5.1
Galvanic compatibility. Above the maximum compatibility corrosion rate (0.009 mil/year), a galvanic couple is deemed incompatible and corrosion protection is required – the CEA or appropriate design authority shall determine the level of required protection. Galvanic compatibility does not imply complete freedom from galvanic corrosion or other types of corrosion, (e.g., crevice or pitting corrosion). This means an assembly or component containing compatible galvanic couples may still require protection against other types of corrosion. The CEA or appropriate design authority shall determine any additional required protection. (Note: Corrosion rate is calculated using the galvanic current obtained from the summation of two polarization curves [i.e., anode and cathode] using a mixed potential theory approach. Galvanic attack is influenced by corrosion current, corrosion potential, kinetic factors such as polarization effects, electrolytic environment, and physical arrangement of the conductive materials. Refer to Appendix C for additional information.)
5.2
Minimizing galvanic corrosion. When dissimilar conductive materials are used in intimate contact, protection against galvanic corrosion shall be applied. In some environments, galvanic corrosion may be appreciable, for example, with conductive materials such as magnesium, steel, zinc, and aluminum (anodes), in contact with copper, stainless steel, and nickel (cathodes), respectfully. For non-continuously immersed couples, the design shall incorporate electrical insulation of the joint and shall include methods to exclude electrolyte intrusion so that both the anodic and cathodic member of the couple are protected. Fatigue may have an impact on the galvanic corrosion rate, and relevant testing should be completed on structural joints where identified by the CEA. Conductive material compatibility shall be demonstrated through the methods outlined in Appendix B. The level of protection required on the conductive materials depends on the level of risk associated with the conductive material couple, both of which are determined by the CEA or appropriate design authority. Sacrificial coatings require different protections to be determined by the CEA.
5.3
Galvanic series in artificial seawater. Tables I, II, and III shall be used to determine galvanic compatibly between conductive materials. The conductive materials are ordered from active (anode) to noble (cathode) based on their measured open circuit potential (OCP). Any combination of conductive materials in the tables has a ranking assigned to it based on the corrosion rate (in mil/yr.) between them, with zero (0) indicating galvanic compatibility. Rankings of one (1) through six (6) indicate galvanic incompatibility and require protection. The range of incompatible rankings does not represent an assessment of risk nor implies a level or hierarchy of protection schemes – the CEA or appropriate design authority shall determine the level of required corrosion protection for their environment relative to sea water. For galvanic couples not included in Tables I, II, and III, data shall be generated according to the procedure outlined on Appendix B, and provided to the CEA or the appropriate design authority for assessment.
5.4
Protection of galvanically incompatible materials. Galvanic couples indicated as compatible do not require protection, unless indicated by the CEA or appropriate design authority to protect against other types of corrosion (see 5.1). Conductive materials that have a galvanic ranking greater than zero (i.e., corrosion rate greater than 0.009 mil/yr.) when coupled, shall not be paired without protection. Protection schemes shall be tailored to the specific application and approved by the CEA. Some recommended protection schemes are outlined in Appendix A and may be accomplished by one or more of the following:
- a. Apply a sacrificial metal coating to the cathodic member with a potential similar to or near that of the anodic member.
- b. Seal to ensure that the faying surfaces are watertight.
- c. Paint or coat all surfaces (anode and cathode) to increase the resistance of the electrical circuit.
- d. Insert a barrier material, which will isolate or insulate the faying surfaces, where the faying surfaces are not required to be electrically conductive. Such a shield must be inert and non-absorbing. This shield may be an organic barrier (such as sealant) to provide interference to galvanic activity, or a compatible ribbon or strip of an intermediate material that will provide a barrier to both physical and galvanic corrosion. Materials used as shields will be application specific.
5.5
Anode to cathode area ratio. Geometries where the area of the cathodic material is larger than the area of the anodic material shall be avoided. Data collected in this standard was collected in a 1:1 anode-to-cathode ratio. The specific anode-to-cathode ratio (A/C) required for compatibility depends on the corrosion rate (CR) between any given galvanic couple, and is given by the following relationship:
Protection of the anodic and cathodic materials is required if the anode-to-cathode ratio from the preceding expression cannot be maintained. A description of the relationship between A/C and CR is provided in Appendix C.
5.6
Additive manufacturing (AM). Conductive materials produced through AM (including cold spray) shall be considered as developmental materials. Until AM processes are standardized, verified, validated, and have established QA/QC procedures, the CEA or appropriate design authority shall evaluate each batch of AM materials and alloys individually to determine galvanic compatibility and required protective treatments. Information on the process used to create the AM materials along with the data listed in B.3.7 is required by the CEA or appropriate design authority for use or consideration for inclusion in future versions of this standard.
5.7
Materials containing conductive components. Materials containing conductive components, such as specialty materials and metal-rich primers, etc., which contain embedded metallic structures and/or powders, can become galvanically active when in contact with a metallic substrate or structure. Thus, the individual metallic components within these matrices shall be tested for galvanic compatibility following the methodology outlined in Appendix B. The CEA shall determine the level of protection required based on the results of the galvanic compatibility test and the intended application of the coating matrix.
5.8
Seawater environments. When joining non-compatible conductive materials, i.e., those with a corrosion rate greater than 0.009 mil/year when coupled (see Tables I, II, III), a barrier approach (e.g., sealant, interposing material compatible with each, shim, etc.) shall be used. Conductive materials exposed to seawater environments shall be resistant to corrosion, including stress-corrosion cracking or shall be processed to resist corrosion and stress-corrosion cracking. All exposed conductive material shall be adequately protected using protection schemes (i.e. sealants, primers, coatings, etc.) sufficient for the level of exposure they will experience.
5.9
Non-metallic materials. Non-conductive materials other than true metals, which must be joined to conductive materials, shall incorporate protection schemes similar to metallic conductive materials, unless approved by the CEA or appropriate design authority. If a material fits the following characteristics, the material may be considered a non-metallic material suitable for joining to metals:
- a. Free of corrosive agents (salts)
- b. Free of acid or alkaline materials (leachant must be neutral pH)
- c. Free of carbon or metallic particles
- d. Are not electrically conductive (>104 ohm-cm resistivity)
- e. Are not subject to significant bio-deterioration
- f. Will not support fungal growth
- g. Do not absorb or wick water in amounts that make them electrically conductive
Some materials classified as non-metallic will initiate corrosion of the adjoining metals, e.g., cellulosic reinforced plastics, carbon or metal loaded resin materials, asbestos-cement composites, etc.
5.10
Magnesium. When magnesium or a magnesium alloy is one of the conductive materials involved in a couple, the maximum feasible protective system as approved by the CEA shall always be employed. Where magnesium or a magnesium alloy is one of the conductive materials involved in the dissimilar conductive material combination, it is required that the edges of the joint
shall be adequately sealed to prevent excess galvanic corrosion. Magnesium shall not be used underwater except as a sacrificial anode. Where electrical contact of the conductive materials is not required, a non-metallic insulating gasket material may be used. This is true whether the combination is to serve in an electrical conducting system or not. Magnesium and magnesium alloys shall not be used in Naval Aviation applications unless approved by the CEA or appropriate design authority.
5.11
Joining methods to prevent galvanic corrosion. Where it is necessary to join conductive materials that are not galvanically compatible as defined by this standard, the following methods shall be used to minimize or prevent galvanic corrosion, as applicable to the design.
- a. Compatible conductive materials shall be selected such that the corrosion rate when coupled does not exceed 0.009 mil/year in immersed conditions and coupled conductive materials shall be designed to ensure that the area of the cathode is smaller than the area of the anode. For example, bolts or screws of stainless steel may be used for fastening aluminum sheet, but not the reverse since stainless steel is cathodic to the aluminum.
- b. For couples that serve as an electrical connection, a compatible conductive gasket or washer shall be interposed between the dissimilar faying surface materials prior to fastening, or the cathodic member shall be plated with a material compatible with the anode.
- c. For couples that are not serving as an electrical conductor, a non-absorbing, inert gasket material or washer shall be interposed between the dissimilar conductive materials prior to connecting them. For immersion applications, full electrical isolation with insulating sleeves and washers on the bolts is required along with an insulating spacer. In addition, an applicable organic coating shall be applied to the faying surfaces prior to assembly. In situations where large faying surfaces are involved, insert a barrier layer such as sprayable or pre-cured urethane sealants which retain flexibility and are resistant to shrinking, aging and cracking.
- d. All faying edges shall be sealed to preclude the entrance of liquids.
- e. For joints that are not required to be electrically conductive, applicable corrosion- inhibiting compounds shall be applied under the heads of bare, previously plated, and treated fasteners that are inserted into dissimilar conductive material surfaces.
- f. Where practicable, or where it will not interfere with the proposed use of the assembly, the external joint shall be adequately protected using protection schemes (e.g., sealants, primers, coatings, etc.) sufficient for the level of exposure they will experience or sufficient for the environment in which they are required to operate and approved by the CEA or appropriate design authority.
- g. Welded or brazed dissimilar conductive material assemblies shall be coated with a paint system or other suitable protective coating that covers the weld and all heat-affected zones.
5.11.1
Humidity controlled environments. Materials in protective, humidity-controlled environments require careful protection. The assumption that corrosion will not occur because humidity control will be maintained must also take into account the end-use environment of the
material. Approval from the CEA or appropriate design authority is required to lessen the stringency of the protective requirements of the couple. Humidity and moisture controlled environments can only be ensured by the use of hermetically-sealed compartments or containers. The moisture vapor content must be reduced to the level at which water will not condense inside the container at the lowest temperature expected in service. If humidity and condensation cannot be consistently controlled, then the dissimilar conductive material contacts shall be protected in accordance with the worst potential environment.