Immersion tests of 0.004% C steel were carried out at 460-540°C. Vacuum melted specimens in the form of 3×40×40mm were abraded, degreased in acetone and dipped into liquid zinc held in a magnesia crucible placed in an electric furnace. After soaked for 24hr, specimens were pickled in 10% HCl solution and weighed to measure iron loss. Metallographic examinations were also carried out. Iron loss increases with increasing temperature and has a maximum at about 500°C. In accordance with the equilibrium diagram alloy layers, consisting of Γ, δ_1C, δ_1P and ζ layer, are formed during the reactions between iron and molten zinc. Compact and well-adhering alloy layers protect iron against the attack by the melt at 460°C. At 500°C ζ layer disappears. δ_1P layer is completely broken up and loses its protective property. Alloy layers are stable again at 540°C. Scanning electron micrographic studies revealed that a compact δ_1P layer formed at 460°C is destroyed at grain-boundaries on etching in a nital solution. From this observation it can be concluded that δ_1P layer is fragile at grain-boundaries, and breaks up readily, when it reacts with molten zinc. Since a continuous layer ζ prevents it from direct contact with zinc, it remains compact at 460°C. At 500°C due to the absence of ζ layer it is attacked and disrupted into fragments at 500°C, which makes it possible for liquid zinc to penetrate through it. As a result enhanced corrosion occurs. This break-up is caused not only by the chemical reactions with the melt, but also by the shear stresses at grain-boundaries arising from the formation of δ_1P crystals from δ_1C crystals. These stresses decrease with increasing temperature to give stable alloy layers at 540°C. Since δ_1P crystals are built up only at δ_1P/δ_1C phase boundaries, expansion forces in a direction parallel with the surface develope in alloy layers at the corners of a specimen, as alloy crystals grow outwards. These expansion forces fracture δ_1P layer to bring about intensified attack at the corners.

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Immersion tests of 0.004-0.20% C steels containing less than 0.002% P and 0.18-0.20% C steels alloyed with 0.002-0.031% P were carried out at 460-540°C in molten zinc. Vacuum melted specimens were used to avoid the influence of other elements, for example, silicon. After dipped in liquid zinc for a given time, specimens were pickled in 10%HCl and weighed to measure iron loss. Addition of carbon up to 0.20% lowered the height of the maximum peak at 500°C, which appears generally in the curve expressing the relationship between iron loss and temperature. The reaction rate at 500°C followed a linear law for 0.004% C steel, but was almost parabolic for 0.20% C steel. The δ_1P layer of the former was completely broken up at grain boundaries, which is responsible for the linear attack. On the contrary that of the latter was stable and possessed protective property. According to scanning electron micrographic examinations the δ_1P layer formed on 0.20% C steel at 460°C is hardly destroyed on etching in a nital solution. Carbon was found to be distributed in alloy layers as mixed carbide, Fe₃ZnC_x. In view of the fact that the break-up is caused by both the chemical reaction with molten zinc and the shear stresses existing at grain-boundaries, it can be concluded from above mentioned observations that precipitated carbides improve the grain-boundary strength to prevent fracture. Since a continuous ζ layer protects δ_1P layer, carbon has no effect at 460°C. At 500°C due to the absence of ζ layer δ_1P layer reacts with molten zinc. 0.004% C steel is attacked violently, whereas 0.20% C steel shows good corrosion resistance. Phosphorus added to 0.18-0.20% C steels accelerated corrosion at 500°C. In other words a high concentration of phosphorus, which makes grain-boundaries fragile, cancels the effect of carbon.

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Pitting behavior of austenitic stainless steels at elevated temperatures has been studied in 0.1M NaCl and mixtures of 0.1M NaCl+0.1M NaNO₃ over the temperature range of 25-300°C in an autoclave. Pitting potentials were determined by the potentiodynnamic method at scanning rates of 25 and 50mV/min under the deaerated condition. The pitting potentials were compared with corrosion potentials under the air-saturated condition, under which pitting corrosion could be initiated spontaneously. With increasing temperature, pitting potentials (V_c′) decreased to less noble values up to 150-200°C for SUS 304, and 200-250°C for SUS 316 and 321. But, above the temperatures the trend was reverse and V_c′ moved to higher potential values very close to the trans-passive potential. Normally, pits were observed on different austenitic stainless steel SUS 304, 316 and 321 exposed under the air-saturated condition at 260°C, however, at 160°C pitting occurred only on the SUS 304 specimens. The attack produced by the anodic polarization showed the characteristic deep pitting at lower temperatures (up to 200°C), but the attack took the form of more uniform nature at higher temperatures. At 260°C, an addition of sodium nitrates to the sodium chloride solution shifts the pitting potentials to more noble values. Thus, nitrates act as a pitting corrosion inhibitor at high temperature as well as ambient temperature.

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