×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Corrosion of Zn plated steel in NaOH solution

Corrosion of Zn plated steel in NaOH solution

Corrosion of Zn plated steel in NaOH solution

(OP)
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.

RE: Corrosion of Zn plated steel in NaOH solution

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

RE: Corrosion of Zn plated steel in NaOH solution

http://hghouston.com/resources/technical-newslette...
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).

RE: Corrosion of Zn plated steel in NaOH solution

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.

RE: Corrosion of Zn plated steel in NaOH solution

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

RE: Corrosion of Zn plated steel in NaOH solution

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.

RE: Corrosion of Zn plated steel in NaOH solution

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources