13.2.1 Introduction
Aqueous corrosion processes generally need oxygen and water in intimate contact to achieve measurable corrosion rates. In space environments, both these substances are virtually non-existent and hence aqueous corrosion processes can be essentially ignored.
However during manufacture, test and storage the structure experiences Earth environments with high levels of oxygen and water, therefore the corrosion process cannot be ignored.
When dissimilar metals are joined, accelerated corrosion can occur at the interface due to galvanic corrosion. This corrosion occurs on a metal (the anode) as a result of current flow from the anode to a less reactive (more noble) metal (the cathode), when they are in electrical contact and in the presence of an electrolyte. The anode corrodes due to dissolution of the metal and the process is balanced (in most practical applications) by the reduction of dissolved oxygen in the electrolyte.
13.2.2 Factors Which Affect the Rate of Corrosion
13.2.2.1 Electrolyte
Electrolyte factors that have major implications for galvanic corrosion are as follows:
- a. Electrolyte composition
- b. Degree of acidity or alkalinity
- c. Electrical conductivity
In general the severity of corrosion often increases with increasing electrical conductivity since in practical situations high conductivity is often caused by the presence of aggressive ions such as chloride or by acid or alkali.
13.2.2.2 Area Ratio
For many situations the rate of corrosion is dependent upon the rate of diffusion of dissolved oxygen to the cathode and is thus proportional to the area of the cathode metal surface. Thus, for a constant area of cathode material, the amount of corrosion of the anode area is constant but the intensity of corrosion increases as the area of the anodic metal is decreased. Since adverse area ratios can occur in threaded fastener joints, it is important that the fastener metal is of the same polarity as the clamped parts, or be relatively cathodic to them.
13.2.2.3 Metallurgical Condition and Composition
Differences in corrosion potential can exist between coupled metals or alloys of nominally the same composition. The cold working of an alloy/metal tends to make it more anodic. Bimetallic corrosion can also occur between generic alloys with slightly differing compositions.
13.2.3 Prevention of Bimetallic Corrosion
The prevention or minimisation of galvanic corrosion is best addressed during the design stage (see Reference 13.1). When dissimilar metals are joined, it is important to take care to protect the anodic member by proper electrical insulation of the joint or by excluding the electrolyte if this is feasible.
Table 13-1 lists metals in the order of their relative activity in an aqueous environment. The list begins with the more active (anodic) metal and proceeds down to the least active (cathodic) metal of the galvanic series. A galvanic series pertains to a particular electrolyte solution and hence for each specific solution encountered, a different series can apply. Usually a seawater environment is chosen as the basis for comparison since it is one of the most corrosive environments and is readily encountered. Galvanic series relationships are useful as a guide for selecting the metals used for a joint. Generally the closer one metal is to another in the series, the more compatible they can be. In a galvanic couple the metal higher in the series represents the anode and can corrode preferentially in the environment. It is therefore necessary to protect metals widely separated in the galvanic series, if they are intended to be joined.
Methods of preventing or minimising galvanic attack are based upon breaking the electrical path in the metallic or electrolytic parts of the system, by excluding oxygen from the electrolyte, or by sacrificial corrosion. This strategy cannot be used if the galvanic couple needs to act as an electrical connection. Electrical insulation can be achieved by interposing a gasket or washer between the dissimilar metals prior to connecting them. If the couple has to act as an electrical connection, a compatible metallic gasket or washer can be placed between the two metals prior to fastening, or the cathodic member can be plated with a metal compatible with the anode. Jointing compounds applied to the contacting surfaces of fasteners and lap joints do not normally provide electrical insulation, they are however very useful in preventing water penetrating the joint and hence preventing galvanic and crevice corrosion.
| Material Category | Variant |
|---|---|
| Mg alloy | AZ - 31B |
| Beryllium (hot pressed) | - |
| Al alloy | 2014 - T3 |
| Al alloy | 1160 - H14 |
| Al alloy | 7079 - T6 |
| Al alloy | 5052-0 |
| Al alloy | 5052-Hl2 |
| Al alloy | 5456-0, H353 |
| Al alloy | 5052 - H32 |
| Al alloy | 1100 - 0 |
| Al alloy | 6061 - T6 |
| Al alloy | 7075 - T6 |
| Al alloy | 1160 - H14 |
| Al alloy | 6061 - 0 |
| Al alloy | 2014 - 0 |
| Al alloy | 2024 - T4 |
| Al alloy | 5052 - H16 |
| Stainless steel | 430 (active) |
| Stainless steel | 410 (active) |
| Copper (plated, cast or wrought) | - |
| Nickel (plated) | - |
| Tantalum | - |
| Stainless Steel | AM350 (active) |
| Stainless Steel | 301 (active) |
| Stainless Steel | 304 (active) |
| Stainless Steel | 17-7 PH (active) |
| Tungsten | - |
| Copper 110 | - |
| Stainless Steel | Carpenter 20 (active) |
| Stainless Steel | 321 (active) |
| Stainless Steel | 316 (active) |
| Stainless Steel | 309 (passive) |
| Stainless Steel | 17-7 PH (passive) |
| Stainless Steel | 304 (passive) |
| Stainless Steel | 301 (passive) |
| Material Category | Variant |
| Stainless Steel | 321 (passive) |
| Stainless Steel | 201 (passive) |
| Stainless Steel | AM 355 (active) |
| Stainless Steel | Carpenter 20 (passive) |
| Stainless Steel | AM 355 (passive) |
| Stainless Steel | A286 (passive) |
| Titanium (annealed) | 13V, 11Cr, 3Al |
| Titanium (solution treated and aged) | 6 Al, 4v |
| Titanium (annealed) | 6 Al, 4V |
| Titanium (solution treated and aged) | 13V, 11Cr, 3Al |
| Stainless Steel | Am 350 (passive) |
| Silver | - |
| Gold | - |
| Graphite | - |
Surface coatings are very effective in protecting metals. It is important that both members of the couple are coated, but where this is not possible then the cathodic metal be coated as this increases the effective anode area ratio and reduces the corrosion intensity.
It important that the non-metallic materials that are joined to metals are treated with caution. For example, composite materials can contain carbon or graphite, which being very noble, can result in corrosion of the metal. Graphite pencil markings on aluminium aircraft structures have given rise to galvanic corrosion problems in the past. It is important to ensure that the non-metallic material does not contain any other corrosive agents. It is important that in particular they are free of the following:
- a. Ionic salts
- b. Acid or alkaline materials
- c. Carbon or metallic particles
- d. An ability to wick or absorb water.
It is important that gaskets used in RF shielding (typically a metallic loaded or metallic mesh supported in an elastomeric medium) are carefully screened to achieve compatibility with the mating surfaces, otherwise the mating surfaces and fasteners can need to be treated in some way.