Materials

Reactive metals such as Zirconium are extremely noble because of their chemically resistant, passive films. Due to these protective films, reactive alloys are attacked by very few substances and are often applied in very corrosive environments. However, due to their high affinity for oxygen and other gases at elevated temperatures these alloys are considered difficult to weld. During welding, special precautions shall be taken to avoid contamination of the weld pool since only minor amounts of impurities cause these alloys to become brittle.

Commercially pure titanium grades feature an excellent strength-to-density ratio and good corrosion resistance. This makes them suitable for the manufacture of components in weight-saving structures with reduced mass forces, and also for components requiring high corrosion resistance. In addition, thermal stresses in titanium structures are lower than in other metallic materials, due to the low thermal expansion of titanium.

Nickel will alloy readily with many other metals, including chromium, iron, molybdenum and copper. This allows for a wide variety of alloys that demonstrate outstanding resistance to corrosion and high-temperature scaling, exceptional high-temperature strength and other unique properties, such as shape memory and low coefficient of expansion. The best-known is 800H (UNS N08810) and its variant 800HT (UNS N08811). Both are members of the group Ni – Cr – Fe alloys with excellent strength at high temperature and the ability to resist oxidation, carburisation and other types of high-temperature corrosion.

Nickel alloys have superior corrosion resistance properties compared to stainless steels. Nickel is an excellent base on which to design alloys because, nickel itself is moderately corrosion resistant, and it can be alloyed with many other elements while maintaining ductility. As a result, over time a wide range of nickel alloys has been developed for various applications requiring superior corrosion properties.

Heat resisting austenitic stainless steels are a special sub-group within the austenitic stainless steels. Their chemical composition has been modified to exhibit better strength and oxidation resistance above 500°C compared to “normal” austenitic stainless steels.

Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel Types 304 and 316. The pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N ranges from 28-38 for duplex and ranges from 38-45 for super duplex. Hyper duplex has a PREN equal to 49.

Stainless steels are a special class of steel alloys known primarily for their corrosion resistant properties. The stainless characteristics associated with these alloys are achieved through the formation of an invisible and adherent chromium-rich oxide surface film that, when damaged, has the unusual ability to heal itself in the presence of oxygen. In order for an alloy to be classified as a stainless steel it must contain at least 11.5 wt% Cr, with at least 12 wt% Cr required for aqueous corrosion resistance. At 12 wt% Cr, they become passive in aqueous solutions. There are five main classes of stainless steels, designated in accord with their crystallographic structure. Each class consists of several alloys of somewhat differing composition having related physical, magnetic, and corrosion properties.