Corrosion in dermacoat coated caron steel 3

[goodbadgood]

Dear All

I am facing some issues related to the corrosion of dermacoat coated carbon steels bolts with grade astm a490m. These bolts are installed in an ammonia plant with 1 year in service. At the same time i have some bolts which are coated with hot dip galvanized coating the grade of the bolts is astm a325m but they are not corroded. Only dermacoated bolts got corroded. Corrosion seams to be like rust with only those surfaces corroded which are exposed to the environment. Any idea why this is happening.

 

[CoryPad]

Can you provide any information about dermacoat? Organic, inorganic, metallic, ?????

 

[goodbadgood]

Metallic coating with thickness 10 micron

 

[MintJulep]

Googling (from the US) for dermacoat returns skin lotion for dogs.

Perhaps a link to the product datasheet...

 

[Maui]

Can you measure the thickness of the coating on the hot dipped galvanized bolts and compare it to the thickness of the dermacoat coating? I suspect that your dermacoat coating may be substantially thinner.

Maui

 

[CoryPad]

You have not provided enough details to determine the true root cause. One possibility is that the thin Dermacoat (never heard of this before, could not find information on the Internet either) is not suitable for your environment. Hot dip galvanizing usually is around 40 micrometers thick, so it can provide higher performance.

 

[TVP]

I believe that the OP is referring to Dacromet coating for ASTM A490 bolts. If so, a potential cause of poor performance is poor adhesion of the coating. Provide some bolts to the fastener supplier and an independent lab for further evaluation.

 

[MintJulep]

Well, if it is Dacromet then...

Dacromet contains Chromium 3 (Cr3+)

Ammonia reacts with chromium(III) ion to precipitate gray-green chromium(III) hydroxide:

Cr3+(aq) + 3NH3(aq) + 3H2O(l) <==> Cr(OH)3(s) + 3NH4+(aq)