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"Why salt spray test hot black oxide parts?"


Q. 1. My customer requests our blackened parts go for salt spray test, and they found white rust came out within one day.
2. The customer put the parts in air conditioned machining room; after 3 weeks, the white rust started appearing.

They reject my parts because it cannot pass the above mentioned test.

Are they performing a reasonable test?

Also, after antirust oil is removed from the surface, we found reddish rust, especially at the corners. The oil removing process is ULTRASONIC, 60 °C, using TCE.

What is the reddish rust, and how to prevent it?

automotive component manufacturer - MALAYSIA


A. Please use lacquer coating process to protect the surface. Solution of lacquer coating (30% with 70% water v/v) can be purchased easily from any chemical supplier.

Above process can pass 24-hour salt spray test for sure. Anyway, need to know what is your requirement for salt spray test? 24-hour, 48-hour, 72-hour or 100-hour? We have another good solution for passing 100-hour salt spray test.

Good luck,

Kaka Ryuuichi
plating - KL, Malaysia


Kaka Ryuuichi,

My customer did not state the requirement, but they not allow any coating/spray/film on the part. Note: bare part with black oxide surface to go for spray test.

What is your suggestion if the part is brake piston, made of SUM24L?

NG Buan Hook [returning]
- Melaka


A. Hi NG. It sounds like a misunderstanding to me. It is well known that removing the oil from a black oxide finish reduces it to essentially zero corrosion resistance. It may flash rust before it even gets to the salt spray chamber :-)

SUM24L is, I believe equivalent to AISI 12L14; I believe it is very rust-prone.

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

sidebar April 2, 2009

Follow on to Ted Mooney's comments, with reference to MIL-DTL-13924 [link is to free spec at Defense Logistics Agency / dla.mil] para. 3.9, "...The black coating, of class 4 (300 series only), prior to the application of a preservative, shall show no signs of corrosion after 96 hours of exposure in the salt spray test..."

Any suggestions to improve corrosion resistance without preservative oil? Does chromic acid sealing helps?

Zainal Bin Hassan
- Singapore

April , 2009

Hi, Zainal. Please note that NG & I were talking about steel parts whereas the section of the spec that you are quoting is talking about stainless steel.

Black oxided steel parts have no corrosion protection without an oil or wax, but black oxided stainless steel parts are still stainless steel. Yes, a chromic rinse will help, but then you may run into RoHS issues, and you must of course follow the spec.


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

May 18, 2011

Q. Dear sir,
I have a similar issue on the black oxide steel parts - screw /bolts /nut which are heavily used in the assembly unit. However, we are facing the rusty issue the moment the product reaches oversea customer (by sea).
No doubt the parts with black oxide come in some oil, but upon handling, the oil layer may be reduced/removed. Should another anti-rust oil be able to solve the rust issue? Or any good oil /wax that can be applied/painted on the screw?
I have done Salt Spray Testing ASTM B117 [affil. link to spec at Techstreet] on the incoming black oxide steel screws and after 24 hours it starts to show rust. What is the right testing hours for black oxide steel as salt spray test process may wipe out the wax/oil during the process.
I am stuck with the solution as what is the right salt spray test time to test and what is the best solution/chemical/wax/oil to prevent black oxide steel from rust.

- Penang, Malaysia

May 19, 2011

A. Hi, Hussain. There are two facts that need to be accepted to address your issue:
1. Black oxide has essentially no corrosion resistance at all. Without oil on it, it can flash rust in a couple of hours, maybe less.
2. Salt spray testing is not intended to predict survival time in real life. Rather it is a QA method designed to let you know if a process that was formerly working correctly has gone south and is out of control. (If you traditionally achieved xx hours of salt spray, and suddenly you achieve much less, something has changed on you).

When you attempt to use a salt spray test to try to predict the real-world anticipated life of an oil or wax, and ask how many hours to test it for, you are not going to solve the problem. The way forward is to pick a reputed corrosion resistant oil from a reliable supplier of black oxide chemistry, and prove to yourself and your customer that the treatment meets his real-world needs. Then you salt spray test it to determine how many hours this proven process should resist. Then you periodically salt spray test it, and a red flag goes up if the salt spray hours significantly decrease. Shipping by sea is another matter, and will probably require special packaging. Good luck.


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

June 12, 2011

thumbs up signThanks Ted.
In short the black oxide is a low corrosion resistance and salt spray test is not suitable and should not be used to determine the corrosive resistance as it is fact is low corrosive resistance.
As the shipment undergo weeks of shipment to the customer, the black oxide may not be the best plating solution to avoid such rusty issue upon receiving the parts,


H.Hussain [returning]
- Malaysia

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