Aluminum alloys corrosion crevice

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. 

Aluminum and alloys are not suitable for (1) alkalis, (2) acids at pH 4.5, and (3) mercury, which can be a significant risk in some liquified natural gas operations. The heat treatable, high-strength aluminum alloys of the 2000- and 7000-series are rarely used because of environmental cracking susceptibility. Aluminum and its alloys are susceptible to chloride pitting and to concentration cell problems such as crevice corrosion and under-deposit corrosion. 

Additional corrosion may occur where the supply of oxygen at interfaces between joined components is limited. This is not bimetallic corrosion, but its effect at bimetallic joints can be as detrimental as true bimetallic corrosion. However, no crevice corrosion occurred in couples of zinc-aluminum alloys and polyethylene in 10-year atmosphere tests (Noranda, personal communication) nor around nylon bolt heads. Jointing compounds are useful in preventing crevice corrosion as well as bimetallic corrosion. Also, stressed parts of the surface tend to be anodic to unstressed parts, but this effect is not usually of practical significance with zinc and zinc-coated steel. 

LIME MORTAR. Contains hydrated lime. sand. Portland cement, coloring. During the period when mortar is liquid, aluminum alloys show etching which ceases when the mortar dries because of the formation of a protective film. It is good engineering practice to protect aluminum alloys contacting mortar in a faying surface to minimize crevice corrosion. See also Ref (1) p. 129. (2) p. 161, (3) p. 72. 

Crevice corrosion occurs mainly (but not exclusively) on passive materials. The most important problem is the crevice corrosion of stainless steels, nickel-base alloys, aluminum alloys, and titanium alloys in aerated chloride environments, particularly in sea or brackish water, but also in environments found in chemical, food, and oil industries. Other cases of crevice corrosion are also known such as the so-called corrosion by differential aeration of carbon steels, which does not require the presence of chloride in the environment. Also mentioned in the literature is the crevice corrosion of steels in concentrated nitric acid and inhibited cooling water and of titanium alloys in hot sulfixric environments. 

SCC is usually nucleated by some form of localized corrosion in chloride-SCC of stainless steels [68] or aluminum alloys [54], cracks start from areas of pitting, intergranular eorrosion, or crevice eorrosion that create die stress concentration and the acidity required for cracking (Fig. 7). In SCC of C-Mn or low-alloy steels, intergranular corrosion occurs along segregated zones rieh in earbon, nitrogen, or phosphorous and provides a stress concentration (helps to achieve A iscc) [ 7]... 

Aluminum and its alloys have been extensively used for structural applications with success. Aluminum resists corrosion from the atmosphere if there is an absence of narrow crevices. Many statues erected, over a hundreds of years ago, have not deteriorated badly which is in contrast with aluminum cables used in seawater. The corrosion resistance of aluminum is due to its tendency to form a compact oxide layer over the surface. The oxide formed offers a high resistance to corrosion. The normal surface film present in air is about 1 nm thick. The film thickness increases at the elevated temperature. The film growth is more rapid in water than in oxygen. 

Actually, the drop of pH is related to more complex reactions and species. Thus, in more sophisticated models, several hydrolysis reactions and metal chloride formation are taken into account but the selection of species and reactions is somewhat different from model to model. Oldfield and Sutton [94] and Watson and Postlethwaite [2] considered only hydroxides as the product of cation hydrolysis. Sharland [96] introduced simple metallic chlorides. The most complete set of species and reactions has been used by Bernhardsson et al. [4], which made available the thermodynamic data of a large number of species, including several iron, nickel, chromium, and molybdenum polycations as well as metal chlorides and hydroxychlorides. Gartland [19] used a more limited set of species (Table 10.3) selected among the Bernhardsson data. According to their experimental results, Hebert and Alkire [95] included Al(OH) " as the hydrolysis product in their model of the crevice corrosion of aluminum alloys. 

Rosenfeld also studied the influence of aluminum idloy on the rate of crevice corrosion and obtained the results plotted in Fig. 5. Aluminum-coppo and aluminum-zinc-magnesium-copper idloys coroded many times fasto than 1100, Sxxx, or Sxxc alloys. Again, crevice gap width is important, because corrosion rates are low for crevice openings gieato than 2S4 pm (10 mils). 

The cathodic protection of aluminum alloys in seawater has been extensively studied (Ref 19, 20). Sacrificial anodes were found to be effective in reducing surface pitting and crevice corrosion without causing cathodic attack. 

Describes a procedure for conducting cyclic galvanostaircase polarization (GSCP) to determine relative susceptibilily to localized corrosion (pitting and crevice corrosion) for aluminum alloy 3003-H14... 

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. 

Initial attempts to use aluminum for automotive trim were unsuccessful due to the corrosion behavior of the metal. It is therefore anodized for automotive trim applications to provide a protective oxide surface which acts as a barrier coating for corrosion pro tec tion. > 2 Aluminum and its alloys are susceptible to pitting and crevice corrosion in chloride containing environments. The corrosion resistance of anodized aluminum is therefore highly dependent on the quality of the anodized surface and the absence of scratches and other damage sites. 

For crevices such as in those in socket welds, the metal in the crevice is likely to be anodic. Crevice corrosion and under-deposit corrosion can be serious problems in oxide-stabilized materials such as aluminum and the stainless steels. Crevices and deposits can also accelerate corrosion in metals (such as carbon steel) that do not exhibit both active and passive states. However, the rate of corrosion is much slower in such materials because they lack the galvanic driving force of the active-passive states characteristic of the oxide-stabilized metals and alloys. The anode areas in crevices and under deposits are typically smaller than the cathode areas. This difference accelerates the corrosion rate. 

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