black substance

[bigTomHanks]

We have a zinc plated part that is exposed to refrigerant. Corrosion has caused the zinc plating to come off and ruin the part. Does anyone have any idea what could have caused this rapid corrosion.

 

[MintJulep]

Acid?

Many refrigerants form acids as a break-down product when exposed to heat and moisture.

More info is needed to take a better guess. What type of refrigerant? What are the service conditions? What is the base metal? Is there a di-chromate conversion coating on the zinc?

 

[bigTomHanks]

R-22
The base material is brass.
An acid flux may have come in contact with the zinc coated part.

 

[IRstuff]

"Corrosion" implies a chemical process. If this is a semantically correct use, then you've already answered your question.

If, on the other hand, the plating is simply flaking off and you're using corrosion simply as a placeholder, then there may be thermal mismatch problems between the plating and the substrate.

Has the plating process changed recently? Did this process work in the past? How do you ensure that your plating is stoichiometrically and structurally correct?

There are other types of plating that result in substantially different mechanical behavior depending on the chemistry of the plating bath, how often the chemicals are replentished vs. replaced altogether.

If the plating is structurally different that your normal process, the thermal shock may be sufficient to crack the plating and expose it to other environmental factors such as high humidity and cause the plating to peel or flake off.

TTFN

 

[Cockroach]

Do the chemistry. You have a limited reagent problem using zinc as an active element. What is the refrigerant fluid? Check the WHIMIS sheets!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[bigTomHanks]

We did SEM analsysis on the substances and the base material to make a comparison. Nothing really jumped out at us. It was mostly zinc mixed in with the base material elements to created a black crud.

 

[btrueblood]

No sulfur?