DETERMINING THE BEST CORROSION COATING FOR THE 21ST CENTURY m3==32 ~ L ( 6 L . d , 0 F

Michael Brantley, CMfgE Manufacturing Engineer

E-Z-Go Div. of Textron, Inc. Augusta, Georgia

Presented at: "Finishing '93 Conference and Exposition" October 25-28, 1993 Or. Albert B. Sabin Convention Center Cincinnati, Ohio

Copyrighted by $ME; Technical Paper Number to be Assigned

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DETERMINING THE BEST CORROSION FIGHTING SYSTEM

INTRODUCTION

Building a good corrosion fighting system is similar to constructing a building. There are three key elements: the foundation, the walls, and the roof. As with any construction project, government regulations and cost/benefit ratio must be considered when making the system selection. - *

FOUNDATION

The foundation to a good corrosion fighting system is the pretreatment system. There are many avenues available to explore ._ when considering pretreatment. There are two phosphate and -. final rinse processes widely used in industry today.

Iron phosphate is the oldest of the phosphating processes. It is the most economical to install and operate, but grovides less corrosion resistance'than zinc phosphate. AE iron phosphate coating weight is typically in the range of 50 - 100 mg per square foot. It is also more forgiving operationally than is a zinc system.

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Typically, zinc phosphate is the preferred method when long life under corrosive conditions is required. Zinc phosphate also enhances paint adhesion because the crystals form a porous surface which can soak up and mechanically trap the overlying paint film. The zinc phosphate film is usually deposited at 200 - 500 mg per square foot. Negatively, zinc phosphate systems usually require more treatment stages, are more difficult to control, and more expensive to install and operate. Because of the added expense associated with zinc, we decided to test iron vs. zinc to determine the degree of added performance provided by zinc.

The final rinse is another area which has multiple possibilities: chrome and nonchrome. A chromic acid rinse stage, at one time, was common in pretreatment systems. In this process, dilute chromic acid is allowed to soak down into defects in the phosphate crystal structure. When it reaches the iron below the phosphate coating, it forms an inert layer that resists corrosion. Stricter governmental regulations have caused many in the finishing industry to abandon the chrome final rinse in favor of a nonchrome final rinse. This change usually requires the addition of a D.I. rinse stage immediately following the nonchrome stage in order to maximize corrosion protection -

The best investigative method is to create a matrix of all the possibilities which could exist and then select the ones pertinent to your needs. For the purpose of this paper, we will consider the following four scenarios: iron phosphate with a chrome seal, iron phosphate with a non-chrome seal, zinc phosphate with a chrome seal, and zinc phosphate with a non- chrome seal. These four phosphate and final rinse scenarios represent the majority of phosphating and rinse systems used in the finishing industry today.

WALLS

F o r our second element, the walls, three avenues were pursued. The first was to select an automotive undercoating to use as either a stand-alone coating or as the primer for a topcoat -

The second consideration was given to electrocoating as the primer. Electrocating is the process in which electrically charged paint particles are plated out of water suspension to coat a conductive object. Two general types of electrocoating systems are in use. They are known as anodic if the part is the anode, or cathodic if the part is the cathode in the - electrochemical process leading to paint deposition. Since it is widely accepted in the coating industry that the cathodic process provides the best corrosion resistance, we decided that the cathodic process would be the primer system for our E-coat evaluation.

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Our third process considered for a primer was powder coating. Powder coating is a dry paint process which does not require a volatile carrier for the purposes of coverage and flow. In the powder coating process, the proper formulation is developed as a finely ground powder. The powder is then applied to the substrate to be coated as a layer of powder- It is then heated which melts the powder layer, allows it to flow and fuse into a continuous coating.* Our initial investigation utilized a zinc rich epoxy powder as the primer.

ROOF

We had already determined that the llroofll of our coating structure would be powder coating. What we had not determined was the resin system that would be employed to afford us the maximum in corrosion protection, flexibility, impact resistance, and weatherability. Testing to determine the proper resin system was accomplished during Phase I1 of our investigation.

PHASE I1

Now that we had established several possible "materials" for some of the components of our "building", it was time to develop a matrix of all the possibilities, prepare samples of each system, and test them for corrosion protection.

Our test matrix was as follows:

AFNC = automotive undercoat, iron phosphate, nonchrome seal AFC = automotive undercoat, iron phosphate, chrome seal AZNC = automotive undercoat, zinc phosphate, nonchrome seal AZC = automotive undercoat, zinc phosphate, chrome seal EFNC = electrodeposition, iron phosphate, nonchrome seal EFC = electrodeposition, iron phosphate, chrome seal EZNC = electrodeposition, zinc phosphate, nonchrome seal EZC = electrodeposition, zinc phosphate, chrome seal PFNC = powder primer, iron phosphate, nonchrome seal PFC = powder primer, iron phosphate, chrome seal PZNC = powder primer, zinc phosphate, nonchrome seal PZC = powder primer, zinc phosphate, chrome seal

1.

\ At this point, it should be noted that all of the above "foundations" and llwallsll were topcoated with a TGIC polyester powder. It was at this moment that we discovered that the automotive undercoating was incapable of withstanding the temperatures required to cure the powder topcoat. In talking to the product manufacturer, this coating was capable of withstanding temperatures of up to 225 degrees F.

* A User's Guided to Powder Coating p.3

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___

The powder curing temperature caused the automotive undercoat to delaminate f r o m the substrate much like paint blistering and peeling on a house, only much worse. The automotive undercoat was tested as a one coat process.

Test Procedure

The next step in the process was to fabricate test samples for coating. The first type of test sample consisted of 4"x 6 " panels fabricated from both cold rolled steel and galvanneal. These two metals represent the substrate of our vehicle. In addition, we fabricated a representative part for evaluation. This part was constructed to have recessed areas where Faraday Cage effect would be maximized. Faraday Cage effect is the phenomenon by which charged particles are prevented from entering recessed areas. Poor penetration is due to the extremely high electrostatic field about the workpiece and the curvature of electric force lines to the nearest ground(See i 1 lust rat ion below) .

/----- ---

of force.

Power Supply

Having now determined the two test part configurations, - samples of each type were fabricated from in-house raw materials. The utilization of in-house raw materials for test material rather than purchasing prepared industry standard panels assures that the pretreatment, primer, and topcoat system tested all work cohesively under actual production conditions. In order to accomplish this, test panels.and parts were sent to our present pretreatment supplier for processing using the various phosphate and final rinse combinations. These panels were pretreated and then sent to various companies for either E- coat or powder primer. After being returned to E-Z-GO, they were topcoated with a TGIC polyester powder. As is standard with a l l our testing, a control sample utilizing our present process was included in the testing. Normally it is not advisable to have two different variables being tested conjuctionally, however, time constraints forced us to do so.

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Having completed the pretreating and coating of these parts and panels, they were placed in a salt spray cabinet for a total of 1,000 hours. They were removed at 250 hour intervals for evaluation. The results of their testing is included in Exhibit I., As you can see, the automotive undercoating exited the testing early in the process.

The zinc phosphate panels with the chrome seal were the clear winners from a pretreatment point of view.

Primer

It was a much more difficult decision with regard to primer. There was only minute differences between the zinc phosphated chrome sealed panels for both E-Coat and primer panels. There were supporters from the task force at E-Z-Go advocating each of the proven primer possibilities.

With the test data and the committee in somewhat of a gridlock, we examined two areas previously mentioned: governmental regulations and cost\benefit ratio. There is an ever increasing tightening of the regulations for air emissions, especially regarding VOC emissions. VOC stands for volatile organic compound. VOC's are any organic compound that participates in atmospheric photochemical reactions. These reactions create ozone and photochemical smog.

Electrocoating systems vary in VOC content from . 4 lbs/gal. to 2 . 5 lbs./gal. Other than stopping the coating process entirely, powder offers the better reduction in VOC emissions. Powder coatings contain no VOC. Switching to powder results in a 100 % reduction in VOC generafion by primers.

The cost/benefit ratio was the most deciding factor. Analysis, based on quotes for both types systems from various potential system suppliers, indicated that an electrocoating system would cost approximately $763,000 more to install than its counterpart with powder as a primer. However, the yearly operational cost of the E-Coat system is approximately $34,000 less than the same system using powder as the primer. Labor and utility costs for the powder primer are the forces which drive its operating costs upward. Exhibit I1 details this brief comparison of the two. Added to this was the potential liability of approximately $225,000 should the E-Coat bath become ~

contaminated.

PHASE I11

Pretreatment

We had now decided to use zinc phosphate with a chrome seal but were a long way from "completing our building". Our pretreatment decision had been based upon our present supplier.

RUST EVALUATION SCALE

I - OVER 10.0 TO 13.0 MM 2- OVER 5.0 TO 7.0 MM 3- OVER 2.0 TO 3.0 MM 4- OVER .5 TO1 .O MM 5- NONE

EXHIBIT I

PRETREATMENT AND PRIMER CORROSION TESTING

. .. .. .~ ~ . . . . . . . . ... I . . _ ; . . . . I . . ,. . . . .

2 - - .

TOTAL VARIANCE FROM CURRENT VARIANCE FROM POWDER

EXHIBIT 2

$2,715,671 $2,137,266 $1,592,905 $578,405 $544,361

$34,044

POWDER VS. E-COAT SYSTEM INSTALLATION COSTS

POWDER VS E-COAT YEARLY OPERATING COSTS

Was his product the best? We now began discussions additional chemical suppliers.

with two

In order to determine the best pretreatment supplier, sample test panels fabricated from cold rolled steel and galvanneal ~

were sent to all three chemical suppliers for pretreatment. ~

After processing, sales representatives returned their respective panels on a mutually agreed upon date. They hung their panels on identical hangers at the entrance of the powder booth and these panels were coated in our present powder system. They were then taken to Quality Assurance and tested in salt spray for 1,000 hours. The results of this testing is found in Exhibit 111.

Coat inss

We now had to focus our attention on the powder primer and topcoat resin systems that would be required to meet our performance criteria listed in Exhibit IV. In order to accomplish this, nine powder suppliers were contacted about submitting samples for testing and evaluation.

Since a zinc phosphate with a chrome final rinse was not available at E-Z-Go, we decided to purchase industry standard test panels. We instructed the supplier of these panels to make sure that all these panels from each substrate, cold roll steel and galvanneal, be from the same batch process. This was specified to eliminate any batch to batch variance that might occur.

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These panels were shipped next day express with the prospective vendors having a work window of one week in which to coat the panels and return them to E-Z-Go. This work window was established in order to minimize the coating time variance between vendors.

Testinq

Upon receipt of the coated panels from the prospective powder suppliers, they were divided into three groups. Group I was sent to an independent lab where 1100 hours of xenon arc testing was performed. The purpose of the xenon arc testing is

This is extremely important to E-Z-Go because our product is exposed to weather the majority of the time in areas like south Florida. EMMAQUA testing was our first choice to develop this information. Unfortunately, EMMAQUA testing is a slow process and time constraints dictated accelerating this test. The results of this testing are detailed in Exhibit V. Sample C and the control were the only two samples which passed this phase of testing.

to determine the gloss retention capability of the coatings. - \ - .~

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EXHIBIT IV

E-Z-GO PERFROMANCE CRITERIA

TEST

CORROSION

ADHESl ON

IMPACT

FLEXIBILITY

GLOSS

HARDNESS

BLI STERl NG

METHOD

ASTM B-117

ASTM D-3359B

ASTM D-522

ASTM G-26B

ASTM D-1474 ASTM D-3363

ASTM D-714

CONTROL

5% SOLUTION, 1000 HRS., NO MORE THAN 1/8" CREEPAGE FROM SCRIBED X

SCOTCH #710 TAPE, 38 PSI

40 FT-LBS., MAX. 1/16" CHIP ON SIDE OPPOSITE IMPACT

CONICAL 1/8" ARBOR, NO BREAKAGE ALLOWED

NO MORE THAN 15% LOSS @ 1000 HRS.

3H

#10 PER INCH

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* e . EXHIBIT V

1 100 HRS. XENOTEST GLOSS RETENTION

SAMPLE I INITIAL I 11 00 HR. I GLOSS I.D. GLOSS GLOSS LOSS SAMPLE A1 72 37 35 SAMPLE A2 16

SAMPLE B1 SAMPLE 82

75 1 74 75 1 74

SAMPLE D1 SAMPLE D2

SAMPLE E l SAMPLE E2

SAMPLE F1 SAMPLE F2

SAMPLE G1 SAMPLE G2

SAMPLE H1 SAMPLE H2

SAMPLE I1 SAMPLE 12

91 89

82 83

97 96

79 80

87 87

96 96

1 1

10 44

2 1

50 60

70 73

1 1

90 88

72 39

95 95

29 20

17 14

95 95

% GLOSS -0ss

48.61 % 25.00%

98.67% 98.67%

98.90% 98.88%

87.80% 46.99%

97.94% 98.96%

36.71 % 25.00%

19.54% 16.09%

98.96% 98.96%

4VG. % ;Loss LOS5

36.81 %

98.67%

98.89%

67.40%

97.95%

30.86%

1 7.82%

98.96%

. . .- x".

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Group I1 panels were divided into two groups for in-house testing. This testing included salt spray(ASTM B117) , adhesion, mandrel bend, pencil hardness, and impact resistance. The results of this testing is detailed in Exhibit VI. Samples B and I were tied at the end of this testing.

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Group I11 panels were sent to a different independent lab for salt spray testing(ASTM B 117). These panels were tested for 1,000 hours with the data from testing being used to verify our in-house test results. There was a positive correlation between the two salt spray tests, therefore, test data from the independent lab is not included in this paper.

CONCLUSIONS

It was clear from a l l our testing that the best foundation for corrosion protection was a zinc phosphate/chrome rinse combination. This foundation can further be enhanced with the addition of a recirculated D.I. rinse followed by a virgin D.I. halo. Due to availability, it was not used in our testing.

Both E-Coat and powder are excellent choices for the llwallsll of a good corrosion fighting structure. From what I've seen in testing, powder offers the better choice in areas where you can be assured of coverage. This is primarily due to the ability of powder to deposit higher coating films than E-Coat.

The roof for this llbuildingll is dependant upon the requirements of the structure. If weatherability is not a factor, then several choices of resin systems seem capable of providing excellent corrosion protection. Should weatherability be an important factor, an acrylic seems to be the best choice as a powder topcoat. Presently, several powder suppliers are developing resins other than acrylics which are very promising from a weatherability aspect.

To summarize, there are three key factors in developing and keeping a good corrosion system for your product. First, use your actual material as the test substrate. Second, test as many variables as you feel pertinent to corrosion protection. Third, testing should be conducted periodically to insure that your

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'. company has the best system available. .. .. - -L

GALVANNEAL 7

Vendor 1- ~- Conical

Topcoat A IlSiZiZK

Primer C

Primer A

Primer D

Pritiier I3 ‘ropcoat 13 Sample F t Primer A To},coat c l-SG$ZZ

Primer E

Primer E Topcoat B 1- Primer B L Topcoat D

Bend FA I LE I 1

PASSED

FAILED

PA1 1,151)

PASSED

FA1 1,ED

PASSED

PASSED

PRIMER

A = Zinc Rich I:poxy B = Hybrid C = TGIC Polyester D = Red Oxide Epoxy E = Epoxy

1% ncil Iardncs

lo

2

3

6

10

6

8

8

EXHIBIT VI

POWDER VENDOR QUALIFICATION TEST

1- I’anel I 250Hrs I 250 I-Irs I 500Hrs I 500 I-Irs 1 750 I-Irs I 750 Hrs I 1000 Hrs 1 1OOOHrs

TOPCOAT

A = I’olycslcr 13 = Acrylic C = TGIC Polyestet D = I’olyitrc~hat~e E = Urethntic

r

..-

I

1’

-

___ Impact PASSED

Panel 250 Hrs. 250 Iirs # Adhesion Creepage A1 SA .NONE-

SA NONE _.____

Adhesion SA SA

._ SA

Creepage . Adhesion Creepage NONE SA 1/16" NONE SA 1/16" NONE SA 1/32"

I'ASSIID 131 SA

132 SA I3 3 SA

-~ ______ ___

____ __

~ _ ~ _ NONE NONE NONE ___._

PASSED

I'ASSI31)

PASSED

F1 SA NONE P2 SA NONE

NONI: 1:3 SA

G l SA NONE SA NONE G2

G3 SA NONE I 1 1 SA NONE

NONE 1 I2 SA 113 SA NONE

_ _ __ ____ ~

- .

~ __-__

_~ -

SA

- SA SA SA SA SA SA

1/64" SA 1/16 1/64" SA 1/32"

NONE SA NONE NONE 5A NONE NONE SA NONE NONE SA NONE SA NONE

US" BLISTER

SA SA SA SA

~

NONE SA 118" BLISTER NONE SA NONE NONE SA NONE SA NONE

US" BLISTER

EXHIBIT VI(C0NT.)

POWDER VENDOR QUALIFICATION TEST

.ED STEEL COLD ROI 500Hrs I 500Hrs 750Hrs I 750Hrs I 1OOOHrs I 1000Hrs 1 Conical Pencil

Adhesion I Creepage SA I NONE

I Hardnes 8

Primer A 'I'opconI A

Primer C

__-~ Sample B

SA I NONE SA NON13 SA I NONE SA - - NONE SA ] NONE SA 1 1/16"

-~ ______

_____

2 SA I NONE I SA I NONE SA I NONE I SA I NONE

Topcoat C

Sample C 10 SA ! NONE Primer A

Topcoat B

Primer D Topcoat E

Prinicr B Topcoat 13

Primcr A

Sample D

Sample E

Sample F

1 C3 ! SA I NONE SA I NONE NONE NONE

SA NONE NONE

6

PASSED 6 E2 SA NONE 1 E3 1 SA 1 NONE

- ~~

SA I NONE SA I NONE r" N O N L

NONl' NONE

SA NONE SA NONE SA NONE SA I NONE

- _. . - . _ . __ - 'l*o~~coa I ('

Sample G . _. .

Primer E Topcoat C

Primer E Topcoat B

Primer J?

_____ Sample H

Sample I

FAILED 1

SA I NONE PASSED 1 PASSED I I 1 I SA I NONJX SA I NONE

12 SA I NONE ! I3 SA NONE

SA NONE SA 1 NONE

PRIMER TOPCOAT

A = Zinc Rich Epoxy B = Hybrid C = TGIC Polyester D = Red Oxide Epoxy E = Epoxy

A = Polyestcr B = Acrylic C = I'GIC Polyester D = Polyurethane

. E = Urcthanc