Scratching Stainless Steel

Hot-rolled or cold-rolled steel is the least sensitive to surface marks. Prepainted, zinc-coated, and uncoated metal with a dull surface are somewhat more sensitive to surface scratches. Stainless steel, aluminum and brass with high-luster finishes are the most sensitive materials to be roll formed. Rollforming of sensitive surfaces occasionally needs unusual tool design, tool material, and good lubrication. 

Figure 53.3 illustrates a pit in a stainless steel such as type 534 or 316 austenitic alloy. Pitting starts at heterogeneity in the steel surface, such as an outcropping sulfide inclusion, the shielded region beneath a deposit or even a discontinuity in the naturally present oxide film caused by a scratch or embedded particle of abrasive grit. This initiation phase of pitting corrosion may take seconds... 

Items of plant fabricated from stainless steels should be inspected before first use and after any maintenance work or unplanned shutdown. All materials that rely for their corrosion resistance on the presence of an oxide or similar passive layer are susceptible to localized attack where that layer is absent or damaged. Damage is most commonly caused by scratching, metallic contamination (nearby grinding or touching with ferrous tools), embedding of grit and weld spatter. 

Potential-time relationships have been widely used for studying film formation and film breakdown, as indicated by an increase or decrease in the corrosion potential, respectively. May studied the corrosion of 70/30 brass and aluminium brass in sea-water and showed how scratching the surface resulted in a sudden fall in potential to a more negative value followed by a rapid rise due to re-formation of the film conversely, the pitting of stainless steel in chemical plant may be detected by a sudden decrease in potential... 

Stainless-steel ware. Beakers, crucibles, dishes, funnels, etc., of stainless steel are available commercially and have obvious uses in the laboratory. They will not rust, are tough, strong, and highly resistant to denting and scratching. 

Materials compatibility. We use scrupulously clean and scratch-free FEP (FEP is the acronym for the copolymer of tetrafluoroethylene and hexafluoropropylene) tubing for handling our mixtures of fluorine and nitrogen at ambient temperature. Corrugated FEP tubing is convenient for making strain-free assemblies. We have found that Monel is excellent for use with dry molten KF-2HF. Mild steel corrodes slowly and stainless steels corrode rapidly. Kel-F polychlorotrifluoroethylene is satisfactory for use with HF and with KF-2HF polypropylene and polymethylpentene are not satisfactory. 

Bonilla et al. (1965) studied the effect of parallel scratches on a polished stainless steel plate when boiling mercury with a small portion of sodium as wetting agent. The mirror-finished stainless steel plate was scored by a tempered steel nee-... 

High vacuum components are made of aluminum or stainless steel. Stainless steel is slightly more expensive but offers a variety of advantages lower degassing rate, corrosion resistant, can be degassed at temperatures up to 200 °C, metal seals are possible and stainless steel is much more resistant to scratching compared to aluminum. 

A minimum chromium concentration of approximately 11% is typical for stainless steel. As more chromium is added, corrosion resistance improves. Concentrations of chromium >20% are found in some alloys. Chromium addition leads to the formation of a tight-forming oxide film on the surface of the metal. This stable film is self-healing, which means that the film will reform if scratched or broken. This oxide is quite resistant to attack by acids, bases, organic compounds, and inorganic salts. 

Stainless steels exhibit the best resistance to corrosion when the surface is oxidized to a passive state. This condition can be obtained, at least temporarily, by a so-called passivation operation in which the surface is treated with nitric acid and then rinsed with water. Localized corrosion can occur at places where foreign material collects, such as in scratches, crevices, or comers. Consequently, mars or scratches should be avoided, and the equipment design should specify a minimum of sharp comers, seams, and joints. Stainless steels show great susceptibility to stress corrosion cracking. As one example, stress plus contact with small concentrations of halides can result in failure of the metal wall. 

The choice of materials available for long-term drug contact is very limited—borosilicate glass, bromobutyl or chlorobutyl rubber closures, certain inert polymers such as polytetrafluoroethylene (PTFE), tetrafluor-oethylene (TFE), and derivatives thereof, and stainless steels. Since the Code of Federal Regulations mandates that it be possible to inspect the drug product after filling, steel cannot be used as the primary drug container. In addition, polycarbonates, which would be ideal materials from durability, scratch-resistance, and cost perspectives, have very poor moisture vapor and extractables profiles. 

Stainless steels are examples of alloy steels, i.e. ones that contain a J-block metal in addition to carbon. Stainless steels have a significant content of the alloy metal and are of high commercial value because of their high resistance to corrosion. All contain a minimum of 10.5% (by mass) of chromium and the resistance to corrosion arises from the formation of a thin layer of Cr203 ( 13 000 pm thick) over the surface of the steel. The oxide layer passivates (see Section 9.4) the steel and is self-repairing, i.e. if some of the oxide coating is scratched off, further oxidation of the chromium in the steel necessarily repairs the wound . A further property that makes stainless steels commercially important is that they can be polished to satin or mirror finishes and this is easily appreciated in the ranges of stainless steel cutlery available to the consumer. 

The effects of scratching tools were studied on three types of tools, 1.6-nim bcills from stainless steel and timgsten carbide, 10-micron sharp diamond stylus, and a novel tungsten carbide micro-blade, which is as sharp as the stylus (10 micron) but veiy wide (0.8 mm) [3]. 

Stainless steel is iron containing a high proportion of chromium, which forms a protective layer of CrjOj over the whole surface of the metal. If the surface is scratched, a new protective layer of CrjOj is formed,... 

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