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SS 316 High performance fittings > Technical Reference > Corrosion Control - Galvanic Table


  Technical Reference
Corrosion Control - Galvanic Table
 
GALVANIC TABLE FOR SEA WATER
Nobe (Least Active or Cathodic)
Platinum
Gold
Graphite
Hastelloy C
Titanium
Carpenter 20 (passivated)
TSS 316 Stainless Steel (passivated)
T304 Stainless Steel (passivated)
17-4PH Stainless Stele (passivated)
Monel 400
Silver
Inconel 600
G-Bronze
70-30 Copper-Nickel
80-20 Copper-Nickel
90-10 Copper-Nickel
Silicon Bronze
Aluminum bronze
Admiralty brass
Yellow brass
Nickel
Naval brass
Manganese bronze
Nickel (plated)
Tin
Lead
Copper
Cast iron
Mild steel
Aluminum 2024
Cadmium
Alclad
Aluminum 6053
Galvanized steel
Zinc
Magnesium alloys
Magnesium
Anodic (most active)
Source: MIL-STD-889.
Galvanic corrosion is an electrochemical action of two dissimilar metals in the presence of an electrolyte (moisture) and an electron conductive path. It occurs when dissimilar metals are in contact. A low energy electric current flows from the Cathode to the Anode causing corrosion to the Anode.
It is recognizable by the presence of a buildup of corrosion at the joint between the dissimilar metals. For example, when steel alloys (mild or plated) are in contact with stainless steel, galvanic corrosion can occur and accelerate the corrosion of the steel.
Galvanic Series
A "galvanic series" applies to a particular electrolyte solution. For each specific solution which is expected to be encountered for actual use, a different order or series will ensue.
Galvanic series relationships are useful as a guide for selecting metals to be joined, will help the selection of metals having minimal tendency to interact galvanically, or will indicate the need or degree of protection to be applied to lessen the expected potential interactions.
Generally, the closer one metal is to another in the series, the more compatible they will be, i.e., the galvanic effects will be minimal. Conversely, the farther one metal is from another, the greater the corrosion will be.
Precautions for Joining Dissimilar Metals
When it becomes necessary that metals widely separated in the galvanic series must be assembled, the following precautions should be considered to minimize galvanic corrosion.
1. Sacrificial - by applying to the cathodic member a sacrificial coating having a potential similar to or near that of the anodic member. If you are designing for a sacrificial element, the sacrificial element should be on the anodic side and smaller. This is not recommended under any circumstances for pressure-bearing components like fittings.
2. Sealing - by sealing to insure that faying surfaces are water-tight and will not come into contact with the electrolyte (sea water).
3. Resistance - by painting or coating all surfaces to increase the resistance of the electrical circuit. When considering this approach with fittings, be aware of differing hardness properties of metals and the increased likelihood of galling dissimilar metals with installation torque. Also consider the friction caused during installation and the tendency to remove protective coatings from less noble materials.
Note: many materials classified as non-metallic will initiate corrosion of metals to which they are joined, e.g., cellulosic reinforced plastics, carbon or metal loaded resin materials, asbestoscement composites.
Design metal couples so that the area of the cathode is smaller (appreciably) than the area of the anodic metal. For example, stainless steel fittings installed in a large aluminum manifold block will corrode more slowly than the reverse combination.
Interpose a compatible metallic gasket or washer between the dissimilar metals prior to fastening.
Plate the cathodic member with a metal compatible to the anode.
Apply corrosion-inhibiting pastes or compounds under heads of screws or bolts inserted into dissimilar metal surfaces whether or not the fasteners had been previously plated or otherwise treated. In some instances, it may be feasible to apply an organic coating to the faying surfaces prior to assembly.