Acceptable pH in CarbonSteel Pipe 1

[ProjEngKLS]

We have a new process that generates a waste water stream. The waste water will be acidic, but we can adjust the pH to suite.

Process piping AFTER the pH adjustment is planned to be carbon steel (grade A106).

We would like to limit the final pH adjusted volume to minimal.

So what is an acceptable pH range for carbon steel?

Thanks.

ProjEngKLS

 

[YLWEA]

In cases like this I normally recommend that the user decide what the maximum acceptable corrosion rate can be and then select the pH that is likely to give that corrosion rate. Considerations include:
1) Is there corrosion allowance in the specified wall thickness?
2) Are there any corrosion prevention systems (cathodic protection and/or coatings?
3) Are there connections between dissimilar metlas that might promote galvanic corrosion?
4) What kinds of corrosion are likely (uniform? localized pitting? stress corrosion cracking?) and how easy is it to detect unacceptable corrosion resulting from "less than optimum" control over the fluid characteristics? If the corrosion is easy to detect then you have time to adust the fluid composition to become less corrosive)
5) What are the consequences of a corrosion failure (inconvenient leaks? catastrophic failure?) This influences how conservative you want to be in the process control.

 

[rsmallwood]

The corrosion rate of carbon steel varies at the same pH depending upon the acidic species present. A strong mineral acid (such as HCl) at pH of about 4.5 will corrode carbon steel at about a 5 mil per year rate at ambient temperature while a weak acid (such as carbonic acid) will corrode steel at the same rate but at solution pH of about 6.0. Depending upon application carbon steel is usually useful at corroion rates 5 mpy or lower but there are many applications where even lower corrosion rates are unacceptable or much higher corrosion rates can be tolerated.