Crevice corrosion conditions

Fig- 7.73 Schematic representation of (a) passive film, (b) passive film rupture by stress-induced slip resulting in exposure of bare substrate, (c) crack initiation by anodic dissolution initiating crevice corrosion conditions before repassivation of exposed substrate, and (d) repassivation of exposed substrate before crack initiation. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

It would not be expected that crevice corrosion could easily occur in demineralized water. However, this exchanger was subject to extended outages with no lay-up procedures. Under such conditions, crevice attack is more likely. 

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

In all cases partial or total hulls of aluminum or stainless steel must be provided with cathodic protection. This also applies to high-alloy steels with over 20% chromium and 3% molybdenum since they are prone to crevice corrosion underneath the coatings. The design of cathodic protection must involve the particular conditions and is not gone into further here. 

Metals and alloys vary in their ability to resist crevice corrosion, and this applies particularly to those that rely on passivity for their resistance to corrosion. Titanium and high-nickel alloys such as the Inconels and Hastel-loys are amongst the most resistant, but even these will be attacked under highly aggressive environmental conditions. 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

In sea-water flowing at slower velocities and more especially in stagnant conditions, pitting and crevice corrosion may develop, particularly beneath deposits and marine growths at the surface of the metal. Some data for the Ni-30 Cu Alloy 400 are shown in Fig. 4.40 the corrosion was mostly pitting. 

Resistance to crevice corrosion Titanium is more resistant to crevice corrosion than most conventional metals and alloys, particularly where differential aeration is involved, e.g. it is very resistant to crevice attack in sea water at normal temperatures. This form of corrosion becomes more severe when acidity develops in a crevice and this is more prone to occur under conditions of heat transfer . Under these circumstances, especially in the presence of halide, even titanium may suffer attack, and the metal should not be employed in strong aqueous halides at temperatures in excess of 130°C. This limiting temperature can be raised to 180°C by use of the Ti-0- 15Pd alloy " or by coating with noble metals. (See also Sections 1.4 and 1.6.)... 

Stainless steel has been tried as an inert anode, mainly under laboratory conditions and with only partial success. Even at low current densities in fresh water the majority of alloys pit rapidly, although others show the ability to remain passive at a low current density . However, at practical current densities, the presence of chloride ions, deposits on the anode or crevice corrosion at the anode support lead to rapid failure , but it may be possible that stainless steel could give useful service under certain conditions and with particular alloys . ... 

Although important contributions in the use of electrical measurements in testing have been made by numerous workers it is appropriate here to refer to the work of Stern and his co-workerswho have developed the important concept of linear polarisation, which led to a rapid electrochemical method for determining corrosion rates, both in the laboratory and in plant. Pourbaix and his co-workers on the basis of a purely thermodynamic approach to corrosion constructed potential-pH diagrams for the majority of metal-HjO systems, and by means of a combined thermodynamic and kinetic approach developed a method of predicting the conditions under which a metal will (a) corrode uniformly, (b) pit, (c) passivate or (d) remain immune. Laboratory tests for crevice corrosion and pitting, in which electrochemical measurements are used, are discussed later. 

Nickel-based alloys withstand chlorides in the feed better than does stainless steel but under extreme conditions, pitting, SCC and crevice corrosion is present [19, 20, 21]. The corrosion takes place in non-annealed structures in the as welded condition. Performance of these alloys may be enhanced by post-weld solution annealing [22]. 

Two recent studies have used surface techniques to examine the protective layers formed in various oxidation conditions and related the nature of the oxide to the corrosion rates in specific environments. Hultquist and Leygraf (43, 44) have examined crevice corrosion resistance in a NaCl solution at room temperatures while Baer (45) has looked at higher temperature (800°C) air oxidation. Oxygen potential, or partial pressure, is an important parameter in both studies where it was found that controlled oxidation can tailor the Cr profile in the alloy and surface oxide for maximum corrosion protection. Specific goals in each experiment were oxide characterization and comparison to corrosion rates... 

Figure 34 The steps involved in determining the depth of container wall penetration under Canadian nuclear waste disposal conditions using data obtained in an electrochemical galvanic coupling experiment. (A) Crevice propagation rate (R cc Ic) as a function of temperature (T) (B) RCc as a function of 02 concentration [02] (C) calculated evolution of container surface temperatures and vault 02 concentrations with time in the vault (D) flux of 02 (Jo2) to the container surface as a function of time (E) predicted evolution of Rcc up to the time of repassivation (i.e., at [02]p) (F) total extent of crevice corrosion damage expressed as the total amount of 02 consumed (Q) up to the time of repassivation (G) experimentally determined maximum depth of wall penetration (Pw) as a function of 02 consumed (Q) (H) predicted maximum value of Pw up to the time of repassivation (fP)-...

Figure 35 Schematic showing the process of hydrogen absorption into titanium under passive corrosion conditions after a period of crevice corrosion.

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