Fresh water corrosion of carbon steel pipe 2

[LinusAD]

I have a new installation of carbon steel pipe in fresh water service. The water is picking up an unacceptable amount of iron from the pipe. The water has a pH of about 7.15.

Is there process or treatment that can be applied to the pipe to reduce the rate of iron pickup?

Linus

 

[TBP]

I suspect that the iron in the water will be due to oxygen corrosion of the steel, rather than pH related. I'm not sure what a cost effective solution would be. Was there a reason that carbon steel pipe was selected for this service? I'm having trouble imagining a material that would be less suitable in this application.

 

[unclesyd]

Is your water treated or is it raw water?
You need to check and see if the "rust" is really iron oxide. Depending on you location there is a Bacteria that causes corrosion to steel pipe and leaves it's calling card which has the appearence of rust. Or as "TBP" states you could be having O2 corrosion.

If you could give a little more information on your system, pipe size, how much, flow rate, etc.


There are people that can line/coat your pipe in place, but it is very expensive.

 

[kenvlach]

What are the source and the use of the ‘fresh water?’

What are the alkalinity and Ca concentration (both in mg/L) or alternatively, the CaCO₃ content (mg/L)? Temperature? Oxygen and iron analyses?

I suspect that your water is too soft; at pH 7.15 and a temperature of 70^oF, a CaCO₃ content of about 500 mg/L is required to give a neutral Langelier index (one that minimizes both corrosion and carbonate scale deposition). Municipal water is usually adjusted to a pH of 7.8, for which only 100 mg/L of CaCO₃ is required.

A good reference is The NALCO Water Handbook, 2nd. Edn., ‘Chapter 4. Water Chemistry and Interpretation of Water Analyses,’ (1988).

Of course, if this is not a potable water system, you may not wish to intentionally ‘harden’ the water. In this case, to avoid having the soft water harden itself by dissolving your pipe, I would bump the pH up to 10-10.5 to minimize the solubility of Fe⁺³. The easiest way is using a pH controller and metering pump to add NaOH solution; it is a bit more complicated to use Ca(OH)₂.

For extremely soft or DI water, use plastic, plastic-lined or stainless steel pipe.

 

[LinusAD]

We've done chemical analysis on the water. It is iron in the water. We are looking for a solution that does not require replacement of the pipe or special treating of the water. I've used carbon steel pipe in similar service at several other installations without any problem.

 

[moltenmetal]

Would phosphating the carbon steel be possible, or help? I've seen it used in boilers, and it's easy to do- I just don't know if it would help much in this particular application.

 

[LinusAD]

What would this involve? I'm not familiar with this.

 

[TBP]

LinusAD, your experience is very different from mine. In every application where I've seen carbon steel pipe used for raw water, the inside of the pipe becomes just one long rusty carbuncle from oxygen pitting.

moltenmetal - phosphate are used in steam boilers to tidy-up the bit of hardness that sneaks past the water softeners. If Linus AD has oxygen corrosion, I don't think it will help anyway. He needs to get a look at the inside of the pipe. If there general wasting of the pipe material, it's likely a pH issue, if there's localized pitting, that's oxygen corrosion.

 

[Metalguy]

LinusAD,

You have been given some excellent advice on what YOU need to measure and do. Just because you may have lucked out before doesn't mean you'll be lucky again, and it sounds like you weren't this time.

 

[LinusAD]

I appreciate all the advice. I will replace the CS pipe with stainless or fiberglass.

 

[Metalguy]

Better check the Cl level before you use SS!