Micro-crevices

Figure 8.27 illustrates the effect of spot welding and continuous welding. Spot welding creates micro-crevices, whereas continuous welding eliminates crevices. 

If the depth of crevices, vias, or similar details on a cathode is small (about 10 tm to 1 cm), the distribution of current and thus that of the deposit should be uniform. In most cases, however, one observes that deposits are thicker over micro peaks (bumps)... 

In an optical micrograph of a commercially available nitinol stent s surface examined prior to implantation, surface craters can readily be discerned. These large surface defects are on the order of 1 to 10 p.m and are probably formed secondary to surface heating during laser cutting. As mentioned above, these defects link the macro and micro scales because crevices promote electrochemical corrosion as well as mechanical instability, each of which is linked to the other. Once implanted, as the nitinol is stressed and bent, the region around the pits experiences tremendous, disproportionate strain. It is here that the titanium oxide layer can fracture and expose the underlying surface to corrosion (9). 

If the depth of the crevices, vias, or similar on the cathode is small (about 10 /zm to 1 cm), the distribution of current and thus that of the deposit should be uniform. In most cases, however, one observes that deposit is thicker over micro peaks (bumps) than, say, micro valleys. Such state of affairs is referred to as bad micro throw. When the opposite is true, one talks about true leveling. From simple geometric considerations it follows that given a V-shaped recess, even in the case of uniform metal distribution, still at the bottom the deposit would be expected to be thicker than at the top. However, pronounced leveling is obtainable by using suitable additives. These kinds of additives are known as levelers. ... 

Production of sulfides. This may involve the production of FeS, Fe (OH)2 etc. and an aggressive chemical agent such as hydrogen sulfide (H2S) or acidity. Micro-organisms may also consume chemical species that are important in corrosion reactions (e.g., oxygen or nitrite inhibitors). Alternatively, their physical presence may form a slime or poultice, which leads to differential aeration cell attack or crevice corrosion. They may also break down the desirable physical properties of lubricating oils or protective coatings. (Stott)5... 

In discussing environment, we can look at its effect on a macro scale, e.g. in the atmosphere, in the ocean, etc. and also examine effects on a micro scale, i.e. what is happening on the metal surface or over short distances. Due to the great variety of environments in which metals are put to use, the range of corrosion problems are equally numerous. Often, similar types of corrosion occur in many environments and may stem from similar mechanisms these have been given specific names which indicate how the corrosion has occurred. For example, under-deposit corrosion and crevice corrosion are related, both being due to oxygen concentration cells. 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

In nature, crevices exist everywhere, not only on the macro-scale, but also on micro-scale. Actually, we could say that there are no material surfaces without crevices. This suggests that crevice corrosion is a very general corrosion, since crevices existing universally can always bring about the corrosion. 

Because Ni-Mo alloys do not develop a passivating oxide film on the surface, they do not suffer localized attack in the classical sense, such as the halide-induced pitting and crevice corrosion of stainless steels. In some service applications these alloys would not corrode uniformly but might develop some shallow cavities on their surface. These cavities could be the result of confined enhanced corrosion as a consequence of micro galvanic couples with oxidizing agents or other impurities. 

FIGURE38.il Micro-corrosion cells, (a) Grain boundaries are anodic with respect to the grain interior, (b) Crevice corrosion due to oxygen-deficient zone in metal s environment. 

Hydrogen is often a by-product of corrosion. A sensor was formed with a micro-mirror of palladium that was responsive to hydrogen concentration in air up to approximately 5 percent. The interaction of hydrogen and palladium reversibly forms a hydride, PdH which has a lower reflectivity than pure palladium. Smyrl and Butler illustrated that this sensor is responsive to hydrogen that is dissolved in water. Thus, monitoring dissolved hydrogen in small areas such as crevices is a potential application for fiber optic micro-mirrors. 

Three-electrode probes are normally used where the fluid conductivity is less than 100 micro ohms (fluid resistivity greater than 104 ohm cm). Flush mounted versions are also avaUahle in various two electrode configurations. As with electrical resistance prohes the flush mounted versions can he susceptible to crevice corrosion at the electrode/potting compound interface and may give unrepresentative corrosivity values. 

Defects such as residual welding flux and micro-fissures create weld metal crevices that are easily corroded, particularly in chloride-containing environments. Some flux formulations on coated shielded metal arc or stick electrodes produce easily detached slags, and others give slags that are difficult to remove completely, even after grit-blasting. 

Micro-fissures or their larger counterparts, hot cracks, also provide easy initiation sites for crevice attack, which wiU drastically reduce the corrosion resistance of a weldment in a bleach plant. 

The micro-fissure provides a crevice that is easily corroded because stainless alloys are more susceptihle to crevice corrosion than to pitting. However, micro-fissure crevice corrosion is often mistakenly interpreted as self-irritiated pitting. 

Micro-fissure corrosion in austenitic stainless steel weldments containing 4 to 6% Mo is best avoided with the nickel-based Inconel 625, Inconel 112, or Avesta PI 2 filler metals, which are very resistant to crevice attack. Some stainless electrodes are suitable for welding 4% Mo steels, but they should be selected with low phosphorus and sulfur to avoid micro-fissure problems. 

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