Exfoliation corrosion

This corrosion may be intercystalline in 2000 and 5000 alloys exhibiting a long-grained structure parallel to the direction of transformation. It can also be found in 7000 alloys with or without copper (which have a low susceptibility to intercrystalline corrosion). It results from the precipitation of parallel stripes of intermetallic phases such as AlgMn, Ali2CrMn, AlFeMn, etc., which are cathodic with respect to the solid solution, and between which there is an anodic zone depleted in Fe and Mn. 

Welded alloys of the 7000 series without copper, such as 7020, in the T4 temper show a high susceptibility to exfoliation corrosion. 

This mainly depends on the conditions of transformation and heat treatments. 

Standardised testing procedures should be used for assessing the susceptibility to exfoliation corrosion [18]. The selection of the test solution and the duration of immersion depend on the alloy series  

When intergranular corrosion occurs in a product with a highly oriented grain structure, it propagates internally, parallel to the surface of the metal. The entrapped corrosion product is about five times as voluminous as the metal consumed. This produces an internal stress that splits off the overlying layers of metal hence the name exfoliation. 

Exfoliation is a very deleterious form of corrosion because the splitting off of uncorroded metal rapidly reduces load-carrying ability. The splitting action continually exposes film-free metal, so the rate of corrosion is not self-limiting. Exfoliation generally proceeds at a nearly linear rate. 

For see to take place, water or water vapor must be present otherwise, cracking will not occur. The presence of halides will accelerate cracking further. 

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Exfoliation corrosion is especially prevalent in aluminum alloys. The grain structure of the metal determines whether exfoliation corrosion will occur. In this form of corrosion, degradation propagates below the surface of the metal. Corrosion products in layers below the metal surface cause flaking of the metal. 

As seen above Moran et a ., has commented upon the exfoliation corrosion of Al-Li-Cu-Zr-Ge . The mechanism has been investigated by Reboul and Bouvaist and a mathematical model suggested by Robinson . The influence of alloy elements in Al-Zn-Mg has been reported by Reboul and it has been shown that exfoliation corrosion of Al-Mg-Si in irrigation water is also governed by alloy and impurity concentration . 

Test method for exfoliation corrosion susceptibility in 2XXX and 7XXX series aluminium alloys (EXCO test)... 

Method for visual assessment of exfoliation corrosion susceptibility of 5XXX series aluminium alloys (Asset test)... 

Certain pre-boiler cupronickels, such as 70 30 alloy tubes, subjected to high temperature, stress, and stop-start operation may suffer oxygen corrosion-induced dealloying followed by exfoliation corrosion, in which oxidized sheets peel away from the solid metal. 

High temperature/stress and stop/start operation effects on 70 30 cupronickel tubes (as found in some FW heaters). A chain of cause and effect including Oxygen corrosion Dealloying Exfoliation corrosion... 

The outer surface of the panel and door after stripping point and removing the catches is shown in Figure 7.13. There appears to be no damage to the plate and the door. The examination of the inner surface showed extensive exfoliation corrosion on the panel and the door in the catch positions (Figure 7.14). Cracking originating from the catch position... 

Figure 7.14 Exfoliation corrosion on the inner surface of the panel and door around a catch location1...

Figure 7.15 Cross-section through the panel, showing the exfoliation corrosion ...

Exfoliation corrosion occurs when the attack occurs along grain boundaries, particularly when they are elongated and form thin platelets. The voluminous corrosion product causes splitting of layers uncorroded material. 

The material conformed to the specification and the aluminum alloy is known to be subject to exfoliation corrosion. The corrosion was extensive at the catch positions and attributed to the stainless steel shims fitted below the catches. The paint between aluminum and stainless steel shims deteriorated, resulting in galvanic corrosion with the stainless steel acting as the noble metal. 

Exfoliation ASTM G34 2XXX (Al-Cu-X) and 7XXX (Al-Zn-X) series Al alloys Exfoliation corrosion... 

Testing for intergranular and exfoliation corrosion in A1 alloys is described in ASTM... 

This encyclopedia and this chapter are focused on electrochemistry. However, a few words about nonelectrochemical techniques are in order for completeness. Exposure testing of samples to service or accelerated environments is widely used, and there are several ASTM standards describing appropriate methods. Several immersion tests for intergranular and exfoliation corrosion were mentioned in the last section. ASTM G31 describes methods for immersion testing of metals [83]. This standard practice contains mostly common sense, but also has information that will help experimentalists avoid common... 

G112-92, Standard guide for conducting exfoliation corrosion tests in aluminum alloys. Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 489, Vol. 3.02. 

Figure 3.24 Exfoliation corrosion around fastener holes in A1 alloy 7075-T6 (7).

Figure 5.15 Cross section through panel showing exfoliation corrosion (Reproduced by permission of the Society of Petroleum Engineers (20).)...

Several aluminium alloys show very good corrosion resistance in various atmospheres. Some pitting occurs, but the pits remain small. Maximum depth seldom exceeds 0.5 mm during 6-20 years of exposure it is usually in the order of 0.1 mm. Some alloys may, however, be attacked by intergranular corrosion or exfoliation corrosion (see Section 7.7). Extensive galvanic corrosion may occur on aluminium in contact with copper, mild steel (in marine atmosphere) and graphite, less in contact with stainless steel, while aluminium is compatible with zinc [8.2]. 

Evans, D. G., P. W. Jeffrey (1974) Exfoliation corrosion of AlZnMg alloys, in R. W. Staehle, B. F. Brown, 1. Kruger, A. Agrawal, eds.. Localized Corrosion, International Conference Corrosion Series, National Association of Corrosion Engineers, Houston, TX, 614—622. 

Exfoliation is commonly experienced on exposure of susceptible aluminum alloys to marine atmospheres. Simulation in the laboratory is accomplished by controlled intermittent spray with 5% NaCl containing added acetic acid to pH 3 at 35-50°C (95-120°F) [28]. In practice, severe exfoliation corrosion has been experienced in water irrigation piping constructed of type 6061 alloy [29]. The cause in this instance was ascribed to pipe fabrication methods and presence of excess amounts of impurity elements (e.g., Fe, Cu, and Mn). 

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