Getting corrosion on ASTM-B-633 plated assy.

[Peva707]

We are having a surface corrosion problem on a welded assembly (13 ga. CRS) that is zinc plated per ASTM-B-633.

The plater (3rd party) says that there is nothing to prevent it due to the chemicals used in the plating process getting trapped between pieces of metal that are welded together (end flap of 13 ga. CRS shelf welded to 13 ga. CRS chassis). The plating is done after the parts are assembled (welded).

Is there no effective neutralizing ar passivating chemical that can be used post-process to prevent the corrosion that creeps out from the welded areas over time (sometimes before the product gets shipped)?

If the answer to that is no, is there an effective coating that could be applied post-process to contain the cause of the corrosion and thus prevent it?

For reference, here is the plating specification as it appears on the assembly drawing: "Finish: Zinc plate per ASTM-B-633, Type II, Fe/Zn 8, color: gold."

Thanks for any help with a solution.

 

[btrueblood]

Switch to hot-dip galvanizing?

Change the weld design to eliminate the crevice?

You could powder coat the assembly, or wet paint with zinc-rich primer.

All of the above would eliminate the crevice/pocket corrosion issue.

 

[Peva707]

Thanks for the advice, btrueblood.

I should have mentioned that we have two separate considerations:
1) What do we do with the existing product, both in the field and in the plant and dsitribution?
2) What do we change to the design/processes for future production?

Some of the things you mentioned have been discussed. They can't do anything to interfere with electrical grounding to the unit (like painting or powder coating). The appearance is important - they mentioned the possibility of zinc chromate, but that its appearance is not consistent enough over a part. I guess similar concerns about galvanizing. I think the "team" has settled on changing the design to use tabs and slots that would be welded in a way that would not trap the chemicals to leach out later.

For the existing product, the solution they're looking at is to dissasemble the parts that are bolted on, wash the assembly in IPA, mask off and paint the joint areas with a special Sherwin Williams primer and coating, then seal those joints with an RTV that is suitable (I know that that has to be selected carefully to not create new problems).

What I was hoping against hope to hear was that there was some magic bullet neutralizing chemical that would fix everything both in existing and future product without changing the design. But - that would have been too easy.