Crevice Corrosion Accelerated Test for Cylinder Head/Gasket/Monoblock Assemblies from Lightweight Engines Exposed to Overheating Cycles 2020-01-1067
Severe crevice corrosion occurring at the joint of cylinder head/gasket/mono-block from lightweight engines causes accelerated dissolution of lightweight material, in particular, in cylinder head producing the linking of the cooling vessels with the combustion chambers or oil vessels. It is conductive to combustion of coolant or oil, and contamination of oil with coolant or vice versa, which is considered as catastrophic engine failure. Since crevice corrosion is dependent of many of the actual assembly characteristics, coolant and engine operation conditions, full-scale tests are the most frequent alternative for this type of evaluations. Nonetheless, they are very long and expensive, and sometimes, unreliable tests. Alternatively, the standard procedure ASTM-G78 is widely used to evaluate crevice corrosion propensity of different metallic materials under certain specified conditions trough immersion tests in a corrosive media in shorter test times. However, the method does not cover the characteristics and conditions existing at the cylinder head/gasket/mono-block joint. Hence, this paper presents an accelerated test consisting on three-electrode cyclic potentiodynamic anodic polarization and polarization resistance standard trials using special assembly samples to replicate the actual cylinder head/gasket/mono-block joint conditions in a corrosive solution. Four lightweight materials (Al-Si-Cu alloys) extracted from different cylinder heads, and a typical cylinder head gasket were used to prepare the specimens. Before the corrosion test, samples were subjected to thermal cycles at 200 and 400°C, respectively, to replicate engine overheating situations. The test duration for each specimen, excluding sample preparation, was around 4 hours allowing the formation of localized crevice corrosion areas at the sealing gap of the samples and obtaining information of corrosion rate and electrochemical behavior at the same time for comparison. This test could be potentially used to evaluate wide range of materials, gaskets, coolants, set or assembly parameters, etc., especially for engine durability purpose.