CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
and at temperature around 1150 °C [33]. These materials can be readily welded and exhibit good
toughness, however in weld metal and heat affected zones, cracking can be observed [34].
2. 7 Chemical properties of austenitic stainless steel
The major chemical properties that affect austenitic stainless steel are high temperature
oxidation, and corrosion resistance. In general austenitic stainless steel has high oxidation
resistance up to high temperature and other forms of high temperature corrosion [34]. Oxidation
resistance of austenitic steel depends upon oxygen partial pressure, temperature, time and type of
service, surrounding atmosphere, and the selection of material for specific application etc.
Austenitic stainless steels are largely known for their corrosion resistance in aqueous media
[17,18]. Always the material selection process for service in corrosive environment begins with
type 304L SS or its variants (Fig. 2.4). The useful corrosion resistance of austenitic stainless steel
is to the fact that it exhibits passivity in wide range of environments. The passive film is thin,
adherent, self-healing, and tenacious having thickness around 10-100 Å. The structurecomposition
of this ultrathin passive film is a vast and complex subject, and depends upon the
alloy composition, and also on the nature of environment they are exposed. Based on the research
over the years, passivity is believed to occur due to the formation of oxide layer or a process that
can be considered as adsorption of oxygen as summarized below.
2.7.1 Oxide film theory
Oxide layer formation is one of the important hypothesis to account for the cause of
passivity of metals, and alloys including different types of stainless steel. The theory is usually
ascribed to Micheal Farady on the study of passivation of iron in nitric acid [17]. Later on, the
protective oxide film theory gained support from the studies carried out by many researchers on
different material and in different environment. In the year 1934, Tronstad and Borgmann showed
that some kind of film exists on 18-8 stainless steel, and assumed that it must be an oxide layer