase by

)sin()( tVtvm

f 2

)sin()( tItim

)sin(

)sin(

)sin(

)sin(

)(

)(

t

tZ

tI

tV

ti

tvZ

m

m

m

Impedance is expressed in terms of magnitude Zm, and a phase shift

Impedance as a complex function

complex number representation of impedance is more powerful for circuit analysis purposes

)exp()(

)exp(

)exp(

jZi

vZ

tjIi

tjVv

m

t

t

mt

mt

using Eulers relationship:

)sin(cos)( jZZm

If we plot real Z on the abscissa and imaginary Z on the ordinate => Nyquist Plot

Real Part Imaginary Part

The Nyquist Plot with impedance vector

each point on the plot is impedance at a given frequency, low frequency data are on the right (no magnitude is depicted, only range) impedance is a vector of length IzI and phase angle is (=arg Z)

eqvt ckt with one time constant

The Bode Plot

It shows the frequency information log IZI vs log (), and Phase angle () vs log()

Fitting of EIS data

EIS data is analyzed by fitting it to an eqvt ckt model Common circuit elements:

resistor: impedance is independent of frequency, has no imaginary component With only a real impedance component, the current through a resistor stays in phase with the voltage across the resistor.

Capacitor: its impedance decreases as the frequency is raised. Capacitors also have only an imaginary impedance component.

The current through an capacitor is phase shifted 90 degrees with respect to the voltage.

Inductor: opposite to that of a capacitor i.e. impedance increases as frequency increases, have only an imaginary impedance component.

Hence, the current through an inductor is phase-shifted -90 degrees with respect to the voltage.

Circuit

Component

V-I

relationship

Impedance

Resistor V=IR Z=R

Capacitor V=C dV/dt Z=1/jC

Inductor V=L di/dt Z=jL

Combinations of ckt elements

series and/or parallel

series: Z = Z1+Z2+Z3+

parallel:

....111

21

ZZZ

Application of perturbation- centered on

free corrosion potential

cathodic protection potential

Modeling of Impedance Spectra

assuming a circuit made of different elements

Find/ fit values of elements

Relate these values to a physical phenomena so that it reasonably represents a corrosion process

Physical Significance of Circuit Elements

elements in the model should have physical significance in the electrochemistry

Solution Resistance, R ~ Resistor, it is calculated when EIS data is fitted to a model

Double Layer : Cdl on a bare metal in an electrolyte ~ 20-60 f/cm2 .

Rp ~ resistor

Coating Capacitance

Virtual Inductor- a curve fitting ckt element formation of a surface layer like passive layer or fouling

(accumulation of unwanted material on surface)

may result from errors in measurement, non-ideal potentiostat

Types of Models

In a physical model, each of the model's components is postulated to come from a physical process in the electrochemical cell The choice of which physical model applies to a given cell is made

from knowledge of the cell's physical characteristics EIS analysts can use the shape of a cell's EIS spectrum to help choose a

model for that cell

Models can also be partially or completely empirical. The circuit components in this type of model are not assigned to

physical processes in the cell. The model is chosen to give the best possible match between the

model's impedance and the measured impedance

possible equivalent circuits (a circuit model is not unique!)

EIS spectrum

physical model should be verified! alter a single cell component (for example increase a paint layer thickness) and see if you get the expected changes in the impedance spectrum

Which is the best eqvt. ckt?

software packages to fit the spectra to analogous circuits

Schematic Nyquist plot showing effects of partial diffusion control (either concentration polarization or diffusion through a coatingwith Warburg impedance W

Validity of Spectra

Before attempting to model impedance spectra, be assured that the spectra are valid.

When a sine wave is used as the perturbation, the relationship between the current and applied voltage can be characterized by the ratio between the amplitudes of the voltage and the current

=> I Z I

and the phase shift between the vectors which represent the instantaneous voltage and current.=> phase shift of the vector (a complex number)

In mathematical terms, the impedance is a transfer function relating (the Laplace transform of) a response (e.g., current) to (the Laplace transform of) a perturbation (e.g., voltage).

Conditions for validity

The transfer function can only become an impedance when the following four conditions are fulfilled: 1. Causality: The response of the system must be a result only of the

applied perturbation. difficult to verify (ASTM => equipment & cell)

2. Linearity: The relationship between the perturbation and response is independent of the magnitude of the perturbation.

easy, generate spectra using higher and lower magnitude and verify the modulus and phase angle values

3. Stability: The system returns to its starting state after the perturbation is removed. generate spectra from high to low freq and then from low to high freq.

(low freq. can upset the system as it takes longest)

4. Finite valued: The transfer function (impedance) must be finite as the frequency approaches both 0 and and is continuous and finite valued at all intermediate frequencies.

Drawbacks Complex & expensive instrumentation Difficulty in quantitative measurements at low Occasional problems with data interpretation

Bode plots for AISI 1010 steel exposed for 2 h to aerated 0.5 N NaCl

Degreased, alkaline derusted, coated with 8 m polybutadiene v. high z

Degreased, coated with 8 m polybutadiene decrease in low frequency z, which measures Rcp rust at coating/metal interface induced coating defects => Rcp Cc (measured by linear portion of curves is same for (a) & (b) => coating shows little/no water uptake that would affect Cc.

Uncoated v. low z (Rcp & Cc are absent, only Rp is present) Rp is low => C. Rate