Corrosion inhibitors for metals are classified into three types, adsorption, oxidation, and precipitation-type ones. Action of the adsorption-type inhibitors is reviewed here. Electrostatic and chemical adsorptions using a polar group of the inhibitor molecule on metals, inhibition effects associated with the linear free energy relationship and the hard and soft acids and bases principle, and action of a non-polar group are described.
Slow strain rate test (SSRT) method was applied to evaluate the SCC behavior of non-sensitized stainless steels in high temperature water. Crack growth rate (CGR) was evaluated by investigating the time dependence of the maximum crack depth in SSRT. The effects of materials and test conditions on the CGRs were studied. As the result, the CGR of non-sensitized 18Cr-8Ni stainless steels (SUS316, SUS304, SUS347) was in the range of (3−6)×10−10 m/s. In 316NG (nuclear grade), the CGR showed the highest at 561 K, and increased with the concentration of dissolved oxygen and conductivity. The CGR in SSRT was higher by 10 times than that obtained in the crack extension test using CT specimens.
The corrosion of silver when exposed to sulfur vapor was studied in a tubular vessel and a boxy vessel. The sulfur vapor released from sulfur flower reacted with the silver plate in the tubular vessel and the silver electrode of the corrosion sensor in the boxy vessel. The thickness of the corrosion product on silver plate was measured using a cathodic reduction method, and that on silver electrode was measured using a resistometry. After exposure, silver sulfide that had formed on the silver plate and silver electrode was analyzed. When sulfur vapor is released from sulfur flower, diffusion of the sulfur vapor is the main factor limiting the rate of silver corrosion. The silver corrosion rate when exposed to sulfur vapor released from sulfur flower was estimated using a diffusion network model that calculated the corrosion reaction. Sulfur vapor reacts with silver at a constant rate corresponding to a reaction probability of 3×10−5. This estimation technique is useful for verifying the effectiveness of countermeasures and can be used to ensure the equipment's reliability.
We examined the effects of temperature and Cr content on the active dissolution behavior of Fe-Cr alloys in a 500 mol·m−3(0.5 kmol·m−3) H2SO4 solution through immersion test and measurement of polarization curves. While the effect of temperature on the corrosion rates of Fe, Cr and Fe-Cr alloys can be approximated using the Arrhenius equation, the values of the apparent frequency factor A of Fe-Cr alloys in different temperature ranges were found to fall between the corresponding values of Fe and Cr, and the apparent activation energy E of the alloys, likewise, between the corresponding values of Fe and Cr. Roughly speaking, both A and E of the Fe-Cr alloys tended to decrease as Cr content or temperature increased. On the other hand, the effect of Cr content on the corrosion rates of Fe-Cr alloys was found to vary as Cr content changed, and the corrosion rates of the alloys drew closer to that of 100% Cr as the Cr content of the alloy increased. For example, the corrosion rate of the alloy increased exponentially as its Cr content increased from 10 to 50%, but the gradient of the curves, which represents the effects of Cr content on the corrosion rate, decreased as temperature increased. To enhance the efficiency of the pickling process in steel manufacturing, it is industrially very important to express the corrosion rate of Fe-Cr alloys in the form of a function of Cr content and temperature. We worked out such an experimental equation of the corrosion rate, and it proved to be effective in accurately estimating the corrosion rate of Fe-Cr alloys containing 10 to 50% Cr in an Ar-deaerated environment of a 500 mol·m−3(0.5 kmol·m−3) H2SO4 solution and in a temperature range of 298 to 353 K. We presume that the reason why the corrosion rate of Fe-Cr alloys increased as Cr content increased and Ecorr of the alloys became less noble is that, whereas the reaction rate of the active dissolution of a Fe-Cr alloy increases as the Cr content increases, the reaction rate of the reduction of H+ into H2 does not change significantly depending on Cr content.