Corrosion and Electrochemical Corrosion Protection
2.3 Potential Dependence of Corrosion Extent
2.3.3 Stress Corrosion
Fig. 2-17 Relation between the time to failure by intergranular stress corrosion cracking and potential for tensile specimens of soft iron: (a) boiling 55% Ca(NO3)2
solution, § = 0.65 Rm o = 0.90 Rm; (b) 33% NaOH, a = 300 N mnr2, at various temperatures.
Nitrate ions have a special influence by inhibiting pitting corrosion in neutral and acid waters at U > Us [Eq. (2-50)] [48,52]. Us corresponds to a second pitting potential and is designated the inhibition potential. The system belongs to group IV, with pitting corrosion at U < U& and transpassive corrosion at U > U".
The system stainless steel/chloride-containing acid media belongs to group II.
Active corrosion occurs at U < U'& and pitting corrosion at U > Upc. In acid media containing chloride and nitrate ions, the following states can exist with increasing potential: cathodic protection - active corrosion - passivation - pitting corrosion - passivation - transpassive corrosion. The different types of corrosion are depen- dent on the potential, with the dependence varying greatly from type to type, which in a particular case can only be learned from chronopotentiostatic experiments.
Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between UH = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10, 40-42].
or through the grains (transgranular). Lifetime-potential curves are used to assess corrosion damage or the applicability of corrosion protection methods. Figure 2-17 shows two examples for soft iron in nitrate solution [65] in (a), and in caustic soda [66] in (b). In both cases the specimens were cylindrical tensile specimens which were subjected to a constant load. There is a critical tensile stress below which stress corrosion does not occur; this can depend on the potential [67],
Stress corrosion systems can be divided into two categories depending on whether crack initiation occurs with constant static loading or whether it occurs only when a critical strain rate is exceeded in dynamic testing. These two types are known as "stress-induced" or "strain-induced" stress corrosion [68-70]. This differ- entiation is important in assessing the effects of damage and in experimental in- vestigation. The critical potential ranges can thus depend on the strain rate.
Systems with lifetime-potential curves of type (a) in Fig. 2-17 can be cathodi- cally protected against stress corrosion. The following metals belong to these systems:
(a) Plain carbon and low-alloy steels in nitrate solutions, particularly at ele- vated temperature. There are critical tensile stresses. The susceptibility range is widened under dynamic loading [71]. The systems belong to group I.
(b) Austenitic manganese steels in seawater [72,73]. The systems belong to group I.
(c) Austenitic stainless steels in Cl~ containing waters at elevated temperature [67,74,75]. The systems belong in general to group I, but at high tensile stresses and more negative potentials a new range of susceptibility (for stress corrosion cracking occurs) occurs (see j).
(d) Sensitized stainless steels in hot waters with a tendency to intergranular corrosion [76]. The systems belong to group I in neutral waters and to group li in acid waters.
Systems with lifetime-potential curves like type (b) in Fig. 2-17 can be protected anodically as well as cathodically against stress corrosion. The following metals belong to these systems:
(e) Plain carbon or low-alloy steels in caustic soda at elevated temperatures [66,77-80]. The systems belong to groups I or IV.
(f) Plain carbon or low-alloy steels in NaHCO3 solutions [77,78,81]. The sys- tems belong to groups I or IV.
(g) Plain carbon or low-alloy steels in NaHCO3 solutions in general with a critical strain rate [77,78,80,82-84]. The systems belong to group I or IV.
(h) Plain carbon or low-alloy steels in Na2CO3 solutions at elevated tempera- ture and with a critical strain rate [77]. The systems belong to groups I or IV.
Fig. 2-18 J(U) curves and critical potential range for intergranular stress corrosion (hatched) for a hardened 10 CrMo 9 10 steel (ASTM P21) in boiling 35% NaOH: — potentio- dynamically measured with +0.6 V rr1; •-•-• potential change after every 0.5 h AU = +0.1 V; x-x-x potential change after every 0.5 hAt/ = -0.1V.
(i) Plain carbon or low-alloy steels in CO-CO2-H2O condensates [82,85], HCN [86], liquid NH3 [87,111].
(j) Austenitic stainless steels in cold chloride-containing acids [88]. These systems belong to group IV on account of the anodic danger of pitting corrosion.
Cases (e), (g), and (h) are of interest in the cathodic protection of warm objects (e.g., district heating schemes [89] and high-pressure gas lines downstream from compressor stations [82]) because the media of concern can arise as products of cathodic polarization. The use of cathodic protection can be limited according to the temperature and the level of the mechanical stressing. The media in cases (a) and (f) are constituents of fertilizer salts in soil. Cathodic protection for group I is very effective [80].
In deciding on the type of protection for the systems shown in Fig. 2-17b, consideration has to be given to the level of protection current, current distribution according to Eq. (2-45), the by-products of electrolysis, and operational safety in conjunction with Fig. 2-15. Figure 2-18 shows the positions of nonstationary and quasi-stationary J(U) curves relative to the critical range for stress corrosion. It is obvious that nonstationary J(U) measurements lead to false conclusions and that anodic protection is more advantageous on account of the small difference among the protection range and the rest potential, the lower protection current density, and the higher polarization resistance [90].