Duplex stainless steel 2205- Local corrosion resistance
Sea Water operations need construction material to avoid corrosion due to hard conditions but it should be economical to buy. The alloys that have 6% molybdenum and super austenitic stainless steel are affordable but these couldn’t work in many of the corrosive conditions. Therefore, a Duplex stainless steel 2205 with excellent corrosion resistance as of UNS N10276 and economical as 6% molybdenum stainless steel alloy is developed that can meet the requirements of seawater industries. A new super austenitic stainless steel is developed that has improved corrosion resistance properties to natural and local corrosion conditions. Its corrosion rate in comparison to other alloys is described in this article.
The new alloys distributed on the commercial scale has a common base that by changing the constituent % , corrosion resistance and mechanical properties can be changed. For an inclusion of element to get suitable increase in the particular property, a reaction occurs. For example, different corrosion resistance alloys need very high annealing temperature above 1200oC and water cooling to prevent secondary phase occurrence whilst other phases crack when cold processed.
Local corrosion resistance is very important for materials that are subjected in the marine operations. Crevice corrosion occurs due to design issue or contamination that occurs on the weakest side of design. However some alloys such as UNS N06686 offer high corrosion resistance feature in the seawater conditions, these are costlier than stainless steel 316/316L and 317 by four times. On the other side, steel S31600 has poor corrosion resistance as compare to nickel-chromium-molybdenum alloys. Super austenitic Duplex stainless steel 2205 sheet has tried to bridge the gap between 300 steel and Ni-Cr-Mo alloy however corrosion resistance is not sufficient.
Final target of new alloy development is to produce an alloy that outstanding corrosion resistance properties as of UNS 276, however doesn’t increase the sigma temperature more than UNS N08926. The disadvantage of addition in alloy is unwanted production of secondary phase while formation like sigma phase. The production of this phase should be strictly avoided to control cracking of alloy while hot processing. Sigma phase in the final product, due to inadequate annealing, bad quenching or long time exposure at high temperatures will seriously inhibit corrosion resistance and mechanical features of alloy.
Sigma phase includes molybdenum and chromium enriched phase that produces while grain development and finally in the entire microstructure. The alloys that get sigma production, the phase produces while welding, stress relieving or fabrication. When it is produced, it is disastrous for mechanical and corrosion resistance properties. It should be solution processed after annealing.
Therefore the solution temperature of Duplex stainless steel 2205 should be kept lower for convenient processing and particularly after annealing. Various alloy constituents have various influences on the sigma solvus temperature but while designing of alloy, care is taken not just to decrease the processing complications but also increase the corrosion resistance properties. In order to increase the corrosion resistance properties, pitting resistance equivalent number is increased while secondary phase temperature is decreased. PREN is a whole number that compares the pitting resistance offered by different alloy’s constituents.