Rivets, corrosion

In reality, aircraft parts can consist of several stacked layers of material, eonnected by rivets or bolts. To avoid corrosion, the layers are often protected by a special coating, so that there is no electrical connection between the layers. If there is a crack for example in the middle layer, no current will thus flow above or below the defect because of the insulating coating between the layers. There is only the possibility for the current to flow around the crack in the x-y... 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Area effects in galvanic corrosion are very important. An unfavorable area ratio is a large cathode and a small anode. Corrosion of the anode may be 100 to 1,000 times greater than if the two areas were the same. This is the reason why stainless steels are susceptible to rapid pitting in some environments. Steel rivets in a copper plate will corrode much more severely than a steel plate with copper rivets. 

As discussed in Section 29.2.5, jointing of two different metals (copper being one) causes electrolysis at the joints, leading to corrosion and failure of the joint. To avoid this, it is recommended that the same procedure be adopted as discussed in Section 29.2, and where the electrode and the connecting ground strip are of the same metal, that the joints are riveted or welded with the same metal after making the surface. Soldering is not recommended. 

For tanks and vessels, welded units are preferred over riveted or bolted designs. Fastener joints provide sites for crevice corrosion. Undrainable horizontal flat tops of tanks should be avoided unless proper drainage schemes are included in the design. Tank bottoms should be sloped toward... 

Corrosion usually results in a leak or failure of a support because a vessel or support gets too thin. It is then not strong enough to withstand the pressure or load. However, rust can cause failure in another way. It occupies about seven times the volume of the steel from which it was formed. V/hen rust occurs between two plates that have been bolted or riveted together, a high pressure develops. This can force the plates apart or even break the bolts or rivets (see Section 9.1.2 g). Corrosion of the reinforcement bars in concrete can cause the concrete to crack and break away. 

The housing is streamlined in design to give good air flow characteristics, ft may be of riveted welded or bolted metal sheets. The usual material is steel, although construction is available to meet corrosion conditions described for the wheel. 

Most fabrication techniques have implications for corrosion performance. Riveted and folded seam constmction creates crevices as shown in Figure 53.8. Those materials that are susceptible to crevice corrosion should be fabricated using alternative techniques (e.g. welding). Care should be taken to avoid lack of penetration or lack of fusion, since these are sites for crevice corrosion to initiate. 

The rate of corrosion and damage caused to the more negative metal will depend upon the relative sizes of the anodic (corroding metal) and cathodic areas. A small anode and a large cathode will result in intensive corrosion of the anodic area. On the other hand, if the anode is large compared with the cathode, the corrosion of the anodic area will be more general and less likely to result in rapid failure. For example, a steel rivet in a copper plate will be rapidly attacked in sea-water, whereas a copper rivet in a steel plate may lead only to slightly accelerated corrosion of the steel in the area adjacent to the rivet. Prediction of the rate of corrosion of the less noble metal... 

The mechanical joining of aluminium alloys to steels using rivets and bolts, a combination which is difficult to avoid in the shipbuilding industry, represents a typical example of a situation where subsequent bimetallic corrosion could occur. Similarly, other examples of an ill-conceived choice of materials, which could normally be avoided, can be found in, for... 

Finally, mechanical joints, e.g. nuts, bolts, rivets etc., are still important joining methods for which attention must be given to compatibility to avoid dissimilar metal corrosion problems and crevice corrosion " . ... 

Figure 16.5 Differential aeration around a poorly fitted rivet leads to intense local corrosion.

Bonding can eliminate crevices that often lead to crevice corrosion in riveted joints. 

In this experiment, polarization curves for carbon steel and copper in 3.5 wt% NaCl will be determined. From these data, the corrosion rates will be estimated for the individual metals freely corroding in solution and for the metals electrically coupled in solution as would be the case for an immersed, riveted connection, for example. 

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