Corrosion can be defined as the attack of a metallic material by its environment. Stainless steels all possess a high resistance to corrosion. This resistance is conferred by the naturally occurring chromium - rich oxide film which is always present on the surface of stainless steel. Although less than 130 Angstrom thick (1 Angstrom unit = 10-8 cm) this invisible film is extremely protective as it is inert and adheres tightly to the metal. The oxide film has the unique property of self-repair which is unattainable in applied films. This means that if the film is removed or damaged or a new metal surface is created by cutting then in the atmosphere or other source of oxygen the protection will be instantaneously re-established. The more highly alloyed grades of stainless steel possess the best corrosion resistance and are able to withstand more aggressive environments.
Selection of the correct grade of stainless steel is the key to avoidance of corrosion problems.
Corrosion takes many different forms. Its initiation and subsequent rate of progress is affected in varying degrees by numerous material and environmental factors. A comprehensive assessment of the exact 'corrosion resistance' of a material is therefore difficult. However, corrosion tables covering a vast range of stainless steels and environments are available.
To achieve optimum corrosion performance care must betaken at the design stage. In particular design should employ smooth contours and radiused corners whilst avoiding sharp edges and crevices. Design should also promote material flow and mixing to avoid localised concentrations and/or stagnant conditions. Other considerations are ease of cleaning and maintenance as well as avoidance of dissimilar metal contact.
Oxidation is the combination of a metal with oxygen to form the metal oxide which occurs in dry conditions. When this process is ongoing the whole of the metal may be converted. Stainless steels are oxidation resistant, even at elevated temperatures. Special heat resisting grades, such as type 310, are operated at temperatures of up to 1100°C.
Pickling and Passivation
To achieve optimum corrosion performance stainless steel must go into service fully passivated. Its protective surface oxide layer must be intact and uncontaminated. In particular weld tint and any fabrication debris must be removed. It is particularly important to remove any Carbon or mild steel contamination. This is done using proprietary solutions, chemical and pastes based on Nitric Acid or a mixture of Nitric and Hydrofluoric Acids.
Forms of Corrosion
Forms of Corrosion
There are many different forms of corrosion.
General Corrosion is the uniform overall attack of a component across its whole surface. It is avoided by correct grade selection.
Other types of corrosion are explained on page 2.