Acceptance criteria after salt spray test

A. Hi Sundar,
The specification or process which you are plating in accord with is the document which defines passing or failure. It will depend on what process you are doing on what substrate. For example, pass or fail for chromating aluminum will be much different than pass or fail for sacrificial plating, barrier layer plating, painting, powder coating, passivation of stainless steel, etc.

If you are processing in accord with specifications, please advise what specifications, and we can review their requirements with you. If you are not processing according to accepted specifications, you and your customer will have to argue it out :-(

Luck & Regards,


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

A. The solution to this problems starts with knowing the probable end-use environment, and then finding a lab corrosion test method that replicates most/all of those conditions. When the test method is known, it is much easier to select a lab corrosion testing cabinet that can perform it. Lab corrosion test methods generally fall into three categories: Traditional salt fog & humidity tests like ASTM B117, Basic Cyclic corrosion tests like ASTM G85 Annex 5, and Advanced Cyclic corrosion tests like SAE J2334. Doing a Keyword search on "how to select" and "cyclic corrosion test chamber" could yield articles that will help you focus on specific test methods and the types of cabinets that perform them.

A. Dear friend,

You are probably from the receiving inspection department, looking for tightening the supplier. Well.

All zinc coated material will have two types of corrosion, namely Zinc and Iron. The corrosion product of Zinc is Zinc oxides which are white in color. The corrosion product of Iron is ferrous oxide which is red in color. That's how you call the results as White rust and Red rust.

If you are getting a white rust, you can interpret the quality of chromate on zinc coatings. All plated products are chromated to protect zinc corrosion.

With the Red rust you can interpret the quality of zinc plating. The duration to start the red rust after white rust if it is low, then the zinc plating is porous.

In plating getting a white rust after 300 hrs is one of the best quality out of my experience. You need to specify the requirement of salt spray resistance, there are no standards. However remember every process has its own limitation.

A. I believe that there are two types of rust called White rust and Red rust.

White rust is nothing but failure phenomena of coated surface and Red rust is failure phenomena of parent material.

White rust and Red rust can be decided by the nature of coating.Generally salt spray specification for white rust will be 46 hrs and red rust will be 120 hrs, of course depends on nature of coating.

A. While assessing Salt spray test results many interpret black spots or patches as failure of Corrosion test.This is only an intermediate stage and this is not to be taken as failure. Only White corrosion product and later on red rust formation need be considered for interpretation of SS Test results especially in the case of Zinc and Zinc alloy coatings.

Thanks, Subramanian! I can agree 95%, but not 100%. Some specifications define failure, and in that case failure is whatever the spec says it is. And, of course, a buyer can write any spec they wish, which can define failure in any way they wish.

But what a buyer probably cannot do (as you say) is fail to define failure, but then claim that black spots or patches constitute failure :-)

Regards,

A. Hi Deepak.

Nickel-chrome plating is a barrier layer type of plating, not a sacrificial plating. Thus, if there is any porosity or any pinhole, the nickel plating will accelerate the rusting of the substrate. You probably need duplex nickel plating or thicker nickel plating to improve the salt spray results.

Regards,

A. Hi Prashant. What specification are the parts plated to? The plating specification, such as ASTM B633 for zinc plating, usually defines what constitutes failure. If not, Subramanian's answer should be helpful to you. Good luck.

Regards,

A. Hi Prashant. White rust is unsightly and it can "gum up" things -- making it difficult or impossible to assemble/disassemble things, operate door latch mechanisms & triggers, fire extinguishers, etc.

White rust is supposed to be forestalled with chromate conversion coatings ... and there is nothing wrong with a spec that demands some given number of salt spray hours before white rust.

Regards,

A. Hi Fai. As Subramanian tells us on this page, black spots do not constitute failure. However, I think the main problem here is that you and the customer must agree what specification you are plating to, and I see no mention of that. Specifications help define what comprises passing and failure.

If your customer is not requiring plating to a specification, I don't see how he can reject anything; nor can I see how you can claim to be offering a satisfactory plating service short of polling the whole world for every vague opinion :-)

Good luck, but please use this disagreement as a launching pad for correcting the omission by referencing specifications in purchase orders and certs. Good luck.

Regards,

A. Hi Claudine. ASTM B117 is basically only the operating requirements for salt spray testing and doesn't define failure. Often, plating specs define failure, but unfortunately I don't have a copy of AMS2404 handy, so I can't comment on what if anything it says about failure. We appended your inquiry to an earlier topic where Subramanian says that black spots do not comprise failure; so, in the absence of anything to the contrary in AMS2404, you are correct that red rust is the dividing line between passing and failing.

Regards,

A. I have a copy of AMS2404, anyone need help please feel free to contact me.

Thanks Davey. It's copyrighted, so I can't ask for a copy, but if you would check how it defines failure, that would be a help!

Regards,

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

A. Hi Claudine,

AMS2404 says the base material shall be free from corrosion. So what you really need to know is whether the black spots are corrosion or whether it is something else causing the staining.

Incomplete oxidation of iron caused the formation of a mixture of the ferrous oxide and the ferric oxide (Fe3O4), this tends to be pretty black in color. The classic red corrosion product is the ferric oxide (Fe2O3).

So, can you analyse the black spot? XRF or SEM with EDX would be good tools to use. If the black spots contain iron then you have corrosion product and you should consider that a failure, AMS2404 does not make any distinctions between corrosion product oxidation states.

A. Hi Mohit. The rust from steel is called red rust, whereas metallic protective coatings on that steel (like zinc plating), if present, corrode to white corrosion products / white rust. In general, you would only get both white rust and red rust if the article is made of steel and has a sacrificial coating of zinc under the paint, and in that case the white rust comes first. But abstract questions are difficult to answer properly because the world if full of possibilities, and we can't cover every "If, and, & but". So if you can give us the details of your own situation, a more complete and reliable answer may be possible. Thanks.

Regards,

A. Hi Sheldon. I'm sorry but your posting seems to have two errors or wrong assumptions. :-)
First, a customer can require any specification for salt spray endurance he wishes (as long as it was referenced in the purchase order).
Second, these "96 hour" and "144 hour" specifications on this page are referring to zinc plated surfaces, not e-coated surfaces. E-coated surfaces do not fail in the same way as zinc coated surfaces, where the sacrificial zinc coating is gradually consumed.

I don't know your particular parts and your particular e-coating, but 96 hours sounds low for any high quality organic coating; are you doing good quality phosphating before the e-coating? 480 hours does not sound impossible, and may not even be unreasonable. Good luck with the issue.

Regards,

A. Hi Pankaj. You are right that a buyer & seller are free to negotiate the performance requirements of the contract but there is an exception: some published specifications forbid you from 'diluting' them. The spec may contain wording that a purchase order can add restrictions or performance requirements but cannot reduce the requirements or selectively quote the spec. For example, if spec XXXX123 requires 96 hours of salt spray performance, the spec organization has declared in advance that a purchase order saying "plate per spec XXXX123 but only 48 hours salt spray resistance is required for 'B' surfaces" is employing the spec invalidly. In the event of a contract dispute you would therefore have a mess on your hands.

Plus, a salt spray test is an accelerated 'advance warning' test which is used to help determine if a process is under control; it usually does not mimic real-life performance, so to spend a lot of time defining allowable percentages of white rust on "A" surfaces vs. "B" surfaces, for example, may not be well applied effort -- real-world corrosion patterns are likely to be entirely different anyway.

Regards,

A. Hi Kay. I think it's as simple as the fact that NaCl is soluble and the stain will wash right off (I am not implying that simple rinsing will remove all salt and its corrosive effects ... just that salt will readily dissolve and the stains disappear).

Regards,

A. Hi Santiago. I'm not understanding your usage of grams of corrosion, and the customer wanting more corrosion than you are getting -- something seems to be worded funny. But, yes, you can certainly get 96 hours of salt spray performance from proper pretreatment and e-coating.

Regards,

A. Hi Vijay. Subramanian says it does not represent failure. I agreed except that if the customer specs says it's failure, then it's failure. Brian Terry tells us that some specs like AMS2404 define the appearance of any corrosion as failure, and that the spots might be corrosion and can be tested. Then too, one person's black spots may not look the same or be the same as another person's black spots.

Please explain your situation so it's less abstract, and send some pics if possible. Thanks.

Regards,

? Hi cousin balaji. Please start with who you are, what you do, what type of plating or coating you are talking about, and what kind of door handle you are referring to (automotive interior, automotive exterior, home interior, home exterior lockset, etc.)

Regards,

A. Hi Yunus. The most important document is the specification which the parts were processed according to. Very often this will tell you how to interpret the results. Interpreting the results for salt spray testing of aluminum conversion coating, various electroplated coatings, powder coating, etc., may be quite different. Please tell us what specification number the parts you are referring to were processed to. Thanks.

Regards,

A. Hi again, Yunus. You probably should find/choose a spec for each of those 6 processes (although I'm not sure what platings you are describing: Zn on Fe, Ni on Fe, Cr on Fe, Ni on Brass, Cr on Zn, Ni on Zn?).

In the USA we have MIL specs, ASTM specs, SAE-AMS specs and probably a few others. There are German DIN specs, Japanese JS specs, British BS specs, and ISO international specs. As previously mentioned, some of these specs will include interpretive guidelines for what constitutes failure in salt spray testing, but there is no single spec which defines what constitutes failure except with regard to itself.

Regards,

A. Hi Vincent. I'd be happy to hear to the contrary from any readers, and a buyer can write any spec they wish and demand compliance with it ... but I side with your supplier. If the parts endured shipment by sea, or storage in a corrosive, damp, or smoggy industrial environment for 6 months they could very well be exhibiting corrosion signs which will fail an examination after salt spray even before they go into the salt spray chamber :-)

Regards,