Forough64,

Note that the surface protective film on steel does not always stay in stable film. The oxide film(ex., Cr2O3) can be decomposed in case it is exposed to specific environment like strong acid. The reason why corrosion resistance of nickel based alloy or titanium alloy is generally higher than SS 316 is due to that they have "more" stable film against "more various" chemicals or strong acids.

I would recommend you to check the compatibility of the SS 316 against the chemical or fluid that was applied and corrosion rate as well.

SiHyoung Lee,

Is the 316 pitted or is is it just 'rusty' looking?
If it is pitted is it more severe at the welds or on the bottom, is there any pattern?
What is the temp, pH, oxygen? What other ions are in solution?
Over time even mildly chemically reducing solutions can lead to enough passive film breakdown to lead to corrosion. Cl and Fl are the most powerful, but many ions will lead to this.
316 is only a very little better at pitting resistance than 304, there are many higher alloy grades with better pitting resistance.
The Cr, Mo, and N are all that matter. The other additions are there just for phase control.
One reason that duplex SS alloys have become more popular is that they use alloying more efficiently, for similar levels of Cr and Mo they develop more corrosion resistance.

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P.E. Metallurgy, consulting work welcomed

Thanks alot.
there is just general corrosion in surface, no pitting. as I know, ammonium carbonate and ammonium chloride in concentrate below 15% is not very corrosive for ss316. But the fluid contents a lot of free iron . Do you have information about the reaction of iron ions with passive film?
thanks

If there is no active pitting then the iron may all be from someplace else in the system and not the SS.
Do you have any pumps? Do you know what the impellers weighed before they were installed? We used to weigh them and stamp it on the back. When they came out for maintenance and people said 'they look fine' we would show them that they were 10% lighter. Un-annealed cast SS impellers can shed a lot of Fe into a system.
Look up Roughing in sanitary service, many papers on the subject.

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P.E. Metallurgy, consulting work welcomed

"One of the first points which should be made regarding the removal of iron residue is that mechanical methods such as abrasive blasting have not been successful. The abrasive merely moves the iron around on the surface; it does not remove it. The only mechanical methods which are successful are those which remove the surface, such as machining or grinding.

The only known methods for removing iron from the surfaces which are not machined are chemical and electro-chemical methods. And not all chemical methods are successful – nitric acid alone does not do the job. The known useable chemical methods include:

Oxidation – This is most readily accomplished by heating the part in air to normal heat treating temperatures. The iron is converted to iron oxide which can then be removed by abrasive blasting. This method is acceptable for unmachined sand castings since the scaling which occurs is not detrimental. The sand blasting abrasive must be free of iron contamination or the part will be re-contaminated. However, heat treating in air is not suitable for parts with machined surfaces and often not for investment castings. Heat treating in vacuum or in protective atmospheres is also not suitable since the iron is not oxidized.
Pickling – This is probably the most commonly used method. Use a solution of nitric and hydrofluoric acids in water. We use ASTM A380, solution D. The formula is specified in A380 as 6 – 25% HNO3 and ½ – 8% HF in water at 70 -140 F (21 – 60 C) for about 30 minutes. This is a strong cleaning solution and may etch highly finished surfaces. Pickling should not be confused with passivating. Stainless steel self-passivates on exposure to air – no special passivation treatment is required. (However, it may be that using a “passivating” treatment such as nitric acid, also described in ASTM A380, may accelerate the formation of the passive film or form a thicker passive film.)
Chemical Cleaning – Some citrus-based cleaners have been shown to remove free iron contamination. However, there are some concerns about the stability of these cleaners since they may be subject to bacteria growth.
Electropolishing – Like machining and grinding, this process removes the surface of the part, including any embedded iron."

Dear EdStainless
the stainless steel part was put in a cast iron tower.Because of sever corrosion of tower body, existing of iron in fluid is normal. I think that this iron ions in fluid has an important role in the rupture of passive film and the corrosion of ss316. Are you agree?

Thanks dear 0707
unfortunately, we didn't remove any surface containment, but it's important for me to know that "can the iron ions in fluid which comes from cast iron and carbon steel parts, hurt the passive film?"

picture of the ss316 part

• https://files.engineering.com/getfile.aspx?folder=11c3d9dc-fabb-437b-8aee-0

In general the presence of any film on the surface will have a negative impact on the corrosion resistance. But if your system is low Cl (<10ppm), pH >6.5, and moderate temp then I don't see serious issues.
In fact if the rest of the system is steel or CI you will never keep these surfaces clean anyway.
This should reach a stable level and remain constant, unless you operating condition are changing.

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P.E. Metallurgy, consulting work welcomed

Thank you so much sir