The corrosion requirements of fasteners are often set without knowing the actual performance characteristics of the coating selected. Witness the use of yellow dichromate parts on the exhaust systems of new cars. Once the engine is started, the finish is gone. The engineering department sets the requirements for the part by looking it up in a standards book that lists the hours of salt spray for the finish, but not any other conditions.
Some engineers know that salt spray requirements do not reflect real-world requirements, but are hampered by the need for a corrosion specification that is readily available. The rest of the engineering department is happy in their ignorance, applying book-value salt-spray hours to any part it handles.
Oils added to tapping screws and parts designed to fit into or against plastic can cause cracking of the plastic through a mechanism that attacks the long molecular plastic polymer chains. As Figure 1 shows, these types of failures, while generally not life threatening, do cause customer dissatisfaction and can result in poor ratings and general loss of business. Hydrocarbon compounds produce failure characteristics similar to oil-based ones and also should not be used on fasteners (washing and degreasing excessive oils is a prime way of introducing these chemicals).
Another corrosion-inducing family of chemicals is the chlorinated solvents. Often used for cleaning greases off of incoming parts, their use on fasteners should be restricted unless great care is taken to insure that the fasteners will not be used in and against aluminum structures.
Secondary treatments such as plastic locking patches and adhesives can damage the finish in the applied area, re-sulting in corrosion failures. These areas should be denoted by the finishing source as being free from corrosion requirements when the order is received.
A lack of proper cleaning is another major reason for finish corrosion failures. Oils, die lubricants and drawing compounds are often not removed properly, or at all, by the heat treater. The film adheres to parts of the fastener, causing poor plating.
Early red rust patch failures of phosphated fasteners at the areas under the head and on shank areas are indicative of burnt-on oil films. Cleaning is a difficult matter, especially for the job shop, as the customers all use varying compounds (on one survey, oils used for die lubricant varied from used automotive oil obtained from a local garage to cooking oil from the owner's other interest, a fast food outlet). These compounds do not react to the shop's basic cleaning cycle and standard cleaners. Since there is little communication between the manu-facturer, heat treater and finishing shop, information as to what compounds are used throughout the manufacturing cycle is non-existent.
Persistent salt-spray failure in one particular area of a fastener is cause to examine and question the materials on the part. Often a change of the cleaning chemical will remove the material. In the case of burnt-on materials, a call to the customer stating the problem and a request for an alternate cleaning method approval is in order. Acidic cleaning, stronger caustics, or even abrasive blasting may be needed in severe cases.blog comments powered by Disqus