-- The Home Page of the Finishing Industry
23+ years of serious education, promoting Aloha,
& the most fun you can have in metal finishing smiley
    no popups, no spam
on this site
current topics
topic 35984

Withstanding 500 hours of Salt spray to white rust

A discussion started in 2005 but continuing through 2017


Q. Dear Sir/Madam,

I have problems on the plating finish of some steel plate.

Could you please help me identify what is meant by:

Can these withstand 500 hours salt spray testing to white rust?


Eddy Mok
- Hong Kong


Steel is often zinc plated for corrosion resistance, because zinc plating is thin and decorative, while providing sacrificial protection to the steel. The plating deters the steel from rusting (red rust). But the zinc plating is invariably provided with a chromate conversion coating to deter corrosion of the zinc plating itself (white rust).

Traditionally, the conversion coating was hexavalent chromate (which is inherently yellow). But European RoHS (reduction of hazardous substances) regulations forced a widespread changeover to proprietary trivalent chromate conversion coatings; these usually require a proprietary organo-mineral topcoat over them to offer satisfactory corrosion resistance. HyProCoat is a trade name for one company's (Pavco's) line of topcoats.

A common accelerated corrosion test is "salt spray/fog testing" in a test chamber designed and operated in accordance with ASTM B117 [link by ed. to spec at TechStreet] for 500 hours or 1000 hours (or other number of hours).

Another trend in recent years is a move away from plain zinc plating toward plating with zinc alloys like zinc-cobalt, zinc-nickel, and zinc-iron because these offer greater corrosion resistance at equivalent plating thicknesses and a moderate cost increase over plain zinc. TK is suggesting zinc-iron alloy plating, with a special hexavalent chromate conversion coating whose color is black; he notes that although many black chromates get their black color from silver (Ag), this black chromate is not silver based.


A. Dear Eddy
If you give details of the kind of plating,I will be able to help you.
Hex yellow chromate is used to passivate a freshly Zinc plated component to protect it from corrosion.
To enhance corrosion life,the plated and passivated component is subjected to top coats like HyProCoat 330.
The component is subjected to salt spray test in a salt fog chamber, usually in plastic coated chambers for hours.
Your component can pass the test up to 1000 hours or more for red rust depending upon the process you adopt.

t k mohan
T.K. Mohan
    plating process supplier 
Mumbai, India


Q. Where could I find that coating to do 1000 hours.
Please let me know.

gary joseph
Gary Joseph
Johannesburg, South Africa


A. Mr. Gary
Zinc-Iron plating (Alkaline non Cyanide) Black passivated with a non-silver (Hex) with top coat is giving 1000 hrs+ Red Rust. In fact test results are available beyond 1200 hrs.
The iron in the bath has to be monitored and should be around 130 ppm/ltr. Other passivations like clear and yellow is difficult to get.

t k mohan
T.K. Mohan
    plating process supplier 
Mumbai, India

Requirement of 500 hrs salt spray hrs for White rust

October 31, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Requirement of best corrosion resistance which is suitable for our product (Chains).
Customer has given a target of 500 hours without white rust and 1000 hours without red rust. Kindly suggest suitable platings or coatings.

Dinesh Ravi
Product designer - India

October 2017

Hi Dinesh. TK has advised that zinc-iron plating with black chromate and proprietary topcoats can achieve this salt spray requirement.

But once again we should remind readers that salt spray testing is a QA measure to insure that a process has not deteriorated; it should not be used for selection of a process. This case offers perhaps a good example because hot dip galvanizing, while it performs poorly in salt spray testing, would easily outlast, outwear, and outperform electroplated zinc or zinc alloys on rough service chains (tow chains, tire chains). Similarly, painted coatings will outperform both electroplating and galvanizing in salt spray tests but would probably be quite unsuitable for those types of chains. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Comment to THIS thread START a NEW threadView CURRENT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2018, Inc., Pine Beach, NJ   -   About   -  Privacy Policy
How Google uses data when you visit this site.