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Breakdown of chromate conversion coating at elevated temperature

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Psa pvtg r n gP r°btem BREAKDOWN OF CHROMATE CONVERSION COATING AT ELEVATED TEMPERATURE ~ " ~ : I have been told that the chromate conversion coating over zinc plating breaks down or is destroyed at high tem- peratures. (These are parts that are paint- ed.) I'm looking for the temperature that this process starts and documentation on this process that can be forwarded to our customers. candy lose its properties-otherwise, nobody would use them under pow- ders or baking enamels. I suggest that you call the technical staff at vendors such as Henkel, Chemetall Oakite and others to validate my answer. A : Specification MIL-C-SS41E Class 1A, "Chemical Conversion Coatings on Aluminum and Aluminum Alloys," states the following: "6.11 Temperature effects on cor- rosion protection. Unpainted con- version coatings will commence los- ing corrosion resistance properties if exposed to temperatures of 60°C (140°F) or above, during drying, subsequent fabrication, or service. In general, as temperature and expo- sure times increase, the corrosion protection of unpainted conversion coated parts decreases. The reduc- tion is believed to result from the coating dehydrating and the result- ing insolubility of the chromates within the coating." In the past I have asked chemical suppliers how this affects parts that are already painted before they are exposed to the elevated tempera- tures. I'm not sure that I ever received a definitive answer. The statement in the MIL specification clearly warns that the fabricator should not raise the drying or curing temperature of the chromate conver- sion coating (CCC) above 140°F before applying paint or powder coating. Since powder coatings and liquid baking enamels are always cured at temperatures well above 140°F (in fact, 250°F - 400°F), I assume that CCC does not signifi- CALCULATING THE PERCENT WEIGHT VOC OF PAINT Q :Could you give me a way to calcu- late the % weight of VOC in a prod- uct using the MSDS? If I have 1.3Ib/gal of VOC what is that in % weight of the product? A : You need to know the weight of a gallon of the coating, and you'll find this information on the MSDS. Look for the WPG (weight per gallon), density (lbs/gal) or S.G. (specific gravity). WPG and density are exactly the same, only expressed differently. Suppose the WPG (or density) of the coating is 9.1 lbs/gal: % Weight ofVOC = Weight ofVOC (lbs/gal) • 100% Density of paint (lbs/gal) % Weight ofVOC = 1.3 lbs/gal • 100% 9.1 lbs/gal = 14.3% If the MSDS provides you the SG, you must multiply that value by 8.33 lbs/gal to get the density. Therefore, suppose SG = 1.09: Density (lbs/gal) = 1.09 ~ 8.33 lbs/gal = 9.1 lbs/gal % Weight ofVOC = 1.3 lbs/gal * 100% 9.1 lbs/gal TRAFFIC MARKING PAINTS Q : I am looking for articles, books, and specifications to deal with water-based paint and epoxy pavement markings. I do not believe that our cur- rent material specification is giving us the most durable product that we could get and would like to revise our specifica- tions. Revising the specifications is the goal, but to get there I need to learn about how the components of water-based paints and epoxy affect the final product on the road. We are not getting the same durability as other states in the region are getting. A : I am not aware of any books that focus on this topic. Several books on water-based coatings and epoxies are available from Amazon.corn, or by going to the bookstore at the website of the Federation of Societies of Coatings Technologies (FSCT). Have you con- sidered speaking to your counter- parts in other regions, especially if you know that their traffic marking paints are holding up better than in Colorado? ZINC PHOSPHATE AND OIL COATING : Zinc phosphate ¢,w oil coating is specified for a small (automotive) strip spring~ for 2S hrs corrosion resistance, a) With the coating weight being specified as 1000 - 2000 mg/ft 2, should an actual coating weight of 2069 mg/ft 2 be considered a manufacturing defect? b) Can this higher coating weight cause "phosphate holidays, " allowing cor- rosion in a situation were 1,000-2,000 mg/ fl 2 would resist corrosion? A : I am not familiar with coating weights of zinc phosphate as high as 1,000-2,000 mg/ft 2. Typically, zinc phosphate coating weights are of the order of 300 mg/ft 2. My concerns would not be holi- www.metalfinishing.com December20071 meta|finishing 143
Transcript

Psa pvtg r n g P r°btem BREAKDOWN OF CHROMATE CONVERSION

COATING AT ELEVATED TEMPERATURE ~ " ~ : I have been told that the chromate

conversion coating over zinc plating breaks down or is destroyed at high tem- peratures. (These are parts that are paint- ed.) I'm looking for the temperature that this process starts and documentation on this process that can be forwarded to our customers.

candy lose its properties-otherwise, nobody would use them under pow- ders or baking enamels. I suggest that you call the technical staff at vendors such as Henkel, Chemetall Oakite and others to validate my answer.

A : Specification MIL-C-SS41E Class 1A, "Chemical

Conversion Coatings on Aluminum and Aluminum Alloys," states the following:

"6.11 Temperature effects on cor- rosion protection. Unpainted con- version coatings will commence los- ing corrosion resistance properties if exposed to temperatures of 60°C (140°F) or above, during drying, subsequent fabrication, or service. In general, as temperature and expo- sure times increase, the corrosion protection of unpainted conversion coated parts decreases. The reduc- tion is believed to result from the coating dehydrating and the result- ing insolubility of the chromates within the coating."

In the past I have asked chemical suppliers how this affects parts that are already painted before they are exposed to the elevated tempera- tures. I 'm not sure that I ever received a definitive answer. The statement in the MIL specification clearly warns that the fabricator should not raise the drying or curing temperature of the chromate conver- sion coating (CCC) above 140°F before applying paint or powder coating. Since powder coatings and liquid baking enamels are always cured at temperatures well above 140°F (in fact, 250°F - 400°F), I assume that CCC does not signifi-

CALCULATING THE PERCENT WEIGHT VOC OF PAINT

Q :Could you give me a way to calcu- late the % weight of VOC in a prod-

uct using the MSDS? I f I have 1.3Ib/gal of VOC what is that in % weight of the product?

A : You need to know the weight of a gallon of the coating, and

you'll find this information on the MSDS. Look for the WPG (weight per gallon), density (lbs/gal) or S.G. (specific gravity). WPG and density are exactly the same, only expressed differently.

Suppose the WPG (or density) o f the coating is 9.1 lbs/gal:

% Weight ofVOC = Weight ofVOC (lbs/gal) • 100%

Density of paint (lbs/gal)

% Weight ofVOC = 1.3 lbs/gal • 100%

9.1 lbs/gal = 14.3%

If the MSDS provides you the SG, you must multiply that value by 8.33 lbs/gal to get the density. Therefore, suppose SG = 1.09:

Density (lbs/gal) = 1.09 ~ 8.33 lbs/gal = 9.1 lbs/gal

% Weight ofVOC = 1.3 lbs/gal * 100% 9.1 lbs/gal

TRAFFIC MARKING PAINTS

Q : I am looking for articles, books, and specifications to deal with

water-based paint and epoxy pavement markings. I do not believe that our cur- rent material specification is giving us the most durable product that we could get and would like to revise our specifica- tions. Revising the specifications is the goal, but to get there I need to learn about how the components of water-based paints and epoxy affect the final product on the road. We are not getting the same durability as other states in the region are getting.

A : I am not aware of any books that focus on this topic. Several

books on water-based coatings and epoxies are available f rom Amazon.corn, or by going to the bookstore at the website o f the Federation of Societies of Coatings Technologies (FSCT). Have you con- sidered speaking to your counter- parts in other regions, especially if you know that their traffic marking paints are holding up better than in Colorado?

ZINC PHOSPHATE AND OIL COATING

: Zinc phosphate ¢,w oil coating is specified for a small (automotive) strip spring~ for 2S hrs corrosion

resistance, a) With the coating weight being specified as 1000 - 2000 mg/ft 2, should an actual coating weight of 2069 mg/ft 2 be considered a manufacturing defect? b) Can this higher coating weight cause "phosphate holidays, " allowing cor- rosion in a situation were 1,000-2,000 mg/ f l 2 would resist corrosion?

A : I am not familiar with coating weights of zinc phosphate as

high as 1,000-2,000 mg/ft 2. Typically, zinc phosphate coating weights are of the order of 300 mg/ft 2.

My concerns would not be h o l i -

www.metalfinishing.com December 20071 meta|finishing 143

pa i nti ngp ro b [e m so [ve r days, but rather that if one were to apply a coating over t h e phosphate layer, it would split due to inadequate cohesion. However, since you are not applying a paint coating over the phosphate, but are applying oil instead, I don't why you would expect a defect, especially at only 69 mg/ f t 2 over the maximum specified limit. From your question I assume that your customer is rejecting your work? If so, I would be pleased to examine the zinc phos- phate for holidays using a stereo microscope, or, at worst, a scanning electron microscope. Have you sub- jected the coating to salt spray? After all, 25 hours of salt spray corrosion resistance is very short.

WHY DO HOUSE PAINTS BREAK DOWN?

a~n ra: We painted a house on a warm day and used a paint sprayer. The paint left spots all over, and the paint swirled n. So, we let it dry and came back another day and did it

again. The paint did the same. It was like the paint was breaking down. We have painted many houses and this is a first. What do we do? Our paint vendor said there was too much moisture in the air. The temperature was approximately 76°F.

A : Painting on a "warm" day when the temperature is 76 o F does not seem unusual. Actually, 76 ° F is a very

average day. What was the humidity on the days that you

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painted? High humidity can be a problem, but in most cases the climate must be very humid for one to notice t h e

defects. I'm not sure that I fully understand your description

of the paint defects. However, if you are able to take some digital photographs, please send them to me and I'll take a look at them. Based solely on what you've written I would not rule out the possibility that perhaps the paint was faulty, but I need to know more. To find out, you would need to take a sample of the paint and have it ana- lyzed for i t s performance properties. This can be costly, but it might be worth your while.

COMPARING POWDER COATINGS WITH EPOXY COATINGS

: I am looking for the pros and cons of these two finishes in a heavy truck application. I'm specifically interested in

Iongevi~y, abrasion resistance, cba&in~ etc.

A : Unfortunately, you are comparing apples with oranges. "Epoxy" describes a resin type, much as

alkyd, acrylic, polyurethane, and more. "Powder Coating" is distinct from liquid coating.

Both powder and liquid coatings are available in a vari- ety of resin systems. Typical powder coatings are available as epoxies, acrylics, polyurethanes, polyesters, TGIC and a few more. On the other hand, liquid coatings are available as alkyds, alkyd-modified acrylics, epoxies, polyurethanes, and wide variations of these resins. There is some (albeit limited) overlap between the resin systems used in both powder and liquid coating formulations.

If you are painting trucks, it seems unlikely that you will want to do so using powder coatings, because the sur- faces that are powder coated need to be baked at 325- 400 °F for 8-12 minutes, depending on the type of powder formulation. If you intend to powder coat the truck itself, and not just components that you can disassemble from the truck, you will need to have a baking oven large enough to house the truck. But be warned: at these high temperatures you will likely melt plastic hoses, ruin the upholstery, and probably blow up the gasoline tank. On the other hand, if you wish to powder coat component parts you will first need to strip the old coatings, prepare the surfaces to remove all grease, oil and other contami- nants. It is always advisable to pretreat the exposed metal surfaces with a phosphate or chromate conversion coat- ing, and only then can the powder coating be applied.

If you intend to paint the truck with an epoxy liquid coating, please bear in mind that epoxy coatings are superb as primers, but chalk and lose their gloss when exposed to sunlight for lengthy periods.

BIO Ron Joseph is a paint ~ coating consultant with Exponent, Inc., of Menlo Park, Calif. You can e-mail your questions directly to him at [email protected].

December 20071 metalfinishing 1 44 www.metaffinishing.com


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