Corrosion in Steel Mills

[waterwise]

We have a severe corrosion problem at one of our steel mills whereby recirculating water used for cooling around the mills recently seems to have increased the corrosion rates of the cast mill stands but more critically we have noticed that the nuts and bolts on the mills have had to be replaced more frequently. This is also true of the "dry" areas in the mill.
Our bulk recirculating water pH is around 7.5 -8, chlorides are around 1500- 2000 ppm and TDS around 4,000 ppm.
There is no bleed or blowdown from the system.
Current corrosion inhibitor program is based on Zinc/phosphate.
The bulk cooling water corrosion rates have been measured at around 3-4 mpy.

Previously the inhibitor program was based on chromates and the perception is that corrosion rates on the mills were not as severe when chromate was used.

A major change recently has been the use of continuous 12.5% hypochlorite injection into the bulk water to control bacterial slime.Total water volume is approx. 300,000 L.

We suspect that it is the chlorine in the water that is causing our accelerated corrosion rates.

The immediate area near the mills is obviously humid. At these high temperatures and obvious aeration, would the chlorine dissipate?
I would like to do some more research into chlorine/chlorides and corrosion of wet/dry splash zones.

Could anyone advise me if research into corrosion specifically in steel mills has already been conducted and where would I be able to get this information?
Are our suspicions well founded?

Your advice and help would be most appreciated.

 

[Metalguy]

I wrote a long message about Cl as a biocide somewhere; I'll have to find it and direct you to it. The very first thing I'd do is use a little acid to get the pH down a little and then reduce the Cl. What ppm do you actually have in the water?

If all that steel is unprotected, you really do have a problem. Chromates were *very* good corr. inhibitors-probably the best.

 

[kenvlach]

See Corrosion of welded carbon-steel tubes
thread338-41296
I think there are other threads on this subject, too.
Chlorine & chlorides are a definite problem. Change the solution –too difficult to remove them.
Disagree with Metalguy about pH. I would raise it to 10.5 to reduce corrosion of the steel (check that this is compatible with the inhibitor package & new biocide).

I use an EPA-approved biocide of the Acticide® type (for a totally different purpose, though). Supplier:

http://www.thor.com/

Hope this helps,
Ken

 

[Metalguy]

Going up to that high a pH will certainly slow down his corrosion, but he'll probably end up with calcium carbonate "cement" all over.

I agree that dumping all that "old" water is the best thing to do. Then he can use a blowdown system or just plan on changing the water every so often.

But until he gets some new biocide/inhibitor package in place, he can lower the pH (just a few tenths) to make the present Cl much more effective as a biocide/algaecide.

Now, *if* carbonate cement won't be a problem or you can use softened water, the cheapest way out is to use NAOH and raise the pH as you suggest. That pH will also be a good biocide-don't know about the algae tho.

 

[kenvlach]

Good reminder that high pH can be a good biocide. In Hoover's Water Treatment and Supply, 11th Edn., pp. 77-80 cover that subject. A pH of 11 kills most strains of bacteria, viruses as well as parasite eggs; in some cases, better than chlorine.
Don't know about algae, though. Copper sulfate?

 

[waterwise]

Thank you to all the expert responses to this question.

My immediate need was for studies or papers written dealing with corrosion of the mills or associated equipment used in the steel mill industry(not actually the steel pipes produced).

It seems to me from your responses, that the concentration of chlorine used ( free chlorine tests will be conducted shortly) as well as the amount of chloride present in the bulk water is responsible for the accelerated corrosion.

One final question-With the cooling water(containing chlorine)heating up to 100 deg.C and flash steam being created at the mill, is the chlorine still going to be present?

 

[Metalguy]

Waterwise,

First, I'm sure yopu can find the studies you want, but be advised that such studies may be very plant-specific and may not apply to *your* plant.

One of the big benefits with this forum is that you get "general" advice, which tends to have broader application.

Now, make sure that whoever does your testing for "free Cl" doesn't use the popular "OTO" swimming pool test kit (turns yellow/orange). This method tests for free and chemically combined Cl (chloramines, usually). Only the free Cl (hypochlorous acid) has the strong biocide effect. One or two PPM continuously at a pH no higher than 7.5 should prevent MIC. You can also add ~50-100 PPM of cyanuric acid to "stabilize" the Cl and help prevent it's loss.

It's not the biocide-level of Cl that corrodes steel--it's that 2000 PPM that's not "free" that does. When you blowdown or change the water, you reduce it way down, depending on the quality of your makeup water. Now, if you can't get the pH down a bit, consider using a combo of Cl and Br. There are many other biocides and corr. inhibitors available, and many factors that only you can supply.