Corrosion of SS fixtures in water valve house 3

[Gungadin]

Hi - we operate a number of valve houses for a large water transmission pipe system. Upgrades at one valve house included fixtures (wall vents, louvers, doors, railings, electrical enclosures) constructed of 316 stainless steel. All of these fixtures are showing some degree of corrosion (some heavy). The humidity of the air in the house is approximately 100%, and condensation is present on all surfaces. The water is from the treatment plant, so has significant chlorine concentrations (avg. dose at the plant is ~1.8 mg/l). Beyond pickling/passivation, is there any other treatment available?

Thanks much for any input!

 

[Golflinks]

Corrosion in humidity under atmospheric conditions in almost due to lower Mo content. Type 316L has specified Mo content of 2-3% Mo. Suppliers tend to keep this as close to 2%Mo as possible. If you can get guaranteed 2.75%Mo containing SS then the observed pitting corrosion would not occur. Since this is a surface phenomenon, either chemically treating with proprietary paste or blasting the surface with glass beads to put surafces under residual compressive stresses would be beneficial.

 

[Gungadin]

Thank you - does the paste serve to bolster the surface layer of the SS by leaving some coating or material, or is it rather repairing the SS natural corrosion resistant surface? can you suggest a few paste products?

Thanks again!

 

[mcguire]

I would think that the alloy content is not the primary problem. If you are dealing with machined, ground or abraded surfaces, they probably need to be pickled with acid( nitric, citric, etc.) to remove the exposed inclusions(usually sulfides). There may also be corrosion promoting ions present such as iron or chloride. Chlorine as a gas is not corrosive at this temperature.
316 does not corrode in the environment you describe unless something's wrong.
Check the sulfur level of the 316 from the source of the components. Specs allow up to 0.030%, but in actual practice it's bad news if the sulfur is above 0.005%.
If you clean the surfaces with acid you will remove any contaminants or exposed inclusions and solve your problem.
Blasting with glass beads will not help in any way.

 

[TEV]

I agree with McGuire on proper chemical treatment of the surface subsequent to any finishing operations. The surface should be passivated per ASTM A 967. If there is any evidence of serious surface contamination (such as would occur if the surface was wire brushed with a steel brush (or something equally gross), the surface should be pickled prior to passivation per ASTM A 380.

Be aware that some commercial cleaning solutions used in facilities contain chlorides or chlorites that can induce corrosion.

I doubt if sulfur level is the problem, although it is probably worth checking. Be aware that the very low sulfur level, <0.005%, will significantly decrease machinability and alter welding of the stainless.

 

[Gungadin]

Are there special considerations regarding pickling and passivation on parts 'in-place'? If the fixtures cannot be removed from the humid environment, are there additional measures required?

Thanks!

 

[TEV]

Pickling and passivation in place can be tough. There are pickling pastes available (talk to a welding supply house for pastes designed to remove welding heat tint from austenitic stainless), but the part(s) must be cleaned and neutralized after the pickling. Nitric acid passivation solutions must be handled with great care for personal safety and potential for damage to other materials. Citric acid passivation solutions are considerably easier and safer to use, but I still think a thorough rinsing after is necessary.

 

[mcguire]

Sulfur is the single worst contaminant in stainless steel. Each 0.001% reverses the value of 1.0% chromium or 0.3%Molybdenum. 316 with 0.008% sulfur has no better corrosion resistance than 304 without sulfur. Benefits to machinability from sulfur aren't seen without the use of very high ( >0.015%)amounts. Its benefit to welding is only to decrease the amount of heat input required for penetration.
The myth of beneficial levels of sulfur comes from producers of welded stainless tubing who dope their stainless with large amounts of sulfur to increase their tube welding line speeds because they use TIG welding. They could achieve the same speed benefit by using laser welding and not ruining the corrosion resistance of the material.

 

[Gungadin]

Thanks again, McGuire - what's the most appropriate method to sample and test for sulpher in SS?

 

[mcguire]

In the mills we relied on spectrometers. If you have purchased a product made of stainless the manufacturer can provide chemical analysis. There may be a surface chemical test, but I don't know of it. Maybe another member can help.

 

[TEV]

McGuire - Please note that I did NOT say sulfur reduces weldability. I said it alters welding.

 

[mcguire]

I stand corrected...sorry, TEV.

 

[Paskenell]

If I may be allowed to add my two cents. I agree 100% with McQuire on his statement that this problem should not be happening under the conditions you mention - if the material is as specified. The problem arises when you mention that &quot;all&quot; of the different items are experiencing some form of corrossion. Unless there is a manufacturer that I am unaware of, louvers, railing, doors and electrical enclosures should have been made by seperate manufacturers usually entirely different lots of 316 bar, sheet or rod. The chances that the metal used in all of these products came from the same mill is minute.
Definetly follow McQuire's and TEV's recommendations in reference to passivation and properly identifying the materials that were used and if this fail's to answer your problem, my next suggestion would be to look very closely at the environment these parts reside in to see if there is an additional chemical present you have yet to identify. Also examine the fasteners attaching these instruments to the structure. Are they also corroding? If not, while unlikely, you may be suffering galvanic corrossion.
Just my two cents worth and good luck.

 

[BarryMartin]

Have you considered electropolishing the stainless steel? Rather than simple chemical pickling the metal can have an applied electrical current in combination with a solution to electropolish the surface. This is commonly done in pharmaceutical industry to provide a mirror finish which means less surface roughness for bacteria. Simulataneously, electropolishing will increase (or enrich) the surface passivation layer of chrome oxides.

 

[TEV]

Check out the article entitled &quot;Effect of Some Surface Treatments on Corrosion of Stainless Steel, A Review&quot; by Gary Coates, in the August, 1990 edition of Materials Performance, published by NACE, The National Association of Corrosion Engineering.