Stainless steel corrosion by chloride ions ?

A. 316 SS has excellent corrosion resistance, but it is not impervious to rust. Chloride is an excellent rust cause with a little damp air moisture. After your NaCl step, increase your rinsing via sprays (mist) or added tanks. A final warm water tank and blow dry step will significantly reduce the corrosion. The probable thing that allowed the corrosion to form was trace smearing of ferric iron in the manufacturing step. Nitric acid or nitric acid/chromate will reduce this problem. Hydrofluoric acid is NOT a recommended step. There are some quite good citric acid proprietary products available now that are a lot safer to use.

A. Rene - I have often seen stainless steel have corrosion because of high chlorine content of the water. Generally, this causes a "white rust" and is very damaging. It happens frequently in cooling applications where biofouling has occurred and the operator throws in a bunch of bleach to kill bacteria and algae in the recirculating tank. I hope this provides a starting point for your investigation. Good luck

A. You may or may not have corrosion of the stainless steel from the chloride exposure, but I doubt that the green color that you are observing is the result of such corrosion. Green corrosion will occur on stainless steel only under very specific conditions, and the green is due to segregation of chromium. If you are not finding Cr or Ni in the green material, this is further evidence that the color is not from the SS. Do you have any copper in the system?

Pitting corrosion of stainless steel due to chlorides would certainly produce a rust-colored product. Passivation can be used to maintain a good corrosion resistant surface of stainless steel process vessel. There are many commercial products for this purpose. In the pharmaceutical industry, this process is often called "derouging", that is to remove the buildup of iron oxides on the stainless steel process surfaces. I would recommend a commercial product with technical support from the supplier, rather than a home-brew passivation treatment.

Inspection of the stainless steel process vessels for pitting corrosion can be very tricky. The pits are often undercut with only a microscopic opening at the surface. Look for small spots of red-brown stain on the surface, then probe these spots for a deeper pit under the surface.

A. First of all, let me confirm that the green products you found are probably not issued from the stainless steel grade 316. At Tocopilla combined cycle power station located in the north of Chile, we face a high corrosion rate of our seawater submerged stainless. The problem is in fact the mixing of to unfavorable conditions: seawater and chlorine injection against biological fouling.

After 3 months only crevice corrosion appeared on the main cooling water pumps impellers made of 1.4408 SS grade (a kind of 316). This was due to chloride attack of the SS passive layer leading to a pitting corrosion. The active base metal remained therefore unprotected and a standard galvanic corrosion occurred between the passive layer (nobler metal or cathode) and the active metal (less noble metal, anode). All this created crevices in the impeller casings. All corrosion products found were red or brown (rusty).

The conclusion is: if you use SS material in a chloride environment, be sure you use the adequate grade of SS. Maybe, under certain concentrations, SS316 is not the best grade to fit your requirements. Have a look SS316HMo or SS317 or other grades with high molybdenum content and even with nitrogen content.

A. Definitely in 304 stainless steel corrosion takes place by pitting corrosion caused due to Chloride ions, But its color is reddish brown, It is not green. Green color is seen in corrosion of Copper metal. So you must look if there is copper.

A. René L,
I have replicated a green color solution with stainless steel by submerging a s/s bolt in hydrochloric acid (muriatic acid, HCl). Did you ever clean the stainless with hydrochloric acid containing cleaning products? If so the HCl may have reacted with the stainless and a resulting green color could have been the result.

A. To the individual asking about the maximum chlorine concentration for an RO Water System, the answer is essentially zero. The corrosion of the 304SS is not the issue, because all RO membranes cannot tolerate even low concentrations of chlorine. Activated carbon filters are used to remove any chlorine prior to the RO system. Even slight leakage of chlorine damages the membranes rapidly.

A. This to hopefully answer the question presented by the student from Hong Kong, Mr.Leung T. The question was why stainless steel reacts with bleach?

1. There are over 50 grades of S.S., some are more expensive than others. S.S. that has higher concentrations of molybdenum in it are considered marine grade and resist chlorides much better. These grades are expensive. 316 is approximately 20 to 30% more expensive that grade 304.
2. What concentration is the bleach? Bleach is sodium hypochlorite (NaOCL)In the U.S.A., it can range across various concentrations. Chlorox bleach is typically 12.5% available chlorine or 125,000 ppm chlorine.
3. The chloride ion penetrates the chrome oxide layer that protects the metal then destroys the rest of the metal components.

Hope this helps. Chlorine is a Halogen and there are other such chemicals the spread destruction. fluorine, bromine, iodine. Note: they all have an "...ine ending in their name.

Pete Churchill

A. Hi, Rechie.

Chlorine is a gas, not a liquid. The gas evolves from bleach solutions. Hydrochloric acid is a gas dissolved in a liquid. So, although no bleach or acid may have splashed on it, the chlorine is still there to attack the stainless steel.

Regards,

Q. Hi,

This is a follow up of my earlier question.The hand-washing sink we use is actually grade 304 stainless steel. The 304 SS stains due to the chlorinated water (calcium hypochlorite of 400 ppm) fumes that dominate the area.

What is the recommended chlorine present in water to not corrode the steel because of its fumes?
Thank you so much

Best regards,
Rechie

A. Hi Gino. Perhaps brass wool could replace the Brillo, but bleach is still murder on 316 stainless.

Regards,

A. I recommend scrubbing with Barkeepers Friend ⇦ this on eBay or Amazon [affil links] with water and dilute bleach, followed by dilute bleach rinse for sterilization, followed by a clean water rinse.

Using brillo may transfer free iron to the surface that will cause rust spots, so it should be avoided.

The combination of bleach and peroxide is counter-productive, since the peroxide will react with the bleach with the disinfecting power of each will be partially or totally destroyed. Use one or the other, and bleach is preferable since it will disinfect at low concentrations, whereas peroxide will not be effective at low concentration. Even 10-25 mg/L of bleach can be remarkably effective.

A. Ted is right, steel-based scrubbing pads should be replaced with something that doesn't contain iron (Scotchbrite perhaps), and bleach is a horrible thing to put on stainless. I realize that it's hard to break a customer off of bleach since they are only thinking about the need for sterilization, so they should be regularly giving the stainless a passivation treatment to counteract the bleach damage. This can be done safely and easily with citric acid based products. Extra measures are likely needed in this case since pit corrosion has already set in. Let us know if we can help.

A. Chloride ions in water will attack many stainless steels. in fact "stainless" is a misnomer - all stainless steels are only corrosion resistant and their resistance is dependent on their working environment and the alloy used. 316 stainless is a general purpose workhorse stainless steel, but it is unsuited to chloride containing liquors. If you seriously want a corrosion-proof shaft, you may want to consider a ceramic and start looking at sol-gel technology to coat a stainless shaft. You will have the strength of the stainless steel, but the corrosion resistance of a ceramic. Sol gel technology is relatively easy to implement and can give excellent performance standards

A. 316 Stainless steel is not a good choice for the high chlorides in your evaporator concentrate. The sulphates are not a concern. For resistance to chlorides in this concentration range, you will probably need to use a Hastelloy alloy.

A. If your heat source is thru the bottom or the walls, You will build up a scale layer that will act as an insulator and cause a faster degradation of the metal as well as wasted heat.

Hi...
Thanks for your answer. But as far as it is the question of SS 316 as MOC in evaporator, there are many companies here which have same MOC and their evaporators are working perfectly fine with it.
And about heating source I am not sure. I will get it confirmed and get back to you.
Thanks again.. :)

A. Neil,
Chloride solutions are always rough on stainless. 316 is the best you can have for this short of duplex. Even so, keep an eye on it and repassivate them as needed. This can be done safely and easily with citric acid based products.

A. If you weld ordinary SS T304, you will get carbide precipitation in grain boundaries at the weld area, and corrosion resistance will be lost. Use type 304L (low carbon) for all welded parts. In the presence of chlorides, use T316L.

A. Shirish,
I don't have any hard data that's applicable, but given 316 and the low concentration you plan to use, I think you will be okay for the the most part. However it would not surprise me if there was at least a little bit of corrosion within a year or so.

A. Gopal,
I would suggest that the HCl source in proximity to your stainless is the major contributor to your problem. Yes, there are safe citric acid based chemical treatments you can use to remove the rust and repassivate.

adv.
Let us know if we can help.

And I recommend you move the HCl system, or increase the ventilation around it.

A. At 1200 ppm Cl and 80 °C, 316L will fail quickly and 317 will fail more slowly, but both will fail.

Use 2025.

A. Usually, Stainless Steel will be a good corrosion-resistant choice to be used in FGD industry. But under the oxidation reaction, the chloride concentration will increase dramatically so that the passive performance on SS 2205 will be moved to negative side fatally.

A. Right, corroding stainless wouldn't normally be green. Though I'm not sure what "copper with a stainless finish" even means.

If it's stainless, citric acid based products should be fine for maintenance treatment to keep things clean and passivated. Accelerating the oxide layer formation with peroxide is hardly ever necessary, and won't make a difference given the constant cleaning with the chloride solution.

? Are you sure it is stainless? The description of green pitting sounds like chrome plated with the use of acid copper to level under a nickel plating.
That is also what I see on worn watercranes.

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Ed. note: watercranes = plumbing fixtures/faucets?

A. It sounds like the fixtures may be nickel plated brass, which is common for plumbing fixtures. Brass contains copper and will corrode green.

A. Adnan,
With high chloride exposure, you will want to re-passivate that stainless often. 316 would have been better for you.

A. Hello,

Washing machine powders generally have a 6.5 to 7 almost neutral pH. Whilst regular washing will not lead to an instant rust formation, but it will lead to a gradual dullness on the finish as time goes by.

It is possible that you see a few spots of rust In Indian hard water after a good many washes, but I would assume that it would be OK in Sweden.

A. Hi Arun. Keep the gas dry. Although wet chlorine gas is devastating to most stainless steels, dry chlorine gas is not particularly corrosive. In fact, plain steel can be used. Good luck.

Regards,