Aluminum alloys galvanic corrosion

Corrosion Properties. In addition to corrosion problems such as pitting and exfoliation ced by conventional (monolithic) aluminum alloys, galvanic corrosion caused by the potential difference between the graphite fibers and the aluminum matrix is a reason for concern in these composites. As shown in Table 1, graphite appears at the cathodic (noble) end of the galvanic series while aluminum is at the anodic (active) end of the series. Graphite/aluminum composites have been shown to corrode 80 times faster than monolithic aluminum alloys in an aerated 3.15 wt% sodium chloride (NaCl) at room temperature (Ref 8). 

Chemical Reactivity - Reactivity with Water Mild liberation of heat Reactivity with Common Materials Corrosive to copper, copper alloys, aluminum alloys, galvanized surfaces Stability During Transport Stable Neutralizing Agents for Acids and Caustics Dilute with water Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Liquid-Metal Corrosion Liquid metals can also cause corrosion failures. The most damaging are liqmd metals which penetrate the metal along grain boundaries to cause catastrophic failure. Examples include mercury attack on aluminum alloys and attack of stainless steels by molten zinc or aluminum. A fairly common problem occurs when galvanized-structural-steel attachments are welded to stainless piping or eqmpment. In such cases it is mandatoty to remove the galvanizing completely from the area which will be heated above 260°C (500°F). 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

Aluminum is not embrittled by low temperatures and is not subject to external corrosion when exposed to normal atmospheres. At 200°C (400°F) its strength is less than half that at room temperature. It is attacked by alkahes, by traces of copper, nickel, mercury, and other heavy-metal ions, and by prolonged contact with wet insulation. It suffers from galvanic corrosion when coupled to copper, nickel, or lead-base alloys but not when coupled to galvanized iron. 

Steel sheets are aluminized by a hot-dip process similar to galvanizing. The principal applications for such a product are furnaces and ovens, automobile mufflers, and other equipment requiring heat and corrosion resistance. When a sheet which has been coated with aluminum by a hot-dip process is exposed to a temperature over l,000°F (538aC). the aluminum forms an iron-aluminum alloy which is heat- and corrosion-resistant. 

Aluminum and Aluminum Alloys. Aluminum can be employed in sea water as a resistant material of construction. Experiments at Fort Bel voir, Virginia, and elsewhere, indicate that by proper corrosion-control practices, aluminum can be used for an entire plant which processes sea water. The sea water entering the plant should be free of all metallic ions, especially copper or nickel. It is essential, in such a plant, that no copper-base alloys be used at all and that galvanic couples to most other metals be avoided. 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

Note the potentials of the graphite and the aluminum alloy that you determined. If these two are connected with an electrical contact, their potentials should move toward each other. Further, since the solution is relatively conductive, and assuming that the electrical lead connecting them was highly conductive, they would come to the same potential. Therefore connect the leads of the two electrodes together and connect them both to the positive (or V) lead of the voltmeter. Measure the potential of this galvanic couple relative to one of the reference electrodes and confirm that the couple potential does indeed rest somewhere in between the corrosion potentials of the two materials. 

Aluminum alloys of the series 7000 and 2000 and 380 die-cast alloy cause serious galvanic corrosion of magnesium in saline environments, especially when the iron content of the alloy exceeds 200 ppm of iron. Data on the galvanic corrosion of... 

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. 

CHEMICAL PROPERTIES corrosive to copper and zinc alloys, aluminum and galvanized surfaces may react with acids, strong oxidizers, chlorine, hypchlorite, halogenated compounds, reactive organic compounds, and some metals reactive with mercury and nitro-sating compounds FP (0°C, 32°F) LFL/UFL (4.9%, 20.7%) AT (430°C, 806°F) HC (253.5 kcal/gmol liquid at 25°C) HF (-47.3 kJ/mol liquid at 25°C). 

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