Preferential corrosion

If pH is unusually low within the tubercle, the floor may be heavily striated. Small, shallow parallel grooves will appear in the depressions beneath each tubercle (Fig. 3.24). The striations are caused by preferential corrosion along microstructural defects such as deformed metal. 

Figure 3.24 Striated, corroded depressions are revealed on a large-diameter pipe after tubercles were removed. Striated surfaces are caused by preferential corrosion along microstructural irregularities in the steel. Such attack can be caused by low pH conditions.

Generai description. Galvanic corrosion refers to the preferential corrosion of the more reactive member of a two-metal pair when the metals are in electrical contact in the presence of a conductive fluid (see Chap. 16, Galvanic Corrosion ). The corrosion potential difference, the magnitude of which depends on the metal-pair combination and the nature of the fluid, drives a corrosion reaction that simultaneously causes the less-noble pair member to corrode and the more-noble pair member to become even more noble. The galvanic series for various metals in sea water is shown in Chap. 16, Table 16.1. Galvanic potentials may vary with temperature, time, flow velocity, and composition of the fluid. 

Figure 15.1 Preferential corrosion of the less noble of two phases in a weld bead (cross section). (Magnification 50x.)...

Figure 15.23 Preferential corrosion of Phase II Phase I remains unaffected. (Magnification 500x, unetched.)...

Preferential corrosion or attack at many other types of crystal defect may also be best illustrated during the etching of metallographically polished... 

Contact between dissimilar metals Galvanic action can occur between two different bare metals in contact if moisture is present, causing preferential corrosion of one of them (see Section 1.7). 

Scrap steel In some fortunate instances a disused pipeline or other metal structure in close proximity to the project requiring cathodic protection may be used. However, it is essential in cases of scrap steel or iron groundbeds to ensure that the steelwork is completely electrically continuous, and multiple cable connections to various parts of the groundbed must be used to ensure a sufficient life. Preferential corrosion can take place in the vicinity of cable connections resulting in early electrical disconnection, hence the necessity for multiple connections. 

Dezincification preferential corrosion of zinc from brass resulting in a copper-rich residue on the surface of the alloy. The term also applies to preferential loss of the zinc component by evaporation at elevated temperature. 

Intergranular Corrosion preferential corrosion at grain boundaries. 

While the positive chemical shift for Ru, indicating thick oxide formation, is virtually absent for x < 0.8 in RuxIr, x,a positive chemical shift is observed for Ir4/ for x > 0.2. Obviously the oxidation and corrosion of the ruthenium component is inhibited at the expense of increased oxidation of the iridium sites. Similarly, no preferential corrosion was observed for a Ru0 8Ir0.2 mixed oxide during 02 evolution at 1.5 Vsce for 8 hours [83],... 

Fig. 6. SEM images of meta-schoepite (a) Precipitation of meta-schoepite after 1 y of reaction (b) After 2 y, preferential corrosion of the U64 phase occurred along specific crystallographic directions.

Tinplate. Immersion of pickled iron or steel in molten tin results in the material known as tinplate, from which cans, caps, pails, pans, and so forth are made. The objective of tin-plating is to obtain a very thin coating of tin that is free from cracks, holes, or other imperfections. The reason for this latter requirement becomes clear if one considers the nature of the galvanic battery cell involving the metals iron and tin. Since iron is more active than tin, an imperfection in a coating of tin results in preferential corrosion of iron hence, an imperfect tin plating,... 

Electrochemical protection can be achieved by forming an electrolytic cell in which the anode material is more easily corroded than the metal it is desired to protect. This is the case of zinc in contact with iron (Fig. 16.11) in this example there is a sort of cathodic protection. Protection of ship hulls, of subterranean pipeline tubings, of oil rigs, etc. is often done using sacrificial anodes that are substituted as necessary. The requisites for a good sacrificial anode are, besides its preferential corrosion, slow corrosion kinetics and non-passivation. Sacrificial anodes in use are, for this reason, normally of zinc, magnesium, or aluminium... 

The selective corrosion of cast iron (graphitization), the preferential corrosion of the steel welding (grooving corrosion), sensitization and knife line attack of welded stainless steels are typical examples of corrosion influenced by metallurgical parameters. 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

Sn7Hg 72 phaseP Newer high copper amalgams reduce the risk of mercury release, as preferential corrosion of the p phase, CueSns, typically occursJ However, release of mercury can still occur even in these materials. Older silver-tin amalgams are based on a silver-tin alloy, while high copper amalgams are based on either a silver-copper-tin alloy or a mixture of silver-tin and silver-copper alloys. 

Fig. 7.51 Interface profile of intergranular corrosion when the precipitate phase is anodic to the matrix phase, (a) Preferential corrosion of continuous AB2 phase, (b) Preferential corrosion of discontinuous DE3 phase...

Galvanostatic None Carbon steels Preferential corrosion of weld metal and of HAZ... 

This type of corrosion was common in austenitic stainless-steel equipment before the effects of carbide precipitation were understood. From some works, it was found that some of the chromium in the austenitic (18-8) stainless steels combined with the carbon to form chromium carbide, which was precipitated at the grain boundaries when the alloy was heated or cooled slowly through the range of 800°F to 1500°F. It frequently occurs in the base metal adjacent to the weld deposit during welding. The rate and extent of the formation of chromium carbide are a function of time, temperature, and carbon content. As a result of localized depletion of chromium at the grain boundaries due to its precipitation as a carbide, preferential corrosion in some acidic environments may occur at the low-chromium grain boundaries. 

---