The Properties of Stainless Steel

Oxidation

High oxidation resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% (by weight) chromium, and up to 26% is used for harsh environment. The chromium forms a passivation layer of chromium oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, and the metal remains lustrous and smooth. The layer is impervious to water and air, protecting the metal beneath, and this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. Corrosion resistance can be adversely affected if the component is used in a non-oxygenated environment, a typical example being underwater keel bolts buried in timber.

When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off, allowing the parts to weld together. When forcibly disassembled, the welded material may be torn and pitted, an effect known as galling. This destructive galling can be avoided by the use of dissimilar materials for the parts forced together, for example bronze and stainless steel, or even different types of stainless steels (martensitic against austenitic). However, two different alloys electrically connected in a humid environment may act as a voltaic pile and corrode faster. Nitronic alloys made by selective alloying with manganese and nitrogen may have a reduced tendency to gall. Additionally, threaded joints may be lubricated to prevent galling. Low-temperature carburizing is another option that virtually eliminates galling and allows the use of similar materials without the risk of corrosion and the need for lubrication.

Acids

Stainless steel is generally highly resistant to attack from acids, but this quality depends on the kind and concentration of the acid, the surrounding temperature, and the type of steel. Type 904 is resistant to sulfuric acid at room temperature, even in high concentrations; type 316 and 317 are resistant below 10%, and 304 should not be used in the presence of sulfuric acid at any concentration. All types of stainless steel resist attack from phosphoric acid, 316 and 317 more so than 304; types 304L and 430 have been successfully used with nitric acid. Hydrochloric acid will damage any kind of stainless steel, and should be avoided.

Bases

The 300 series of stainless steel grades is unaffected by any of the weak bases such as ammonium hydroxide, even in high concentrations and at high temperatures. The same grades of stainless exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some etching and cracking, especially with solutions containing chlorides such as sodium hypochlorite.

Organics

Types 316 and 317 are both useful for storing and handling acetic acid,especially in solutions where it is combined with formic acid and when aeration is not present (oxygen helps protect stainless steel under such conditions), though 317 provides the greatest level of resistance to corrosion.Type 304 is also commonly used with formic acid though it will tend to discolor the solution.All grades resist damage from aldehydes and amines,though in the latter case grade 316 is preferable to 304; cellulose acetate will damage 304 unless the temperature is kept low.Fats and fatty acids only affect grade 304 at temperatures above 150℃ (302 °F), and grade 316 above 260 ℃(500°F), while 317 is unaffected at all temperatures. Type 316L is required for processing of urea.

Electricity and magnetism

Like steel, stainless steel is a relatively poor conductor of electricity, with lower electrical conductivity than copper. Other metals in contact with stainless steel in a damp environment may suffer galvanic corrosion even though the stainless metal may be unaffected.

Ferritic and martensitic stainless steels are magnetic. Annealed austenitic stainless steels are non-magnetic. Work hardening can make austenitic stainless steels slightly magnetic.

3D printing

Some 3D printing providers have developed proprietary stainless steel sintering blends for use in rapid prototyping. One of the more popular stainless steel grades used in 3D printing is 316L stainless steel. Due to the high temperature gradient and fast rate of solidification, stainless steel manufactured via 3D printing tends to have more refined microstructures. This in turn results in better mechanical properties; however, stainless steel is not used as much as materials like Ti6Al4V in the 3D printing industry, this is because stainless steel manufactured via the traditional methods tends to be more economically competitive.