The two main grades of stainless steel used in the industry are 304 and 316 (see Table 2 for equivalent designations)

Generally speaking 304 grade is used for product contact of dry components and 316 is used where liquid contact is involved.

Grade 316 has virtually the same mechanical, physical and fabrication characteristics as 304 with better corrosion resistance, particularly to pitting corrosion in chloride environments. If an application requires greater corrosion resistance than grade 304 can provide, grade 316 is the 'next step up'.

Applications

Outside the industry, typical applications for 316 include boat fittings and structural members; architectural components particularly in marine, polluted or industrial environments; food and beverage processing equipment; hot water systems; and plant for chemical, petrochemical, mineral processing, photographic and other industries.

Corrosion Resistance

Grade 304 has excellent corrosion resistance in a wide range of media. It resists ordinary rusting in most packaging applications. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

In warm chloride environments, 304 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50°C. However, it can be successful in warm chloride environments where exposure is intermittent and cleaning is a regular event.

Grade 316 also has excellent corrosion resistance in a wide range of media. Its main advantage over grade 304 is its increased ability to resist pitting and crevice corrosion in warm chloride environments. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

Neither 304 0r 316 should be cleaned with chlorine containing compounds.

Magnetism

Like other austenitic grades*, 316 in the annealed condition is virtually non-magnetic (ie. very low magnetic permeability) whereas 304 can have magnetic attraction after being cold worked. This may be a reason for selecting grade 316 in some applications.

Austenitic

A family of alloys containing chromium and nickel (and manganese and nitrogen when nickel levels are reduced), generally built around the type 302 chemistry of 18% Cr, 8% Ni, and balance mostly Fe. These alloys are not hardenable by heat treatment.

Pickling

Stainless steel can corrode in service if there is contamination on the surface or if there has been a process that has altered the composition of the surface, usually welding or some other heating process. Unless a fabricator is meticulous with his/her handling of stainless steel, pickling should always be carried out after fabrication for maximum corrosion resistance.

Pickling or acid cleaning restores the corrosion resistance of the surface by removing any surface contamination such as grease or dirt, as well as any embedded iron or exposed inclusion particles in the steel. The most common solutions contain nitric acid, although others are available. Some of the surface contaminants may be removed by grinding or polishing, but sometimes particles of polishing compounds can be embedded in the surface, and hence damage corrosion resistance.

Where the steel has been heated, by welding or any other means, to the point where a coloured oxide layer can be seen, there is a chromium depleted layer on the surface of the steel underneath the oxide layer. The lower chromium content gives lower corrosion resistance. The layer can be ground off, but it is safer to pickle it off. Hydrofluoric acid must be included in the pickling solution, since nitric acid alone will not dissolve the surface layer. Pickling is best done by immersion in the pickling solution or by using pastes where the solution is mixed with an inert carrier to allow selected areas to be treated.

Passivation

Passivation is the regeneration of the passive film that protects stainless steel. It is not an essential step of the cleaning process since the passive film will form spontaneously over time after the part leaves the pickling tank. It has been reported that, without any special action, it starts to form instantly but takes about 24 hours to achieve its maximum effectiveness on a dry component in air. Passivating solutions aim to accelerate this action so that it will occur before the part leaves the passivating tank. Most passivating solutions are based on nitric acid, which is highly oxidising and instantaneously restores the passive layer. Most pickling solutions and pastes are formulated to contain both hydrofluoric and nitric acids, and where they do contain nitric acid it is not essential to have a separate passivation step.

Both pickling and passivation solutions employ dangerous acids that can damage both the operator and the environment if not handled correctly. Stainless pickling acids are highly corrosive to carbon steel. It is also essential that all acids are thoroughly removed from the component after completing the process - residual hydrofluoric acid will initiate pitting corrosion.

Further sources of information on both pickling and passivation can be obtained by contacting ASSDA.

Condensed from: http://www.assda.asn.au/