Metals/Corrosion of stainless steel
I have been asked to provide a ball park figure for the maximum chloride content of water when in contact with stainless steel. I know that there are all sorts of factors involved with the corrosion rate on stainless steel and particularly with respect to chloride levels. The grade of steel I think that is used is 304. However I have not been able to find any real answers. Can you help?
Many thanks in anticipation
Hello Dave and thanks for the question.
You are correct that there are a great many factors involved with determining what stainless steels can withstand. While you are no doubt looking for a simple "the maximum is ___%", it is indeed not that simple. I will provide you with observations from my experience along with information on the steels, which is my expertise.
I will tell you Chloride or Fluoride concentrations are major factors to corrosion of stainless steel. You will need to evaluate everything coming in contact with the stainless; process water, cleaning solutions, etc. The level of heat is a great factor in stainless corrosion, as increased heat levels typically raise corrosion rates significantly.
As an example, one of my customers produces smoked hams. They have 60 smokehouses, which operate at about 150 degrees F. Cleaning the stainless everyday over a period of years with a Chlorine rich cleaner to remove the smoke buildup from the walls, resulted in stress-corrosion cracking in the 316 stainless steel walls.
Looking closely it appeared the stainless was etched with spider web cracks. At the worst, you could punch the 20 gauge stainless wall and put your fist right through it. Once it gets to that point, looking for an effective cleaner that is low in Chlorine is far too late. It is time to replace the material.
That is also why there are great varieties of stainless steel grades available. The standard "Use the right tool for the job", has never been more true than with the stainless steels.
When evaluating alloy steels, start at the beginning - the specific chemistry of the steel.
Increased resistance to corrosion is usually accomplished with higher levels of Nickel.
Your 304 series stainless steels contain: (nominal) 19% Chromium, 9% Nickel, maximum of 0.08% Carbon, maximum 2% Manganese, trace amounts of Sulfur and Phosphorus and the balance is Iron.
Standard food processing grade is 316 stainless steel. The difference between 304 and 316 is 316 contains 12% Nickel and 17% Chromium. The additional Nickel provides greater corrosion resistance.
The ham producer found their solution was to replace the 316 stainless with Avesta 254 SMO stainless. It contains 6% Molybdenum, 18% Nickel, 20% Chromium with 1% Manganese. Molybdenum works with Chromium and Nickel to enhance the best of each of those elements. It is a more expensive material, but if you are having similar experiences, it could be the answer for you as well.
I hope this has been helpful.
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