Deterioration of Nickel Spark-plug Terminals in Service

FRANCIS B. SILSBE,* Washington, D. C. (written discussion?). ?This paper is an interesting example of the -solution by the methods of metallography of what seemed at first a purely chemical problem. The authors have shown: in detail, how the well-known corrosion of spark-plug terminals is due to the dropping out of the individual crystals of the metal as it becomes surrounded by an oxide film. This corrosion is usually quite slow but in special cases may become a serious factor in limiting the life of spark plugs. There still remains the problem for the metallurgist to produce a material that will be entirely free from, this trouble. One of the most startling results of the work is. the demonstration that it affords of the deleterious effect of a mechanical stress continuously applied to material that is at the same time subject to adverse chemical or thermal conditions. The particular style of spark plug shown in the illustration, and in which serious trouble was caused by this corrosion, has the side terminals fastened at both ends and the method of assem-bling is such that a considerable initial tension is applied to the wire. It is without doubt this superimposed tension that causes the inter-crystal-line network to line up, as it were, into a few continuous and deeply penetrating transverse fissures instead of forming a great number of shallow surface cracks. This result is clearly shown both in the case of