|Standard Duplexes - Corrosion Resistance|
The duplex stainless steels have general corrosion resistance ranging from similar to 304L to being superior to the 316L types, and this is dependant on the corrosion media. For example, 2001 has significantly better corrosion resistance than 316L-1.4404 in sulphuric acid (H2SO4) solutions.
Pitting resistance is important, mainly in applications involving contact with chloride solutions, particularly in the presence of oxidising media. These conditions may be conducive to localised penetration of the passive surface film on the steel and a single deep pit may well be more damaging than a much greater number of relatively shallow pits.
Where pitting corrosion is anticipated, steels with high pitting resistance equivalents (PRE), such as the duplexes, should be considered. The below diagram shows the critical temperature for initiation of pitting (CPT) at different chloride contents for 304, U-316L and 2205 types (potentiostatic determination at + 300mV SCE, pH = 6.0).
The atmospheric corrosion resistance of duplex stainless steels is unequalled by virtually all other uncoated engineering materials. 2205 should be used in areas where the atmosphere is highly polluted with chlorides, sulphur compounds and solids, either singly or in combination.
The duplexes have good oxidation resistance, both in intermittent and continuous service, up to 980°C for 2205. However, continuous use of the duplexes between 300°C and 950°C may embrittle the steel and lower the corrosion resistance. At the lower temperature range, the embrittlement is due to the precipitation of α’ (475°C embrittlement) and nitrides or carbides. In the high temperature range, χ and σ phases precipitate. However, during normal production and fabrication procedures, the times at these critical temperatures are such that the risk of embrittlement and/or a decrease in corrosion resistance are small.
In addition, this effect does not necessarily affect the behaviour of the material at the operating temperature and is less pronounced in thinner gauges. For example, heat exchanger tubes are used at high temperatures without any problems. A full anneal and rapid cooling treatment will restore the toughness and corrosion resistance of the duplexes.
Sensitisation may occur when the Heat Affected Zones of welds in some austenitic stainless steels are cooled through the sensitising temperature range of between 450°C and 850°C. At this temperature, a compositional change may occur at the grain boundaries.
If a sensitised material is then subjected to a corrosive environment, intergranular attack may be experienced. This corrosion takes place preferentially in the heat affected zone away from and parallel to the weld. Susceptibility to this form of attack, often termed ‘weld decay’, may be assessed by the following standard tests:
a) boiling copper sulphate/sulphuric acid test as specified in ASTM A262, Practice E.
b) for non titanium stabilised grades only, boiling nitric acid test as specified in ASTM A262, Practice C.
The Columbus Stainless Cr-Ni-Mo austenitics have low carbon contents and are resistant to sensitisation and can be specified for welded structures unless the higher carbon types are required for their increased strength at elevated temperatures. In this case, 316Ti should be specified.
STRESS CORROSION CRACKING
Stress corrosion cracking (SCC) can occur in austenitic stainless steels when they are stressed in tension in chloride environments at temperatures in excess of about 60°C. The stress may be applied, as in a pressure system or it may be residual arising from cold working operations or welding. Additionally, the chloride ion concentration need not be very high initially, if locations exist in which concentrations of salt can accumulate.
Assessment of these parameters and accurate prediction of the probability of SCC occurring in service is therefore difficult. Where there is a likelihood of SCC occurring, a beneficial increase in life can be easily obtained by a reduction in operating stress and temperature. Alternatively, specially designed alloys, such as duplex stainless steels, will have to be used where SCC cracking is likely to occur.
Austenitic stainless steels are attacked by erosion corrosion if exposed to flowing media containing highly abrasive solid particles, e.g. sand, or to media with very high flow velocities. Owing to its combination of high initial hardness, work hardenability and corrosion resistance, the duplexes displays very good resistance under such erosion corrosion conditions.
The duplexes possesses higher strength and better corrosion resistance than ordinary austenitic stainless steels. The duplexes, therefore, also possess better fatigue strength under corrosive conditions than such steels. For example, in rotary bending fatigue tests in a 3% NaCl solution (6 000rpm, 40°C, pH 7), 2205 required 430MPa stress in the unnotched condition to bring about rupture after 2x107 cycles, while 316N failed at only 260MPa. The corresponding notched figures were 230MPa and 140MPa for 2205 and 316N respectively.