Black spots on weld areas of zinc plating

A. Hi Rajendraprasad. We appended your question to a thread where Monica O. seemed to have pretty much the same problem.

Luck & Regards,

Rajedraprasad,

Black or brown spot on part after zinc plate: strip and run this part again.
If the parts come out ok that means you have cleaning problem or hydrochloric acid dip is very low concentration, raise concentration to help cleaning process.

A. Hello Shawn,
I think I know what is causing you a problem, two questions first. (1) Is the flat metal hot or cold rolled?
(2) Is the welding tig rod stainless steel?

Regards

A. You really need to talk to your plater or another plater. The plater needs to determine a pre treatment cycle (cleaning, acid dip, etc) that will remove the flux you are leaving on the part. this cycle will vary depending on the manner in which the plater processes the parts (manually or automatically) and the type of plating process being used.

If I was to guess, I would say that the flux has been burnt into a porous area of the weld and the plater is having trouble thoroughly cleaning this area.

If product specific treatments are needed or additional operations you may find that the cost will be higher.

A. Hello!

Well we were also facing the same problem ,but only during winter season. I did lots of R&D but couldn't find the solution.Now I tell you a story which helped me to find my solution to the problem.

I always go to my office on bike and when I came to home, my face got black due to pollution (particles got stick to my face).When I wash with water , I couldn't got clean enough and when I wash with hot water my face got cleared.

So I thought that if after plating ,if I dip the plated material in hot water for few minutes so that the chloride laden chemical could be washed out from the welded parts and after that I did de-hydrogen embrittlement for at least 5 hrs. after that I again dip the same part in hot water and after activating it with Nitric acid and washing it with again hot water by immersing it in that hot water say for 4--8 mins. and passivating it at a slight warmer rate, my problem got solved.

So the conclusion is that when we did plating the chemical is in the pores of the welded spots and after plating, the chemical couldn't got washed out effectively and when it came out, it takes out the plated chemical. So when we did hot water dip the extra chloride comes out ,as it got melt instead of disturbing the plated metal.
So during summer season this type of problem didn't occur because room temperature is enough to melt down the extra chloride deposited.

For more better results we can also use centrifugal hot drier in order to dry the plated parts more effectively.

I hope this could help you to solve the problem.
This type of problem can also came if you are manufacturing threaded articles, because the same problem came in the threads of nuts and bolts. Do the same technique and you will be out of problem.

A. From your original picture, it looks like flux or slag that was not chipped off. All people that weld are not welders! I would look at hiring a certified welder trainer as a consultant for a day or two to improve the weld process.
A company that I worked for had a serious cracking problem with 718 welds even tho it was "in spec" . It took a talented person less than half a day to fix the problem - permanently.

A. Hi,

I had these problems as well while I was working in the car industry. Fact is, that due of higher Temperature (or the flame is too close to the to be welded part) the flux is getting burned in and of course won't be removable that easy. One of the help was, that after the welding process the parts get dipped in a cleaner and afterwards rinsed and dried so the parts won't corrode prior plating process. If the parts are in the plating process it is very difficult to eliminate. Therefore it should be also considered which kind of pickling are in use (Hydrochloric acid, or a mixture of different acids such like phosphoric acid and sulfuric acid). The pickling of course should contain inhibitors to avoid strong attacks and other un-welcome effect.

Regards,

A. According to the picture I agree with James. This problem has little to do with the plating shop. If a weldment is not chipped or blasted to get rid of the slag, little can a plater do with chemicals to get rid of it. Make sure your welders do their job properly or add a shot or glass bead blasting cycle. Minute amounts of flux slag can be cleaned away in pre-treatment but this is obviously way too much. A respondent mentioned TIG. This certainly looks to me like a coated arc electrode weldment.

? Hi Vageesan. I'm not sure what the picture is trying to show. There appears to be some blackening on the pipe near the top of the photo, but it's hard to tell whether it's a defect or just lighting. I see no rust or discoloration of the weld area here. What finish is on the fastener, and what is the green mark on it?

I'm sure our readers will try to help you, but when you have discoloration, rust, short salt spray life, no established re-work procedure, and you claim no knowledge of expected shelf life, proper handling, or proper storage, and your needs are urgent, I think you should retain a plating consultant.

Regards,

A. Hi Monica. As you see, we attached your inquiry to a prior thread where the discoloration may look a little different but the questions & answers sound highly applicable.

Although it might be possible for your welders to do things just exactly right, so the slag is only on the fillet, and then manually chip off / brush off the slag, the general problem seems to be that the welding process is producing slag which isn't getting brushed off, and which the plater can't remove chemically, but must remove by blasting.

I doubt that the plater is doing anything wrong, or that this is rust. And I doubt that the discoloration is on top of the plating, so I share your concern that after you remove the discoloration you are probably down to bare steel. To me it makes more sense to remove the discoloration (by blasting) before plating rather than after. Good luck.

Regards,