In previous times it was considered necessary for periodical repair of mechanical and electrical components. Today greater consideration is being given to the reduction of maintenance. In cases where a fastener must be removed, corrosion resistance is particularly important to avoid corrosion hindering or preventing fastener removal.
The cost of dealing with corroded fasteners is more expensive than using corrosion resistant fasteners originally. This is especially so when other costs resulting from fastener failure, such as lost production or down time, are taken into consideration.
A low cost option for designers considering corrosion resistance are plated fasteners. These are usually manufactured from mild steel and given a corrosion resistant coating of gal. or zinc. Whilst these coatings may be adequate in non severe corrosive conditions, many designers consider the additional cost of an inherently corrosion resistant material, such as stainless steel, to be an inexpensive insurance against possible failure or discolouration.
In considering the cost factor in using stainless steel fasteners, it is interesting to note that the overall cost of the finished product will generally be affected only by an insignificant amount if we choose stainless.
Where a different colour or surface finish is required for aesthetic appeal this requirement can usually be accommodated in stainless steel. The designer should consider a fastener as an integral part of the design because a joint is normally under stress and possibly where a failure is likely to occur. Therefore designers should start with the best fastener and design the joint around it.
The best way to combat the destructive character of corrosion is by prevention. One common practice in industry is to use a fastener material that is more corrosion resistant that the joining materials. The reason being, the fastener is the key to joint integrity and corrosion affected fasteners may cause a major failure, whereas the same amount of corrosive attack elsewhere may not cause a problem. There are several types of corrosion that can occur in fasteners. The most prevalent form is called galvanic corrosion. Galvanic corrosion can occur between dissimilar metals in the presence of an electrolyte which could be no more than polluted water. When the two metals are in contact, in effect, a battery is created. Current flows and one of the metals corrodes. In considering galvanic corrosion it is important to know which of the two metals is more anodic (least noble). This metal will have corrosion accelerated. See following table.
| Fastener Metal Base Metal | Zinc & Galvanised Steel | Aluminium & Aluminium Alloys | Steel & Cast Iron | Brasses, Copper, Bronzes, Monel | MartensiticStainless Type 410 | Austenitic Stainless Types 302/304, 303, 305 |
|---|---|---|---|---|---|---|
| Zinc and Galvanised Steel | A | B | B | C | C | C |
| Aluminium and Aluminium Alloys | A | A | B | C | Not Recommended | B |
| Steel & Cast Iron | A, D | A | A | C | C | B |
| Terne (Lead-Tin) Plated Steel Sheets | A, D, E | A, E | A, E | C | C | B |
| Brasses, Copper, Bronzes, Monel | A, D, E | A, E | A, E | A | A | B |
| Ferritic Stainless Steel (Type 430) | A, D, E | A, E | A, E | A | A | A |
| Austenitic Stainless Steel (Type 302/304) | A, D, E | A, E | A, E | A, E | A | A |
A. The corrosion of the base metal is not increased by the fastener.
B. The corrosion of the base metal is marginally increased by the fastener.
C. The corrosion of the base metal may be markedly increased by the fastener material.
D. The plating on the fastener is rapidly consumed, leaving the bare fastener metal.
E. The corrosion of the fastener is increased by the base metal. NOTE: Surface treatment and environment can change activity.
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