plating, anodizing, & finishing Q&As since 1989
Is this galvanic action?
Q. For about 10 years we have had two mild steel powder coated tanks that hold our "reclaimed" (water from a lactose RO polisher unit and condensate from milk evaporators) water. This water has been treated with chlorine, bromine and of late chlorine dioxide over the course of 10 years to keep its microbiological growth under control.
Over the years the powder coating hss been scratched up and a good amount of corrosion has taken place in these tanks. We do inject sodium hydroxide to keep the pH in a neutral range.
Control or proper control has always been an issue. Either we have too much oxidizer and jeopordize the metals or not enough and have bacteria problems. As well we have had some episodes of pretty low (2.2) pH's from time to time.
These tanks are piped at the outlets through a common discharge header of stainless steel to a series of pumps and distributed throughout our large faciility. Recently within the past few weeks we have noted surface rust on much of our stainless throughout areas of our plant. We are being told that this is due to "galvanic action" from the mild steel tanks connected to the stainless. Is this possible to begin so rapidly in the past couple of weeks when over the past 10 years we have seen nothing of this sort? There are literally thousands of feet of stainless lines from the mild steel holding tanks to the silos in our plants where we are seeing this occur.James Hrusovszky
food processor - Mesa, Arizona, USA
A. Never say never, especially if you haven't seen the actual conditions yourself, but this isn't galvanic corrosion. In the first place, it is the base metal (steel) rather than the noble metal (stainless steel) that corrodes in galvanic corrosion situations. Look elsewhere. Cleaning of the pipes with steel brushes, fumes in the air... don't know, but find another theory :-)
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
First of two simultaneous responses -- 2004
A. Ted's right, this isn't galvanic corrosion. It is still corrosion though, so now you have to figure out the driving force and stop it. If your stainless is a 300 series material like 304, 304L, or 316 then there will be no red rust coming from the material itself. Any red rust will be on the surface and from a different source. Once you clean a section thoroughly and inspect it for pitting wrap it with plastic or something to keep environmental dirt away. Leave it covered for a few days and check for the red rust. If it is present you have iron embedded into the surface, if not it's environmental and you need to look for what changed... That's probably the best question you can answer, "What is different now?" Look for any source of iron or steel or anything that would give that appearance.
- Pearland, Texas
Second of two simultaneous responses -- 2004
A. With some of the lower pH's you mentioned, the chlorine and bromine will break down into chlorides and bromide, both of which attack stainless steel.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
December 9, 2012
Q. We are using RO water in our 15 ton ice machine for ice and as cooling water in the shell condenser. This is a new 304 stainless condenser. after only 6 months I'm having serious electrolysis problems. It has a sacrificial plate and 2 zincs in it. The old condenser was copper, lasted for 20 years with some plating of copper on the steel end plates. We do use marble chips to help make the water less aggressive. I'm not a water wizard. helpMicah Laroche
- Wadmalaw Island, South Carolina, USA
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