Corrosion of Zn plated steel in NaOH solution

[daviddem]

Hello, I am new here and I would like to submit a problem.

The situation is as follows:
- a braided steel cable lies inside a steel pipe (not sure of the steel grades, but this is mild, not stainless)
- the steel of the cable is zinc-plated supposedly because the Zn layer acts as a sacrificial anode, which protects the steel of the cable from galvanic corrosion
- inside the pipe a NaOH solution (pH 10) flows almost continuously
- pressure inside the pipe is about 5000 psi
- temperature inside the pipe is about 250 degF

We observe severe corrosion of the cable. Within about four months, the steel braids show visible signs of corrosion and have become very brittle. We also observe corrosion on the low side of the pipe (where the cable lies), to the point that pinholes can develop.

We have been at odds of identifying the exact causes of the problem. However we recently stumbled on the below redox equation, which would suggest that the NaOH could possibly rapidly eat up the Zn protective tlayer, leaving the bare steel open to galvanic corrosion:
Zn + 2 H2O + 2 NaOH → Na2Zn(OH)4 + H2

Since fresh NaOH solution is constantly supplied by the flow (this is not closed-circuit), I would think that the reaction would be complete until no Zn at all is left?

So the question is: is it possible that this is what is happening? Can steel and steel (cable and pipe) corrode each other once the Zn has been chewed up by the NaOH?

Any help, insight, comment, suggestion welcome.

 

[daviddem]

I also forgot to ask: is it also possible that the hydrogen produced by the reaction causes hydrogen embrittlement?

 

[cloa]

http://hghouston.com/resources/technical-newsletters/corrosion-in-caustic-solutions.aspx

For 250F, it should have been high alloy stainless steel- too high for 304.

Even without the Zinc corrosion- the steel is well beyond the safe range for caustic corrosion due to hydrogen embrittlement.
The Zinc corrosion will only compound the problem. Good find of the alkaline corrosion reaction for zinc (the hydroxyl ion complexes the zinc ion).

 

[TomDOT]

Yep, that's an inappropriate application for zinc. Even in less aggressive conditions, plating is generally too thin to provide much corrosion protection.

Can you install an impressed-current cathodic protection system inside the pipe? They are more often used on the outside of buried or underwater pipelines.

 

[daviddem]

No we can't install cathodic protection, however we are thinking of replacing the NaOH with soda ash (Na2CO3) for pH control.

 

[cloa]

If you are trying to achieve a　pH of 10 and temperature of 350 F, I don't think it will make a difference- its still basically NaOH with bicarbonate ions.

 

[TomDOT]

I don't suppose it would be helpful to add some calcium carbonate to achieve a Langlier index barely above zero in order to precipitate a thin layer of protective scale.