Tin plating: how to improve Salt Spray hours and high humidity corrosion resistance?

A. Hi Abubacker
I'm probably not understanding the phrasing of your question because if your customer is presently getting 5 years of life with the current system, how could they possibly get double that life without changing anything? :-)

But we've appended your inquiry to a thread which I believe has the answers you seek.
Reading between the lines of Danny Neidel's posting, if 5 microns gives them 5 years of life, 12 microns can probably be expected to give them 10 years, although I don't really understand the corrosion conditions / environmental conditions the tin plated components will be exposed to. Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi, Ben.

Although tin plating over nickel plating is possible and is done, I think you will find that tin plating is not as corrosion resistant as nickel plating. If the original nickel plating was not corrosion resistant enough, replating with tin instead of nickel should not be expected to improve the situation. I don't think you will find the documentation you seek because corrosion resistance depends on so many different factors.

The problem seems to me to be that the nickel plating was not free from porosity despite the fact that it was spec'ed as reasonably thick. You say the components are "battery contacts"; if we're talking batteries for electronics in a benign environment, I'm having trouble understanding what went wrong that they are rusting. But if this is for some sort of battery exposed to the environment, 5 microns is not a lot of plating. Good luck.

Regards,

A. Dear Sergio,

Yes, it can resist 240 hr in SS per ASTM B117. It depends on thickness and porosity of the tin layer. Tin will protect steel only if it is pore-free. Where that is not the case, the iron becomes anodic and strong corrosion follows.

According to ISO 2093 coatings are classified in four categories; group 4 extremely demanding conditions, group 3 demanding cond., group 2 moderate cond. and group 1 mild cond. Over steel the following thicknesses are required : 30, 20, 12 and 5 micron. Corrosion resistance can be improved by a copper-strike interlayer.

Test for porosity with Kesternich test (included in ASTM B545) or ferroxyl test, plate some test parts and check corrosion resistance in the salt spray chamber to set up process parameters.

A. Hi Sergio,

In the standard mentioned you need to plate 30 micron (group 4), 20 micron (group 3), 12 micron (group 2) and 5 micron (group 1).

To reach 240 hours in salt spray over steel you have to plate at least 20 micron. Chromates avoid staining in the parts, but it doesn't improve really corrosion resistance. If it is permitted (parts are not soldered after plating), you can use a organic sealer or lacquer. That is easy to make for rack parts, small parts in barrel tend to stick with each other. An organic sealer improves corrosion resistance 2-3 times, commercial products are available from local vendors.

I hope that's helpful.

Saludos

A. Hi Dianne,

Salt Spray test is not normal for tin coatings. Salt Spray test is a typical corrosion test for zinc or zinc alloy plating. Tin is not a common corrosion protection on parts that are exposed to severe environmental condition. For your application where tin is plated on components/parts NOT exposed to environmental condition, salt spray test may not be relevant.

A. Hi Dianne,

You don't mention the Salt spray test duration? But with the thickness that you mention, I don't think that the parts will sustain even 24 hours.

Moreover, with the thickness as you mentioned, this part only can be used in an indoor and dry environment (no condensation can occur). So, if you do Salt Spray test, it's normal if the result is failure.

A. Hi Rishabh.

As you see, we appended your question to a thread that pretty much answers it for you. But if you now have more questions, please get back to us. Thanks.

Regards,

A. Hi, Nilesh.

As you see, we appended your inquiry to a similar thread. As you will read, you are going to require rather thick tin plating to solve this problem, and possibly an organic sealer if the specs permit. But your current 5 hours may also indicate that you are not processing the parts quite correctly; the plating needs to be free of porosity.

Regards,

A. Hi Eury. There are a number of open questions:
Condition B is 48 hour salt spray, but doesn't define what constitutes passing vs. failure; that you need to determine from ASTM B545 or another spec. Are you sure your actual minimum thickness is 0.0004" as required by B545 SC2? Danny notes, and the literature does too, that you are pretty susceptible to porosity and thus galvanic attack at this thickness; bumping it up even a little can reduce porosity. Are you applying a chromate or an organic sealer, or nothing on top of the tin?

Regards,

A. Hi again. Sorry, but after 2 postings I still don't know your situation, so it is difficult to respond. I assume you are from the plating shop not the buyer, i.e., you are doing this plating yourself to someone else's requirements? And you know that the thickness everywhere is no less that 0.0004"? The substrate is cold finished steel? And you are experienced in tin plating but have just never salt spray tested it before? You are conducting the salt-spay testing in house? Do you do any other testing of your work, such as adhesion testing, perhaps bending the parts over a mandrel?

Do you do any soldering of the parts? Successful soldering in itself can be a good adhesion test and indicate that the surface is not oxidized or soiled, since poor pretreatment will certainly cause porosity and corrosion.

48 hours is probably not unreasonable for 0.0004" deposits; but you won't pass if there are pores in the plating due to excessive surface roughness, poor cleaning, poor activation, or poor plating practices. Generally the plating process supplier who is selling you your tin plating chemistry and pretreatment chemicals can advise you what sealers they suggest -- is your process MSA or stannate or what? Thanks!

Regards,

A. Hi Rajesh. You added your question to a thread which already contains some great info on the subject. I've personally never heard of testing for white rust on tin plating, but hopefully a more knowledgable reader will chime in. If there is such a thing, it would have to be prevented with some sort of post-treatment or topcoat. Please send some pics of this white rust to mooney@finishing.com for posting here if you can.

What constitutes passing or failing a salt spray test is usually defined by the specification. What specification are you plating to?

8 microns is less than the 12 microns required for even moderate service, so it's not a lot of thickness. And it's important to realize that tin plating is a barrier-layer coating, not a sacrificial coating like zinc plating. So, whereas pores and porosity in zinc plating may not be a major issue, they are crucial in tin plating because if the environment "sees" any steel base metal, the tin plating will galvanically force the underlying steel to rust.

It is normal for specifiers and shops to have acquired, due to previous experience with zinc plating, a mindset about salt spray testing along the lines of x thickness equals y hours. I would urge you and your customer to take a quick look at thread 1744. It's about salt spray testing of stainless steel, but it conveys an important idea: whereas salt spray testing of zinc galvanically "wears through" the coating, salt spray testing of stainless steel and barrier-layer coatings like tin plating is more a case of finding or not finding a defect.

Because of this pinhole and porosity situation, the shape of the parts and their surface condition can seriously affect corrosion resistance and salt spray results. If your customer claims and demonstrates that other shops pass their requirement with 8 microns, there may be a need for you to mechanically finish the parts before plating.

Luck & Regards,

Sir, thanks for your valuable addition on the topic and for the detailed explanation.

It is great that you are sharing your knowledge to future generations, and I sincerely appreciate that.

Best Regards,