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From: [email protected]
Date: 06 Jan 2003
Time: 19:52:21
Remote Name: 202.67.96.43
As promised - here's the rest. Sorry Bill - a steel wire brush WILL cause galvanic corrosion under any amount of paint. Chromate or Zinc primers are the go. Beware, though, these are very toxic. Some people have very low thresholds at which they willbecome very ill from exposure. Don't test your limits. For serious corrosion exclusion anodising can,t be beaten. However this is impractical on a whole car and near impossible to pint over sucessfully. The next best is a surface pre-treatment called alodine. Once again this acid, so safety first and remember to wash the surface thouroughly with water then dry before applying surface finishes. Probably the best thing to do regardless of subsequent coatings is to scrub the whole area with a PLASTIC scourer and water to achieve a "break free" surface on the water. That is a continuous surface of water across the substrate with no patches that "clear" easily, like a silicone is present. With regard to joins to steel - unavoidable, but reduce the galvanic action with a sealer. Resist the urge to use RTV, this contains acetic acid - corrosive again! Beware of cutting and welding. Welding is intensified oxidation. This resultant oxidation is exceptionally hard to keep in check. Aluminium alloys vary and disimilair matal corrosion can occur between them. (though very slowly) Different alloys will have different suscertability to corrosion. eg; Duralumin is so susceptable to corrosion that it cannot be used alone, so is sandwiched between a quite pure form of aluminium, forming Alclad sheets. So what to do to reduce future problems? An earth strap is very good idea. A return earth from all electrical bits to the CHASSIS, and a bond between the body and chassis. The point where the strap is attached will get most corrosive activity, so attach a sacrificial ali plate between. Remember that every rivet between steel and ali will also be an "earth" contact and therefore a point of possible electrolysis. The best things to think of with rivets are the chances of fretting corrosion from the head moving on the soft ali. Boats use a zinc block to reduce corrosion, particularly on outboard motors (sacrificial anode). This can be applied to 7's too, and is particularly useful in the engine cooling system when an ali head is used. One last hint - where cracking occurs, drill a fine hole at the end of the crack. This will spread the load of the weakening force and help prevent further migration. Hope this helps, Courtenay