Painting Stainless Steel and Corrosion 5

[metalman8357]

Over the years I have seen some stainless steel part manufactures specifically state to not paint stainless steel, but I'm trying to figure out why this might be an issue? Can anyone suggest a possible mechanism or example in the literature for how paint might accelerate or be harmful to the corrosion resistant properties of stainless steel? The only thing I can really think of is that sometimes people use acidic primer and/or roughen the surface with sandpaper prior to painting and this might be detrimental to the protective oxide layer...

Any thoughts?

 

[btrueblood]

Google crevice corrosion.

 

[TVP]

Why pay for the added alloying elements and mill processing costs associated with stainless steel if you are just going to rely on a coating for corrosion resistance? Properly treated carbon steel using a multi-stage clean + zinc phosphate + sealer + inorganic zinc basecoat + organic topcoat has excellent corrosion resistance. Zinc-nickel plating + chromate + organic coating is another option.

 

[weldstan]

Sometimes painting is required to mitigate stree corrosion cracking; however, if the wrong coating type is used, the probability of stress corrosion cracking will be increased. Manufacturers of specific equipment have (in many cases) determined that the services where the equipment could be used preclude the need for coating. Also see btrueblood.

 

[metalman8357]

Anything in the literature on how painting stainless steel might facilitate crevice corrosion? I can't seem to find much. I figured if it's a known problem there should be a lot of information on it.

 

[metengr]

Seek and ye shall find............................. Look under Chapter 1.

http://www.diva-portal.org/smash/get/diva2:566900/FULLTEXT01.pdf

 

[weldstan]

Pores will be present on paint coatings over stainless steel. The pores may act as a crevice themselves or as initiating sites for localized disbondment of the coating from the stainless surface, thereby producing a crevice.

Effective coating selection, application and maintenance will provide added protection from crevice corrosion in locations susceptble thereto.

 

[EdStainless]

In most cases where SS is painted to protect it against external SCC the alloy was selected because it is ideal for the internal conditions. In most processes it is the internal process requirements that drive material selections. If extra steps must be taken for external protection then so be it.
I have seen cases where only part of a SS structure was painted, there was sever crevice corrosion at the edges of the paint. I have seen a similar effect when a fully painted unit had surface damage that was not repaired. I can envision problems with paints that have high permeability or poor adhesion.

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Plymouth Tube

 

[Maui]

Here are some good examples of crevice corrosion in stainless steels:

http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/25077/SGNO78.pdf?sequence=1

Maui

http://www.EngineeringMetallurgy.com

 

[EdStainless]

This is my favorite.
The inspector used a paint marker.
It sure was OK.
We pulled the other one out of service and it was identical.

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Plymouth Tube

 

[moltenmetal]

EdStainless: what were the exposure conditions? We used to be able to etch out paint marker marks where they were left in the HAZ of a weld, but never seen anything like that photo!

 

[moltenmetal]

Damn, Ed, now I'm really curious! I showed that one around the office. What was that pipe's material, and what conditions was it put in, that led to that beautiful OK etched into the pipe wall?

 

[EdStainless]

These were SS pump shafts (Nitronic 60 I think) used in brackish water service.
It was all crevice corrosion, took a few months.
The unit was pulled because of a bearing issue.

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Plymouth Tube

 

[moltenmetal]

Thanks Ed- beautiful example.

 

[tbuelna]

Just came across this thread, and it caught my interest.

I work in the aerospace industry, so I have been exposed to the issue of SCC and stainless steel alloys. While I am familiar with common surface treatments used on stainless steel components such as passivation, anti-seize or anti-galling coatings like silver plating or MoS2 dry film, or wear resistant coatings like thin-dense-chrome, I cannot recall ever seeing an example of a stainless steel part being primed and painted.

SCC issues with stainless steel materials are usually addressed by selecting the appropriate alloy and heat treatment.

 

[EdStainless]

tbuelna,
In a chemical plant where you are near the coast (salt fog) and have thousands of feet of 4", 6", and 8" lines handling hot chemicals that are perfectly compatible with 304L external paint is the best option. Any attempt to use alloy selection to solve the problem will at least double your metal cost.
In smaller scale applications I agree, changing the alloy is a more reliable method.

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Plymouth Tube