Corrosion in Steels – Its Types and Testing...

Corrosion in Steels – Its Types and Testing Corrosion is a universal natural process. The effect of corrosion is seen in every-day life in the form of rusted steel parts. Corrosion has a huge economic impact. About a fifth of the global annual steel production goes towards simply replacing steel parts damaged by corrosion. Even though it involves higher up-front cost, correct and efficient corrosion protection at the source helps save money and resources in the long run. Failure due to corrosion can result into dramatic consequences. Corrosion is the gradual degradation of a metal by chemical, often electrochemical reaction with the surrounding environment. Corrosion results into loss of material properties such as mechanical strength, appearance, and impermeability to liquids and gases. Whether steel is corrosion resistant in a specific environment depends on the combination of the chemical composition of steel and the aggressiveness of the environment. As per ISO 8044:2010, corrosion is the physicochemical interaction between a metal and its environment, which results in changes in the metal’s properties and which may lead to significant functional impairment of the metal, the environment, or the technical system of which they form a part. Corrosion takes place when there is a change in the steel’s or system’s properties which may lead to an undesirable outcome. This can range simply from visual impairment to complete failure of technical systems which cause great economic damage and even present a hazard to the people. The typical corrosion process can be regarded as the thermodynamically favoured reverse reaction of the metal-winning (extraction) process (Fig 1). Like all chemical reactions, corrosion processes take place when conditions favour the related chemical reactions (thermodynamics). Then, potential other factors drive the speed of the reaction (kinetics). Fig 1  Chemical reactions of iron during...

Corrosion of Stainless Steels...

Corrosion of Stainless Steels Stainless steels (SS) are alloys of iron (Fe) which containing a minimum of 10.5 % chromium (Cr). With increasing content of Cr and with the presence or absence of many of other elements, SS can provide an extraordinary range of corrosion resistance. Different grades of SS are being used since several years in atmospheres which are mild (open air, in architectural applications) as well as extremely severe (chemical-processing industries). Stainless steels are classified in five families as per the crystal structures and the strengthening precipitates. Each family of SS shows its own general features in terms of mechanical properties and corrosion resistance. Within each family, there is a range of specifications which varies in composition, corrosion resistance, and cost. Stainless steels are vulnerable to several types of localized corrosive attack. The avoidance of such localized corrosion is the focus of most of the efforts made in the selection of SS. Also, the corrosion performance of SS is strongly influenced by practices of design, fabrication, surface conditioning, and maintenance. The selection of a grade of SS for a specific application involves the consideration of several factors, but the main factor remains corrosion resistance. It is the first necessity to specify the likely service environment. Besides considering the design conditions, it is also necessary to consider the reasonably anticipated exposures or upsets in service conditions. The suitability of a specific specification can be assessed from laboratory tests or from the documented field experience in similar atmospheres. Once the specification with satisfactory corrosion resistance has been identified, it is then appropriate to consider other factors such as mechanical properties, ease of fabrication, the types and degree of risk present in the application, the availability of the necessary product forms, and cost. Families of...