Service Tests Solve Aluminum Cylinder Head Corrosion Problems 490093
Aluminum cylinder heads have been widely used on automobiles during the past fifteen years to increase the efficiency of engine operation. The great majority have given satisfactory service during many years of use and high mileages. In 1938 their use was greatly curtailed because of competitive costs. The competitive situation now prevailing favors the re-adoption of aluminum cylinder heads. Because of this, it is fortunate that the two problems formerly associated with the use of aluminum cylinder heads have been solved. One was difficult removal resulting from corrosion at the stud clearances. Tests have shown that this can be prevented easily by the use of cap screws, larger stud clearances, or stud coating compounds.
The other problem was corrosion at the coolant inlet ports. The causes and means of prevention of this difficulty have been determined recently by means of road tests of seventeen automobiles, and cyclic tests of V-8 engines with divided radiators. It was learned that the corrosion can be prevented by simple methods which can be applied by the engine manufacturer at little or no added cost. The corrosion was found to result primarily from the galvanic couple formed with the iron cylinder block. Corrosion did not occur in road tests of engines having aluminum blocks as well as aluminum cylinder heads.
One of the major factors affecting the rate of galvanic attack is the electrode potential of the cylinder head alloy. To achieve the low potential desired, the alloy should contain an adequate concentration of copper, and no more than a few tenths of a percent of zinc. Also, the alloy should be in the as-cast or solution heat-treated condition to retain sufficient copper in solid solution.
Another effective means of reducing port corrosion was found to be the use of a protective design of cylinder head gasket which covered the port shoulders of the cylinder head and preferably extended into the ports. Such gaskets protect the shoulders against erosion and lengthen the paths of the galvanic currents.
Proprietary alcohol and ethylene glycol antifreezes, and inhibitors used to prevent rusting of the iron cylinder block, were found to be important accelerators of port corrosion, particularly when mixed with water containing several hundred parts per million of chloride. These conclusions are contrary to those previously drawn from continuous engine tests before the development of cyclic testing. In the recent cyclic and road tests, the high chloride water when tested alone caused practically no corrosion. Also another municipal water high in chloride, sulphate, alkalinity, and copper, caused no appreciable corrosion. Straight ethylene glycol and methanol were less corrosive than the proprietary products, which contain inhibitors of iron corrosion. The amount of dilution of alcohol and glycol antifreezes with water proved to be an important factor, 70 to 80% of water causing maximum corrosion. The preferred non-freezing coolant of these types is water containing at least fifty percent by colume of any proprietary ethylene glycol antifreeze. These vary somewhat in corrosiveness depending on the inhibitors of iron corrosion employed. Sodium benzoate and certain oils were the most satisfactory inhibitors from the standpoint of aluminum cylinder heads. Kerosene showed promise as a non-corrosive, non-freezing coolant under special conditions.
Inhibitors are sometimes used in tap water coolants to prevent rusting of the block and consequent clogging of the radiator. They cause corrosion of aluminum heads, even in the absence of antifreezes. Of the inhibitors tested in high chloride water, soluble oil was the least harmful to aluminum heads, and is therefore recommended where an inhibitor of iron corrosion must be used. Sodium benzoate has not yet been tested by the authors for such use but appears promising in view of the results reported by Vernon.(1) Chromates caused severe corrosion and their use with engines having aluminum cylinder heads and iron blocks cannot be too strongly condemned.
If the engine manufacturer uses aluminum cylinder heads of the recommended alloy composition and condition, and a protective design of gasket, harmful corrosion will be avoided with only moderate attention to the recommendations concerning antifreezes and inhibitors.