Steel Pickling Line does unwanted Copper Plating
We have a hot rolled steel sheet pickling and slitting facility near Cleveland, OH. We've been pickling steel here for almost 25 years. We are facing a dilemma, and somebody who knows plating may be able to provide the key to solving our problem. We've talked to several steel experts who have all said they've never heard or seen anything like it.
Our customer has found a copper coating apparently "plated" on the steel we processed for them. SEM analyses of rejected samples confirm the Cu presence in a 1-2 micron thick coating. The copper coating on these samples only seems to come off with heavy abrasion. Large portions of the steel tend to be affected on both the top and bottom surfaces, with exception of the ends and edges of the 1300 - 1500-foot x 42" wide strips.
Indications are that the copper is being applied in our hydrochloric acid pickling process or upstream at the steel mill who casts and hot-rolls the steel. Up to this point, we have not been able to find a source of the copper and the conditions by which it could be plated here. Tests of our pickling solutions don't seem to indicate any significant amounts of copper in our system - our hydrochloric acid tanks have been tested several times with the highest reading of Cu being 132 ppm. (We regenerate our acid.) We've found no significant electric current, and the pickling lines are well-grounded. Analyses of the base metal steel indicate 0.01 - 0.03 wt% Cu.
I am hoping someone can provide me with information on the process of plating and conditions and requirements necessary to plate copper on steel. Perhaps it will give us a lead, or it will allow us to say that the conditions don't exist here to cause this to occur. Can copper be plated in a ~18% HCl bath with no electric current running through the steel strip? If so, would it be possible with Cu levels in the acid tanks being 40 - 130 PPM? Does anybody have any suggestions for places to look for the copper source?William Brahler
- Valley City, Ohio
Copper will immersion plate on steel. Your concentration seems to be low for that to happen, but it is probably your best guess. The edges of the sheet will be slightly positive compared to the center of the sheet in an acid solution,thus no plate on the edges. My guess and it is only a guess is that you are on the hairy edge of it plating and not plating. What does it do in the lab? What does it do if the copper is reduced by say 50% in the lab? There is a remote possibility that there is some other chemical or element that is acting as a reducing agent.James Watts
- Navarre, Florida
What you're seeing is probably an immersion copper deposit. It doesn't require current. If this deposit is a problem, you've got to lower the Cu concentration in the pickling bath.
- Tallahassee, Florida
Copper will definitely immersion plate onto steel because copper is far more noble than iron. Many students have done the experiment of observing copper sulphate deposit onto an iron nail. It's a tricky calculation, but you could try the Nernst equation to predict how many ppm of copper will cause this problem.
Ted Mooney, P.E.
Pine Beach, New Jersey
William, it is possible to plate soluble copper on to steel (iron) without any external electrical current. The process is call immersion plating and occurs when soluble metal ions in a solution are less reactive then a metal surface contacting that solution. The classic example of this is putting a nail into an acidic copper sulfate solution. As the iron in the nail is dissolved by the acid, copper ions are reduced to metallic copper and plate out on the nail.
To me, it appears the same thing is happening when you pickle the steel strip. The areas which are the most active on the strip are the edges. These areas are attacked faster by the hydrochloric acid and as a result, have the largest amount of copper plated on them.
The easiest solution to your problem is to find the source of the soluble copper and replace that component with an other material which will not be dissolved producing soluble metal ions. Whatever is made of copper now is slowly be dissolved away and will likely need to be replace anyway.Roy Nuss
Trevose, Pennsylvania, USA
It is common for copper to plate out on steel with out any current. Platers call this immersion plating. It occurs because copper is more noble than iron. It is also possible that if there is a high concentration of ferric chloride in the pickling solution, the copper that is immersion plated on the surface of the steel is redissolved by the ferric chloride.
The real question is how copper got into your pickling solution. Typically running a coil of steel through the pickling solution should reduce the copper contamination down to less than 5 ppm if no ferric chloride is present. Does the hot mill process both steel and copper slabs?
The pickling solution contains some ferric chloride and this is a very aggressive etchant. Do any parts of your pickling or regeneration tanks have items made from copper?
Hope this gives you some places to look for sources of the copper contamination.
St Paul, Minnesota
We have seen Copper Plating on our Pickle Line. At time it is only on one side. What would cause this?Sam Piraino
steel - Sault Ste. Marie, Ontario, Canada
The copper you see on the sheet may not be the result for your pickle bath at all. I have seen this condition, in more than your pickler, and believe it occurs in higher Mn./C. steels. The pure Copper molds that oscillates at the caster, at temperatures, above 2500 degree F, would be an excellent source for the Copper. Your pickling bath and the Hot Strip water has to low a PPM of Cu for the amount of plating that is on the sheet.Gerald Mannarino
- Dearborn, Michigan
April 19, 2008
There's a polarity to plating or electroplating. If you switch that polarity you can remove the plating. SO why not try grounding your machine that rolls the metal and adding a current to the pickle or vice versa?
If current is right you might even roll back though and remove all ready deposited copper.
It's possible that something "copper" has fell into your pickling tank.
October 4, 2009
The Cu plating is not very common in the HCl pickling lines. however if some of the equipments are having Cu plates like guiding plates, threading tables, run-out table having cu liners on top to reduce the scratches. then in such cases the sheet may rub on this and may lead to cu patches on the strip.
pl check all the equipments carefully.
March 12, 2010
Any new developments with this?Mark Tibbs
- Delta, Ohio
March , 2010
Hi, Mark. No two situations are ever really the same. What is your situation?
Ted Mooney, P.E.
Pine Beach, New Jersey
December 26, 2010
Copper plating is common when the Cu concentration in the hydrochloric acid used for pickling exceeds a certain threshhold. It is also more likely to occur on highly Mn alloyed strips.
The only known solution is to use acid produced by technologies yielding low copper levels.
Among the common acid regeneration technologies applied by large steel mills are Spray Roaster and Fluidized Bed pyrolysis. Spray Roaster Plants yield regenerated acid low in copper. Fluidized Bed Plants only yield low copper levels if scrubber wash water is continuously disposed. "waste water free" Fluidized Bed Acid Regeneration Plants are the known copper-troublemakers.
- Vienna, Austria
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