Stainless steel pickling is the most common pickling process used at Metapic’s facility. The most common application for stainless steel pickling is to “ pickle / passivate ”. This is a one-step process which removes heat tint and weld scale while simultaneously passivating the surface of the parts being processed. Most customers are surprised to find out that pickling also accomplishes passivation. However, where required by our customers, Metapic will pickle and passivate in two complete and separate steps.
The importance of pickling, passivating and cleaning Stainless Steel cannot be stressed enough. These processes remove rusts, heat scales, weld contamination and can detect faults in the product. Furthermore, stainless steel can acquire surface contamination even in non-corrosive environments. Therefore, the possibility of corrosion is greatly reduced and the life of the treated stainless steel increased. Metapic procedures are generally in acordance with ASTM A380.
Significant Added Value
Chemical pickling, or pickling for short, has a long history of use for cleaning the corrosion from stainless steel. The process is somewhat flexible. A chemical pickling agent made from nitric acid and hydrofluoric is applied to a surface to remove contaminants. Microscopic images in Fig. 3 (a, b, c and d) were captured by using an optical microscope (OM) proved that immersion in acid pickling up to 15 min could remove oxide scales. The effect of acid pickling treatment on stainless steel samples, slightly roughen the surface.
In relation to the material and labour costs incurred by stainless steel production, the relatively low additional cost of pickling is insignificant. Yet without this important proccess the integrity of the product is likely to be seriously deminished in the future.
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Pickling is a finishing process in which heavy oxide crust or scale is removed chemically.
That is done by immersion in a bath, called pickle liquor, of suitable mixtures of acids.
Scale develops on the surface of iron or strip steel, steel wire, and some other forms of steel, during hot forming or heat treating in air.
The most used solution to treat Carbon steel products, with an alloy content less than or equal to 6%, contains either hydrochloric (HCl) or sulfuric acid (H2SO4).
Hydrochloric acid is more expensive than sulfuric acid, but its action is much faster while minimizing base metal loss.
It avoids the formation of smut and decreases the danger of excessive attacking.
It also permits to economize on heating expenses because the process runs at lower temperature than that based on sulfuric acid.
Pickling steel (not only cucumbers..)
Scale must be thoroughly removed before applying further finishing layers like plating or paint, that otherwise would not make contact with the metal.
Most important such layers would not protect the metal surfaces from further corrosion.
Results of this cleaning process depend on the type, strength and temperature of the acid mix employed.
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Sheet steel processed by acid-pickling will oxidize (rust) rapidly when exposed to moist atmospheric conditions.
To prevent this, a thin film of oil or similar waterproof coating is applied to avoid contact of pickled steel with moisture in the air.
The oil film must be removed for any further fabrication, plating or painting processes.
Inhibitors are natural or synthetic organic compounds that are added to acid solutions to protect plain carbon steels being cleaned.
They are added for slowing or stopping the chemical action of the acids on the base metal.
Inhibitors:
- reduce the loss of iron,
- minimize hydrogen embrittlement,
- protect the surface from pitting attack caused by excessive pickling or from poor surface condition quality,
- reduce fumes resulting from excessive reaction of acids on metal, and
- help economize on acid consumption, without affecting appreciably the rate of scale or rust removal.
For alloy steels, uninhibited acid solutions are preferred because a more energetic chemical action is required to remove the oxides.
If the surfaces covered with scale and rust are generally and uniformly clean from additional contaminants and if the processing time is not critical, no other pretreatment is required.
Rusted or scaled surfaces submitted to cleaning that are contaminated by soils that do not react with acids, like grease, oils, soaps, lubricants and other coatings, must be precleaned by an additional process.
Usually alkaline cleaning is especially advantageous when acid immersion time is limited to a few seconds in a continuous working line.
Limitations of acid cleaning consist in the difficulty to handle the products because of their corrosiveness, and in the problem of hydrogen embrittlement that may significantly affect some alloy and high-carbon steels.
The hydrogen liberated from the acid reacts with the surface, makes it brittle and may cause cracks.
To assure desired scale removal rates without endangering these steels, acid concentrations and solution temperatures must be kept under stricter control.
Sludge is the waste product from this process, that includes acidic rinse waters, metallic salts and waste spent acid.
EPA considers all these as hazardous waste to be treated before being disposed of. Sludge from steel processes should be neutralized with limeTorrent crack for en route definition de. before being discharged in a land fill.
Those engaging in these processes should adopt responsible attitudes, be familiar with environmental protection requirements and make sure to meet them always in their small or big industrial activities.
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