Stainless steel corrosion with oxidation/reduction conditions 1

[Boysie]

I have a textile operation using Alkaline Hydrosulfite to remove dyestuff from fibers. Machinery is type 316 SS, though some portions (flanges) are marked type 304.
Process uses 5 g/l Caustic pH 10 + hyrosulfite @ 220 deg F 40 mins.
This is followed by 1 g/l Sodium Chlorite, at ph 3.5 and 210 deg F for 45 min. I am also using 3 g/l Sodium Nitrate as a corrosion inhibitor
Does this sound like a destructive process, as far as Alkaline and Chlorine attack.?
I have been told that Pickling was done in the textile industry to reverse the effect of alkaline, or chloride attack. Is pickling necessary?
Does the fact that I sequence an acid oxidation, with an alkali reduction, help to minimise corrosion, or make it worse.?

 

[mcguire]

The alkali isn't hurting nor is it preventing the damage done by the Cl/NO3 combination which very similar to the solutions used to pickle stainless. Hot chloride solutions of low pH are deadly to stainless.
You will not reverse the damage done by a harmful solution; you will only arrest the progress until the material sees that solution again.

 

[Boysie]

The Chloride step is very critical to the process, and I have used the Sodium Nitrate at the request of a textile bulletin claiming it would be an anti-corrosive agent.
My problem is, how do I determine if the amount of chlorite, and the pH , and the exposure time is creating a corrosion which is dangerous.
I have run about 20 cycles of the procedure so far, the machinery is 316 stainless.
If I do have a real corrosion concern, is there any way to minimize it. Which would be most effective. Less Chlorite, less temperature, higher pH. any chemical additions?

 

[mcguire]

Some of the aspects of the environment that determine how severe the attack will be are summarized in an FAQ in this section.
1. changing pH won't do much; pits create their own acidity.
2. decreasing temperature will protect the stainless.
3. decreasing chloride concentration helps exponentially.
4. 10F equals roughly a factor of ten in chloride concentration.
5. Inhibitors do help. SO4, NO3,ClO4, CrO4 all inhibit pitting attack.
If none of these are feasible or you can't change the process enough, consider cathodic protection.

 

[rustbuster1]

The damage mechanism you could see is pitting of the stainless steels. You are doing 2 bleaching processes - one is under reducing conditions (hydrosulfite) and the other is oxidative bleaching (chlorite).
There is a theoretical possibility of pitting under the chlorite conditions in the exposure time of 40 minutes, especially at the temperatures you use. BUT because you change the environment back to the reducing conditions after 45 minutes, things will be probably be fine.
You do not have to worry until or unless you see some pitting or crevice corrosion - it will affect the 304 parts first.