Problems in zinc plating of cast iron

"Zinc Plating"
by Herb Geduld

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A. You could try doing anodic cleaning in NaCN solution after acid pickling and prior to plating it may help; please let me also know if you succeed.

A. You have not mentioned the grade of Casting, assuming the grade to be 25, before Yellow passivation dip in 2% HNO3 and rinse Dip in Yellow passivation and spin dry in a Centrifugal dryer at about 50 °C.

A. Many castings are porous. It is common for chemistry to leak back out of these pores after finishing, causing the described types of failures. One technique which I have used in the past has been to alternatively dip the finished part in clean cold, then hot rinse water tanks, repeating several times. This will cause the metal casting to expand and contract slightly, thereby drawing in and expelling the water, and rinsing to some extent the pores. Thorough drying must be achieved with time in a hot air box, fan-forced type dryer.

A. Hi, John. The impregnation should help on the bleedout issue. But you can't give the plating shop a pass; it is they who must satisfactorily plate the parts.

It is possible that the shop you are working with uses an alkaline zinc plating process whereas acid zinc plating may give better coverage on cast iron. In any case, careful rinsing will minimize staining, alternately rinsing in hot and cold water can be effective in getting the plating solution out of the pores.

Regards,

A. This kind of spotting out is normal for castings that will be zinc plated. Degassing or vacuum impregnation may help minimise but will not be a 100% fool proof solution to this problem. Try E- Coating the part instead.

A. We have had many years experience with plating cast iron, what we have found that this defect can arise when the part is overpickled. His results in surface porosity and the corrosion is from inside out. Try using a weaker pickle or utilize inhibitors to limit metal attack.

Q. Thanks for your interest, I've already spoken with our customer and he told us that he use a hydrochloric pickling bath for about 15 minutes. I asked him what he think about and he said that we cannot decrease pickling time 'cause our body needs it. Which kind of inhibitors shall we use? Shall we decrease our level of acidity or do we need another kind of acid bath?

Thanks a lot,

A. If you cannot seal the porosity adequately and your client will allow a copper plate, castings can quite often be sealed with a cyanide copper plate and then subsequent zinc plating. Cyanide copper is nearly 100% efficient and there for similar to acid zinc the castings will usually cover, and being highly conductive the zinc deposit will readily plate over the copper. In the galvanic series zinc preferentially corrodes to both iron and copper and therefore the zinc will still corrode preferentially. Iron on the other hand corrodes preferentially to copper.

The only two drawback here is possible migration of the zinc and copper at the interface that over considerable time may spot out, and the other would be how critical the dimensional issue may be with say a .0002" copper deposit in addition to your zinc deposit.

? Hi Brandon!

What type of steel parts? Is this cast ductile iron or SAE 1212/similar? Leaded steel?

Can you give us a hint of how you prepare the parts prior to plate them? Maybe we can help you that way.

Regards!

A. Hi Brandon!

It is a tough one... These black spots appear only in the non machined surface, or are they all over the part?

The rough surface in the non machined area is like cast iron, is it really SAE 1215?

What thickness is required for this part?

Do you use hex or trivalent chromate?

I think there are too many questions and few answers...

Best of luck!

Hi. Although I'm no metallurgist, I have to agree with Daniel that this certainly looks like a casting, not a rolled or forged item … in which case, yes it is probably bleed-out from the pores. Vacuum impregnation can probably eliminate the problem, but who pays for it -- you or your customer?

Regards,

A. Hi, the problem is in cast iron and solution is shot peening and humidity and casting sand.
Controlling humidity and spheroidal graphite is necessary.
70% trapezium grit and round grit 30%.
Magnesium content in smelter controls spheroidal graphite and lowers pores.

----
See also letter 38946, "True that cast iron can't be Zinc Plated?"

A. Hi Jozef. I would anticipate neither hydrogen embrittlement nor significant change of dimensions from over-pickling of cast iron parts which will be zinc plated. But what I would anticipate would be difficulty in successfully zinc plating them. Over-pickling can leave carbon (graphite) on the surface and greater porosity which can lead to excessive staining, bleedout, etc.

I think the correct answer is, unfortunately, that nearly any breakdown will generate "out of spec" components, which means that the particular lot must be isolated and evaluated as to salvageability rather than making a salad of good and potentially bad parts. As the world increasingly moves towards treating electroplating as a critical process rather than as a 'commodity', we'll find that 'breakdowns' become financially unacceptable and will stop happening. Best of luck.

Regards,

A. Hi again, Jozef. Yes, multiple rework processes can cause problems even if we don't fully understand why :-)

Way back when automobile bumpers were usually nickel-chrome plated, and there were lots of bumper manufacturers, there was sort of a standard about reworking vs. scrapping that many shops followed: a rejected bumper was stripped (often just of its chrome plating) and sent for a second trip through the nickel plating and chrome plating tanks. If it was rejected a second time, it was scrapped. The reason it was scrapped after two tries probably had less to do with potential functional problems from three trips through the nickel tanks, and more to do with the simple economic premise that we don't exactly know why it failed the first time, then it failed a second time and we don't exactly know why -- so the likelihood of it failing a third time due to something we don't understand or haven't accounted for is probably quite high, and high enough that we'll probably have better luck with a random raw part.

Although the economics, logistics and type of plating are different in your case, the same general thoughts might apply: if a given part was rejected, then replated, then rejected a second time, it may be best to cut your losses. I can't prove that it would fail on its third time through, but I can say with pretty good confidence that the rate of success drops with each re-do.

Regards,