Mechanism of NH4HS corrosion

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Mechanism of NH4HS corrosion

NH4HS corrosion is well known in the literature, but very little quantitative information is available. Corrosion rate data have been reported by Piehl (1976), Damin and McCoy (1978), Scherrer et al. (1980) and Foroulis (1993). No electrochemical data are available that would make it possible to determine the mechanism of corrosion in an unequivocal way. Therefore, we propose here a tentative mechanism of NH4HS corrosion that is capable of quantitatively reproducing the available corrosion rate data.


A characteristic feature of NH4HS corrosion of steels is a threshold concentration of NH4HS above which the corrosion rate rapidly increases. This indicates that a local accumulation of a corrosive agent is responsible for the rapid increase in corrosion rate with concentration. At the same time, it is known that Fe and Ni in contact with NH4HS solutions form FeS and NiS layers, respectively. This leads to a corrosion mechanism in which the cathodic and anodic areas may be spatially separated by the sulfide layer. Without the influence of NH4HS, the anodic reaction is:


Fe + xH2O → Fe(OH)x(2-x)+ + xH+ + 2e-    equation (1)


In the presence of a large concentration of HS- ions that may locally accumulate under the FeS layer, another reaction may contribute to the anodic process:


Fe + xHS- → Fe(HS)x(2-x)+ + 2e-    equation (2)


Local accumulation of HS- ions is made possible by their migration through a porous FeS layer. To satisfy the electroneutrality condition in the local anodic area, the NH4+ ions can also migrate. At the same time, HS- ions participate in the reduction process on the surface of the FeS scale, i.e.,


HS- → 0.5H2 + S2- - e-    equation (3)


The enhancement of the anodic process due to the presence of the FeS layer may also involve the migration of other active ions in addition to HS-. In particular, chloride and cyanide ions may accelerate the anodic dissolution, i.e.,


Fe + aH2O + bX- → Fe(OH)a Xbc+ + aH+ + 2e-    equation(4)


where X- stands for Cl- or CN-. Reactions such as eq. (4) are common in localized corrosion. They involve the complexation and hydrolysis of corrosion products and lead to a local acidification of the anodic area. Thus, they prevent the precipitation of corrosion products and enhance the corrosion rate.


The above mechanism indicates that the effect of NH4HS is mediated by the presence of an FeS layer on the surface of the metal. Therefore, all alloys that have a tendency to form FeS can, in principle, be subject to NH4HS corrosion. This conclusion is corroborated by the data of Damin and McCoy (1978), who showed that both carbon steel and type 316 carbon steel suffer catastrophic corrosion above a certain threshold concentration of NH4HS. Although the corrosion rates for 316 SS are lower, its behavior is qualitatively similar to that of carbon steel. Metals that do not form FeS layers should not suffer from the same kind of catastrophic corrosion. In fact, Al and Ti do not show the same behavior as carbon steel or 316 SS (Damin and McCoy, 1978). Titanium is particularly resistant. Incoloy 800 was also shown to be resistant under the conditions of Damin and McCoy’s (1978) experiments, but it may be, as an Fe and Ni-containing alloy, susceptible to corrosion at more severe conditions.


The effect of The effect of NH4HS concentration on the corrosion rates of carbon steel and type 316 stainless steel at 93 °C is shown in Figure 1. It is noteworthy that the rate increases very rapidly for concentrations above approximately 10-11 m. The experimental corrosion rate data for carbon steel show some scattering, which is fairly typical for measurements at such conditions. The calculated results are in agreement with the data of Damin and McCoy (1978) at higher concentrations. At low concentrations, the data of Scherrer et al. (1980) and Damin and McCoy (1978) are not in agreement with each other and the predictions are close to the data of Scherrer et al. (1980). In the case of the 316 stainless steel, the two sets of data are in agreement with each other and are accurately reproduced by the model. concentration on the corrosion rates of carbon steel and type 316 stainless steel at 93 °C is shown in Figure 1. It is noteworthy that the rate increases very rapidly for concentrations above approximately 10-11 m. The experimental corrosion rate data for carbon steel show some scattering, which is fairly typical for measurements at such conditions. The calculated results are in agreement with the data of Damin and McCoy (1978) at higher concentrations. At low concentrations, the data of Scherrer et al. (1980) and Damin and McCoy (1978) are not in agreement with each other and the predictions are close to the data of Scherrer et al. (1980). In the case of the 316 stainless steel, the two sets of data are in agreement with each other and are accurately reproduced by the model.


Fig1.png
Figure 1. Corrosion rates for carbon steel and type 316 stainless steel at 93 °C as a function of ammonium bisulfide concentration. The experimental data are from Damin and McCoy (1978) and Scherrer et al (1980).


It is noteworthy that the behavior of type 316 and 304 stainless steels is fairly similar. This is illustrated in Figure 2. This is understandable because the mechanism is essentially the same in both cases.


Fig2.png
Figure 2. Corrosion rates for type 304 and 316 stainless steel at 93 °C as a function of ammonium bisulfide concentration. The experimental data for type 316 are from Damin and McCoy (1978).


As mentioned above, NH4HS corrosion is enhanced by the presence of Cl- and CN- ions. This is illustrated in Figure 3 for carbon steel in moderately concentrated NH4HS solutions. The model calculations are in excellent agreement with the data of Foroulis (1993). As shown in Figure 3, the increase in corrosion rates due to the presence of active ions is very substantial. The effect illustrated in Figure 3 is fairly nonspecific. Other active species, such as Br- or SCN- ions, would cause a similar effect. No experimental data on the effect of Cl-, CN- or other active ions are available for stainless steels. The effects on stainless steels are qualitatively similar although they are smaller.


Fig3.png Figure 3. Effect of chloride and cyanide concentration on the corrosion rate in moderately concentrated NH4HS solutions. The data are from Foroulis (1993).


References

Damin, D. G., McCoy, J. D. Materials Performance, Dec. 1978, 23.
Foroulis, Z. A., Corrosion Prevention and Control, 40, no. 4 (1993).
Piehl, R. L., Materials Performance, Jan. 1976, 15.
Scherrer, C., Durrieu, M., Jarno, G., Materials Performance, Nov. 1980, 25.