Alclad

Aluminum alloy (and in particular alclad aluminum) tubing is sometimes used in water service. The alclad alloy has a sacrificial aluminum-alloy layer metallurgically bonded to a core alloy. 

It should be noted that a number of aluminum alloys are available (see Table 28-16). Many have improved mechanical properties over pure aluminum. The wrought heat-treatable aluminum alloys have tensile strengths of 90 to 228 MPa (13,000 to 33,000 Ibf/in ) as annealed when they are fuUy hardened, strengths can go as high as 572 MPa (83,000 Ibf/in"). However, aluminum alloys usually have lower corrosion resistance than the pure metal. The alclad alloys have been developed to overcome this snortcoming. Alclad consists of an aluminum layer metaUurgicaUy bonded to a core alloy. 

Alclad a composite in which a thin layer of aluminium, or an aluminium alloy of good corrosion resistance, is bonded metallurgically to a high-strength aluminium alloy (of lower corrosion resistance) to provide a combination of these two properties. 

The Alclad alloys have been developed to overcome this shortcoming. Alclad consists of a pure aluminum layer metallurgically bonded to a core alloy. The corrosion resistance of aluminum and its alloys tends to be very sensitive to trace contamination. Very small amounts of metallic mercury, heavy-metal ions, or chloride ions can frequently cause rapid failure under conditions which otherwise would be fully acceptable. When alloy steels do not give adequate corrosion protection—particularly from sulfidic attack—steel with an aluminized surface coating can be used. 

Panels of 24-ST alclad aluminum, 6x6 inch 0.032 sage, are coated with the fire-retardant system (only one surface) and placed in the vibrating test rig. 

Aged in nitrogen on 17-7 pH stainless steel - a Aged in air on 2024T-3 alclad aluminum o Aged in air on 17-7 pH stainless steel ... 

Aluminum Pipe) A Heavy Wall Alclad Aluminum "Spool Piece" Will Avoid Galvanic Corrosion of Pipe... 

Reprinted from reference 27, pp. 111-145 by courtesy of Marcel Dekker, Inc. bCladding for Alclad products... 

Fresh water. Aluminum and its alloys are not prone to corrosion on exposure to distilled water up to 180°C. Thus storage tanks, piping and fittings of the alloy can be used for handling distilled water. The composition of natural fresh water is variable. In spite of this restriction, the alloys are not attacked, even at 180°C in natural waters. It should be noted that pitting may occur when a small thickness of the sample is exposed. In this case Alclad 3003 is recommended for use to avoid failure due to pitting. 

Atmospheric Corrosion. The aluminum-based alloys in general are corrosion resistant to outdoor exposure with the exception of copper-bearing alloys. The Alclad alloys gave the best performance. The loss in tensile strength has been used as a measure of corrosivity and the loss of 1-2% of tensile strength over a period of 1 yr and in particular a loss of 17% was observed with 2017T alloy in 1 yr of outdoor exposure. 

The aluminum-based alloys such as 1100, 3300, 5052, 6053, Alclad 3300, Alclad 1017-T and Alclad 20-24-T are highly resistant to most natural environments alloys of thickness <0.076 mm should not be used. Lead and bismuth added to alloys 2021 and 6262 improve chip breakage and other machining characteristics. 

Some measures involving cathodic protection of aluminum using zinc as the sacrificial anode,47 and protection of aluminum ship hulls are found in the literature.37 Overprotection may lead to alkali (cathodic corrosion) attack. Alclad alloy systems are also some form of self-contained cathodic protection systems. 

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