Excessive Corrosion on Zinc Plated Parts

[thom9446]

We've been having some major corrosion issues with zinc-plated parts recently. We've been manufacturing and shipping equipment all over the world for years and within the last year our equipment is arriving totally rusted. We've been using an SC2 Type III zinc coating and are looking into switching to Type II (colored chromate) but are unsure if this will solve any problems. The attached picture (see link below) is especially confusing in that of the four identical parts, only three are rusted. Here's some facts that might be of interest.

- We box our equipment ourselves for shipment with desiccant packets inside the boxes (boxes as large as 15' X 25' X 8').

- These corrosion issues have only been happening recently on shipments to China and India even though we've shipped plenty of products to these countries before without issue.

- We currently use an SC2 Type III coasting (ASTM B633).

Here are some specific questions and ides I've had.

- Switch to a Type III coating (we're getting samples from our supplier to examine this coating)

- Has anyone else noticed an increase in the corrosive environments of India and China lately?

- Has anyone noticed a decline in the quality of their zinc plating? (we have wondered this ourselves, but we also have bare steel components that used to arrive in great condition that are now fully rusted)

- Is there anything anyone has to offer on a more reliable corrosion protection system for overseas shipment? Could the high electical activity in the area our equipment is being used be a factor?

I'd greatly appreciate any input. Our equipment goes into aluminum cast houses and any kind of rust is unacceptable (rust + molten aluminum = dangerous explosions). On top of that it of course just looks terrible.

Thanks, Mike

http://files.engineering.com/getfil...6c57-4afb-a764-8f2c3d570ca7&file=IMG_1020.JPG

 

[KENAT]

I haven't looked up the zinc plate standards you state but, is there any chance it's due to changes to the chromate related to RoHS compliance?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[mjl2023]

After the Zinc plating, what supplementary treatments are on top of the Zinc? IIRC the Spec- SC2 Type III coating per ASTM B633 only applies to the Zinc, not the supplementary treatment usually chromating.
Agree w/ Kenat, verify w/ supplier they didn't change chromate treatment related to RoHS compliance

 

[thom9446]

Yes, we have verified that the supplier has not changed the chromate treatment due to RoHS. In fact, the type II coating is not compliant (hexavalent chromium).

mjl2023 - The "type" I'm referring to is the chromate coating. Type II is for colored chromate coatings and Type III is for colorless chromate coatings.

 

[TVP]

thom,

There is significant base metal corrosion (red rust) occurring, which means that you need to confirm that the zinc layer is correct instead of only looking at the chromate layer. Verify that the zinc thickness is correct, that there are no cleaning/adhesion problems, etc.