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"Chemical Reaction of SST, Chlorine & Hydraulic Fluid?"


Ladies and Gentlemen,

I work for an engineering consulting firm. Thus I am not a chemist, nor do I have excellent knowledge of coating processes. With that said...

We have a customer that is having a problem with a stainless steel fitting. The fitting is used to route light hydraulic fluid to a device that is permanently submersed in a chlorinated solution (swimming pool water). The specific problem is that the stainless fittings are catastrophically failing under intended use. There is an obvious increase in brittleness. We are under the impression that the brittleness is caused by some type of galvanization process taking place from the reaction of the stainless, chlorine and hydraulic fluid. Any information remotely related to this subject would be greatly appreciated.

William T [last name deleted for privacy by Editor]
- Middletown, Ohio



Your comment that the fitting has been embrittled leads me to suspect that the fitting may be suffering from stress corrosion cracking. This would most likely come from the water side, unless there is water contamination in the hydraulic fluid.

Without some evaluation, however, this is just a guess. If this is a serious problem, a metallurgical evaluation to determine the failure mode is probably needed. A good evaluation could tell you the mode of failure, whether it is coming from the inside or outside, and options for prevention.

larry hanke
Larry Hanke
Minneapolis, Minnesota

First of two simultaneous responses-- 2000

Depending on the stainless alloy and the surface area/mass (i.e., is this a thin piece of stainless?), it seems likely the chlorine is attacking the steel. Stainless steels are susceptible to attack by chlorides, and a thin item subjected to 0.5 mg/l as free Cl (aq) might easily be destroyed in no time. The hydraulic fluid is generally compatible and should not be the issue, as these are usually mineral oil or similar hydrocarbon products. You may need to use another metal, i.e. nickel, to reduce the susceptibility to halogens. As long as the part is not coming under abrasive conditions, nickel or precious metal plating (gold, platinum, rhodium) will help, but might not be enough to make the part last indefinitely unless absolute coverage is achieved.

Dale Woika
- Bellefonte, Pennsylvania

Second of two simultaneous responses-- 2000

Mr. Thorn:

One of the worst conditions that stainless steel can be exposed to is chlorinated compounds like swimming pools, bleach, etc. We have seen many situations like this. We can recommend that you talk to an expert on stainless steel corrosion, and can put you in touch with one if you are interested.

Best regards,

lee kremer
lee kremer sig
Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois

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Before I begin writing any type of piping specification, I like to know the specific chemistries of the materials the system is in contact with (not just a generic descriptor), the pressures that the system will be subjected to (max., min. and nom.) and temperatures the system will be subjected to (max. min. and nom.), not too mention the fluid dynamic characteristics (potential erosion problems here as opposed to corrosion problems). All of these are inter-related and will affect the decision of the materials selected as well as required thickness of the system components. Then there is the requirement of the ASME Pressure Vessel and Piping code requirements that ought to be taken into consideration.

To make matters a little simpler, for material compatibility, when it comes to corrosion, I normally check out the "corrosion allowance tables" in "Chemical Engineers Handbook" [affil. link to book on Amazon]. This will normally get you into the ball park when it comes to process mediums and material compatibility.


Neil E. Hatfield
- Franklin Park, Illinois

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