49018

Steel wire pickle green surface  

June 12, 2008

Hello All,

I am in the wire processing industry and I have a query about our pickling cycle. We have a batch pickling cycle in sulfuric acid. Our bath concentrations of sulfuric acid usually range between 12- 18% acid at around 140 - 170 degrees F (60 - 77 degrees C). For our Low to Mid carbon content grade steel wire we always notice a green surface that can be easily rubbed off after pickling and rinsing the coil. One can see the bare metal after the green surface is rubbed off.

Bryson Hayes
wire processing intern - Fort Wayne, IN, USA

Sounds like excessive attack of the wire by the acid (instead of just taking off the scale), followed by inadequate rinsing, thus leaving a film of ferrous sulfate.

You don't mention an inhibitor, which would minimise attack of the steel itself. One of the supporters of this site (Henkel) has an inhibitor Rodine 85 which we have found excellent in 8-12% sulfuric running at 70 degrees C in a new hot-dip galvanizing plant.

I suggest an inhibitor to minimize metal attack, and improved rinsing as an added precaution.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

I forgot to mention we use an inhibitor made by Crown Technology. It's called ACID AID LG10. Could the green surface just be a product of the Iron Sulfide that hasn't dissolved in the acid tank?

Bryson Hayes
- Fort Wayne, IN

Can't see any way ferrous sulfide could be present. Sulfur in the steel is present as manganese sulfide inclusions which are grey-coloured and microscopically small - there shouldn't be any iron sulfide at all. The sulfuric will generate hydrogen sulfide gas from the small amount of sulfur in the steel, and that gas simply escapes into the air.

Given that you use an inhibitor, and assuming that it works OK (I don't know it, so can have no opinion on its effectiveness), there will be little or no reaction on the surface of the steel as you come out of the pickle, so we're left just with a rinsing problem. Can't see your contaminant as being anything other than ferrous sulfate on the surface - problem is where does it come from? If you use a tank for rinsing, is the acid concentration in your rinse a bit high? Whether you use a tank or a spray rinse, if you leave some acid on the steel after the rinse then the inhibitor concentration in that residual acid will be too low to prevent reaction between the acid and steel, and you could form ferrous sulfate while the steel is drying.

Lots of suppositions and possibilities, I know, but may be some help in deciding where to run specific tests.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Sir,
I prefer Rodine 95 for sulfuric acid. The concentration is normally between about 1 part per 10,000 to 3 parts per 10,000 of mixed acid. Additionally you need a simple inhibitor test to know what is in solution. This test appears in an article of mine, now published in the journal, Metal Finishing.
Regards,

Dr. Thomas H. Cook, Galvanizing Consultant Hot Springs, South Dakota, USA