Hydrogen Embrittlement

Question:

We have a cathodic epoxy electrocoat line, and a customer has asked if it will have problems with hydrogen embrittlement. I assume that there is some hydrogen trapped in the part considering we have a cathodic electrocoat tank. But considering the length of time in the bath, I’m not sure if it is enough to worry about. T.F.

Answer:

Hydrogen embrittlement in steel parts is usually only a concern in parts that are pretreated in an immersion zinc phosphate or are plated. The coating weight of the phosphate is also a factor. Concerns of hydrogen embrittlement are usually present only in parts that have lengthy immersion times to process the part due to their size and/or mass or need for a “heavy zinc” phosphate coating.

The “correction” of hydrogen embrittlement in steel parts is to “stress relieve” the part by an extended bake after pretreatment processing. As a custom coater, I have pre-baked parts before electrocoating them, but only if the parts had been off-line immersion zinc phosphated or processed for heavy zinc (300+ mg/ft2 coating weight) phosphate.

To the best of my knowledge, the hydrogen generated during cathodic epoxy electrocoat process is not likely to cause hydrogen embrittlement. Any hydrogen formed while electrocoating may also be trapped in the coating and not the part. As you noted, the immersion time is usually short (two minutes or less). Even if the part is immersion zinc phosphated or plated on the same line (square transfer design), the phosphate or plating time is also only about two minutes and embrittlement is unlikely. The usual cure (baking) cycle of electrocoated parts would be beneficial in “baking out” any trapped hydrogen in the coating or part.

If you have a specific part that your customer is concerned about, review it with your pretreatment and paint suppliers.