letter 5183

Corrosion resistance of zinc plating

.

I run a zinc plating operation. The question of corrosion resistance comes up often. In zinc plating, protection to white corrosion products is the function of the chromate. Protection to red rust is the function of the chromate plus the thickness of the zinc plate. ASTM B201-80 [link is to spec at TechStreet] address corrosion resistance to white corrosion products. Is there any published information on corrosion resistance to red rust on zinc plate over ferrous metal with NO chromate post treatment?

Don P
- Elkhart, Indiana

Zinc and zinc alloy plating needs some sort of passivation, most often this is hex-chrome. I can not imagine any part being zinc plated and NOT passivated after plating, so I do not think you will find that type of data around.

The zinc plate protects the base metal. The chromate protects the zinc plate. Once the chromate is overcome, the plating oxidizes resulting in white corrosion. Once the zinc plating is used up, the base metal oxidizes resulting in red rust.

There are different types of chromate passivates. Maybe you are wondering what the zinc plating's contribution alone and worry about the chromate's added benefit later. Well, this is not A + B = C. The chromate and the zinc plating act together to give more corrosion protection than either would achieve alone. You need to test the zinc WITH a chromate in order to yield useful information. This might mean trying out different combinations of chromate and plating, but that is the correct way to assess corrosion resistance.

Timothy Neveau
- Auburn Hills, MI

Hello Don I don't know where to get data for zinc plating with and without chromate, but I do know of some dealing with tin/zinc. Shahin, G.E. " Alloys Are Promising as Chromium or Cadmium Substitutes" from Plating and Surface Finishing 85(8):8-14;1998. Where tin with 20% zinc has a 25 fold increase in time to white rust and a doubling of time to red rust, under salt spray test with the addition of a chromate coating. There may be some data on straight zinc with and without passivation, however I don't have it handy.

I seen zinc with chromate compared to zinc with a zinc phosphate conversion coating, though these were not tested to white/red rust. The paper is on the web, www.aappso.com/hazmat/AMSSV2/techref.htm [Ed. note: URL no longer works] there are a few papers there, this one is called "Preliminary Evaluation...Ion Vapour Dep'...." by Cramer, Ault and Hartline. Terry is right there is an interaction between the chromate and the plating, though the zinc might well be oxidising it does not display the white corrosion product as quickley when used in conjunction with a chromate coating. I haven't answered really what you asked but I hope this has been helpful.

Ian Brooke
university - Glasgow, Scotland

Using this plating spec. ASTM B-633 [link is to spec at TechStreet]-98; Fe/Zn 12, SC 3, Type I. Any Idea what the corrosion resistance to red rust would be when following ASTM B-117 [link is to spec at TechStreet] procedures? (.0005" zinc, as plated without supplementary treatment)

Don P
- Elkhart, Indiana

Hi Don ,
I am sure that if you contacted the "Zinc Development Association" they would be able to help .

Have a nice day

John Tenison - Woods
- Victoria Australia

MFSA's Quality Metal Finishing Guide for Zinc Plating (www.mfsa.org) will have this info.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

hi don

I too am suffering from the similar problem. some authorities are debating that salt spray life of zinc plating (till white rust only) can be improved only by increasing the thickness of zinc layer. as you have studied more in this line, can you help me in this context. as far as i learned WHITE RUST LIFE can be increased only changing the type of passivation and should be immaterial of thickness given. please try to answer this. thnx.

vrishali d
- mumbai,india

I am in need of a detailed explanation about the following finish with the process data

FINISH: ZINC CHROMATE PER ASTM B633-98,TYPE 3 ,SC3,CLEAR

KARTHIK
service industry - INDIA

Hello, Karthik. ASTM B-633 [link is to spec at TechStreet] is a copyrighted specification, and you must start the process by ordering a copy. You will see that we have hotlinked to one place on line where you can purchase and download it immediately. After you have the spec in hand, please get back to us with any specific questions. However we must have your full name and your city in order to continue the discussion. Thanks!

Ted Mooney finishing.com Brick, New Jersey

Post a question on a different subject.

 

Answer or follow-up on this subject (in non-commercial fashion).