Intergranular corrosion of 0MO28Ni65Fe5(hastelloyb) alloy

Although Hastelloy B alloy has good solid solution corrosion resistance, especially hydrochloric acid, sulfuric acid, etc., once it is welded and used in hydrochloric acid or sulfuric acid, the edge corrosion occurs at the weld, and the intergranular region appears in the heat affected zone. corrosion. Table 7-27 is the results of some tests. As can be seen from the figure, the Hastelloy B alloy has two sensitizing regions: a high temperature sensitizing zone of 1200 to 1300 ° C and a medium temperature sensitizing zone of 600 to 900 ° C. When passing through two sensitizing zones, the Hastelloy B alloy produces intergranular corrosion, which not only causes a significant decrease in corrosion resistance, but also increases hardness. The intergranular corrosion of Hastelloy B alloy is different from that of ordinary austenitic stainless steel, and it cannot be prevented even if it is solution treated and rapidly cooled.

Table 7-27 Relationship between intergranular corrosion of Hastelloy B alloy and cooling rate after solution treatment

Studies have shown that the intergranular corrosion of Hastelloy B alloy has a close relationship with the composition and structure and structure of the precipitated phase in the alloy. In the high temperature region of 1250 °C, the precipitates in the alloy have M6C, M2C and other carbides with high Mo content and o phase; in the intermediate temperature range of 550~900 °C, there is Ni-Mo intermetallic phase (higher than 850). °C is Ni2Mo type, lower temperature is Ni4Mo type) and carbides such as M6C and M2C. These intermetallic phases and carbides all contain higher Mo, which are severely depleted along the grain boundaries, resulting in intergranular corrosion of the Hastelloy B alloy. At the same time, the strength and hardness of the alloy are improved due to the strengthening effect.

In order to solve the intergranular corrosion problem of Hastelloy B alloy, the Corronel-220 alloy containing V was developed in 1959, which was not popularized because it reduced the corrosion resistance of Ni-M0 alloy in HCI acid. Later, due to the development of smelting technology, Hastelloy B-2 alloy with low C, low Si and low Fe was developed, which made the problem of resistance to intergranular corrosion more perfect. However, the moderate temperature aging brittleness of B-2 alloy led to the birth of hastelloy 3 alloy and hastelloy 4 alloy.