Chemical surface treatment
All finishing of stainless steel entails a risk of weakening the natural corrosion resistance of the steel. If this weakening is greater than what the chosen material can handle, the corrosion resistance needs to be re-established which is done in the safest (and cheapest!) way by applying the chemical surface treatment. Blueish, tarnished welding from a tank constructed by 4301. In these kinds of tarnishing the loss of corrosion resistance is critical and to re-establish the corrosion resistance, these groups of tarnishing need to be removed. This is done in the best and most cheap way by pickling.
Pickling of stainless steel
Pickling is by far the most important chemical surface treatment of stainless steel. The pickling bath itself typically contains 10-20% nitric acid (HNO3) and 1-8% hydrofluoric acid (HF). Highly alloyed steel requires an aggressive pickling bath with high contents of hydrofluoric acid while “normal” stainless steel of the 4301 and 4401 grades are most efficiently pickled with a relatively high content of HF. Hydrochloric acid (HCI) can, in theory, substitute hydrofluoric acid. However, this results in partly a more aggressive pickling (aqua regia!), and partly a higher risk of consequential damages in the form of pitting corrosion. Hydrochloric acid containing pickling cannot be recommended.
At room temperatures, the pickling time is usually between 30 minutes and several hours depending on the degree of the tempering which is to be removed (the darker, the more time) and the concentration of metal contaminations in the bath. After the pickling, you once again have a clean steel without any sorts of corrosion-based weakening.
The predominant disadvantage of pickling is the fact that we are dealing with an actual etching. This means that the surface roughness of the steel will increase especially when dealing with very smooth subjects, which can be extremely inconvenient in the medical and food industry. In these industries, the roughness is required to be held down for the sake of microorganisms. Often times, you operate with a maximal roughness (as Ra) of 0,6 µm.
Moreover, variations in the surface roughness can make the subject appear less homogenous which, though, is mostly of cosmetic importance.
Pickled welding seam in a subject of 4404. Note the fact that the steel gets a dull appearance. This is because pickling involves etching of the steel which increases the surface roughness.
The application of the pickling is, with the most ease, done by dipping the subject in the pickling bath. If this is not an option, you can apply a thickened pickling paste that can be “painted onto” the subject by use of a brush. This is extremely useful in e.g. field welding where it is not desired to pickle the entire plant but just the welding.
Note the fact that the lowest alloyed ferritic steel types (e.g. 4003 and 4512) are not suited for pickling. Neither sulphur alloyed machining steel nor martensitic knife steel can be pickled. The higher alloyed ferrites such as 4509 or 4521 can be pickled, however, you should apply a milder and less aggressive pickling compared to the corresponding austinites.
Further note that the use of hydrofluoric acid in Denmark requires a poison certificate (they are issued at police stations).
Pickled welding seam in a subject of 4404. Note the fact that the steel gets a dull appearance. This is because pickling involves etching of the steel which increases the surface roughness.
The application of the pickling is, with the most ease, done by dipping the subject in the pickling bath. If this is not an option, you can apply a thickened pickling paste that can be “painted onto” the subject by use of a brush. This is extremely useful in e.g. field welding where it is not desired to pickle the entire plant but just the welding.
Note the fact that the lowest alloyed ferritic steel types (e.g. 4003 and 4512) are not suited for pickling. Neither sulphur alloyed machining steel nor martensitic knife steel can be pickled. The higher alloyed ferrites such as 4509 or 4521 can be pickled, however, you should apply a milder and less aggressive pickling compared to the corresponding austinites.
Further note that the use of hydrofluoric acid in Denmark requires a poison certificate (they are issued at police stations).
Passivation of stainless steel
The purpose of passivation is to enhance the naturally protecting oxide film and as an additional advantage exposed inclusions and corrosion-based impairments are dissolved. Both things enhance the corrosion resistance of the steel.
The bath is often clean, consists of 18-25% nitric acid, and the processing time is normally an hour. Sulphur alloyed machining steel (4305) and lowly alloyed ferritic steel can only be passivated in dichromate inhibited nitric acid.
A great advantage of passivation is that the surface roughness is not influenced, and passivation is, therefore, brilliant for the treatment of e.g. polished or glass blown surfaces. A great disadvantage, however, is that tempering around the welding cannot be removed. If tempering is apparent, you do not get the liberty of avoiding either a pickling or a fine polishing followed by pickling or passivation.
Decontamination of stainless steel
“Decontamination” means de-poisoning– and that is exactly what happens. All impurities are removed while the actual steel is completely unaffected – neither tempering, the de-chromed layer or the free surfaces. Decontamination is a form of advanced cleaning and neither affects the surface roughness of the steel nor the most regular rubber and plastic materials, which makes the process especially suitable for cleaning of medical equipment or other things where pickling or passivation is not applicable either for the good of the actual steel or sensitive packaging. Most synthetic materials do not go well with nitric acid or other acid oxidants.
Decontamination is typically conducted with a 2-10% dissolution consisting of a semi-weak acid (phosphoric acid, citric acid, nitric acid, formic acid, oxalic acid, etc.). The temperature can be 20-90 °C and the processing time up to many hours if the demand is to remove something more recalcitrant.
Iron traces are a special problem that often can be handled with decontamination. Rust (ferric oxide and hydroxides) is only slowly soluble in nitric acid but decomposes far better in a warm mixture of citric acid and phosphoric acid. Metallic iron, however, is better decomposed in nitric acid.
Electropolishing of stainless steel
Electropolishing is the only surface process that requires an external power source. The bath is typically a strong mixture of sulphuric acid and phosphoric acid, the temperature is often above 50 °C, and the actual subject is anodically coupled by use of a rectifier.
During the process, a part of the steel is decomposed and since the dissolution primarily occurs on the top of the micro-roughness of the surface, the process will result in a slow levelling. Additionally, the surface roughness declines which, again, results in the surface becoming more shiny. Electropolishing can theoretically also dissolve tempering, however, that is far from always the case which is why it is recommended to conduct a pickling before an electropolishing.
Besides producing a beautiful and shiny surface, electropolishing will also increase the corrosion resistance of the steel. Actually, electropolishing is the only process, which is capable of enhancing the corrosion resistance remarkably compared to a normal 2b-surface. This is caused by the low surface roughness, which makes the surface very unsuitable for collecting chloride and other contaminations. Above the waterline, the fine roughness entails that salts and other corrosive substances are not easily stuck onto the steel and there are instances of electropolished 4301 performing just as well as 4401, 2b.
The disadvantage of electropolishing is especially the price. It is a complicated and expensive process partly due to the difficulty of installing the cathodes in the correct positions. Electropolishing is, therefore, a process that almost solely is applied to especially critical equipment in industries such as the medical industry. In that industry, it is desired to have a fine roughness for the sake of the steel being easy to clean as well as the minimal risk of microbial growth.
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