Reactor MoC : High Salt Concentration

[TiCl4]

I have a potential small reactor design that will alternate between high pH (10% by weigh NaOH, so 14+ pH) that is later taken to a slightly acidic side with HCl, resulting in a 6-10% salt (NaCl) solution. The reactor temperature will be ambient, so no high temp conditions. I've reviewed some literature on chloride pitting, and have mostly settled on C276 as my prime material of choice. I'm exploring different options, of course, but budgetary estimates for an Alloy 20 reactor were only about 10% less than a C276.

Before settling, I have ordered several corrosion tabs so I can place them in this reaction mass (acidic side), seeing what, if any, corrosion develops over the course of 6-8 weeks.

A couple of questions:
1. Given my reaction is ambient temperature, will heating the material for the corrosion tabs testing be beneficial for accelerating corrosion to see it in a short timeframe? Or should I keep the tab testing near temps the reactor will see?
2. Are the below charts generally correct? Any other materials or other corrosion mechanisms to watch out for?

 

[EdStainless]

Generally a "C" type alloy would be selected for your service.
But what do you mean by 'slightly acidic'?
A pH of 5, or of 3?
Even with highly corrosion resistant alloys the alternating conditions will lead to some corrosion.
Generally a PREn of >40 is used for seawater service.
You could warm you test slightly (what is the hottest day that you expect? 100F) but if you heat too much you may change the corrosion mechanism and be testing the wrong thing.

 

[TiCl4]

Ed,

This is a developing process through R&D at the moment, so I don't have the exact pH. Given the nature of some of the reactants, I would expect a final pH of near 4 to be required as a starting baseline. Would going lower change material selection from C276 to something else?

How do alternating PH conditions lead to corrosion over and above simple exposure to high/low PH environments?

 

[EdStainless]

I am going from memory here, but I want to recall that 686 should stand up better in the alternating pH situation.
You are going to need very solid weld procedures.
And you will need to qualify those procedures with extensive corrosion testing.
Even if I used C22 I would weld with 686.

 

[TiCl4]

I am going from memory here, but I want to recall that 686 should stand up better in the alternating pH situation.
You are going to need very solid weld procedures.
And you will need to qualify those procedures with extensive corrosion testing.
Even if I used C22 I would weld with 686.

Having not spec'd a vessel for corrosive service before, what would typical specifications before for welding for corrosive environments? I assume there would be a limit on heat to limit the HAZ as well as certain cleanliness requirements, but haven't really delved into this area before.

 

[EdStainless]

You need to have raw material specs.
Your weld procedures will have a bunch of extra required variables.
Your procedure qualification should include microstructural evaluation (no detrimental phases) and corrosion testing.
For a corrosion test I would use G48-C at 85C for 72hrs.
There should be no corrosion attack.