Galvanic processes occur between different metals and between different areas of the same metal in the water environment. Water is an electrolyte, a poorly conductive one at the low dissolved solids content of fresh waters, and a highly conductive one at the high dissolved solids content of sea water.
When two different metals are immersed in an electrolyte and connected through a metallic path, current will flow. Oxidation occurs at the anode and reduction (normally oxygen reduction) occurs at the cathode. These reactions and the hydrogen reduction reaction that occurs in deaerated waters are represented in the usual form below.
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Oxidation (corrosion)
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M → M+ + 2e
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Reduction (deaerated waters)
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O2 (dissolved) + 2H2O + 4e → 4(OH-)
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Reduction (deaerated waters)
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2H+ + 2e → 2e
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The electrons flow through the metal path from the anode to the cathode. The circuit is completed by transport (migration) of the ionic species (OH) from the vicinity of the cathode to the vicinity of the anode. In the absence of other species, the rate at which these reactions occur, and consequently the rate at which the anode corrodes, is controlled by the rate at which oxygen can be reduced at the cathode.
The rate of reduction of oxygen at the cathode in turn is determined primarily by the resistance to electron flow in the circuit, the cathodic surface area available for oxygen reduction and the amount of oxygen available at the cathodic area. The galvanic current (corrosion) is directly proportional to the cathodic area when the cell is under cathodic control as it normally is in water.
Conductivity plays a major role by limiting galvanic corrosion to the immediate area of contact in low conductivity fresh water and by spreading the galvanic effect over rather large areas in highly conductive waters such as seawater.
Painting the anode requires all of the anodic corrosion to occur in the very small areas where coating breakdown at scratches, welds, etc. occurs and exposes the steel. Painting the cathode reduces the area available for the rate-controlling oxygen reduction reaction and the amount of oxidation (corrosion) that can occur at the anode. Stainless steels are positioned towards the cathodic end of the galvanic series in seawater.
The potential range shown for each alloy should be interpreted as the range within which the metal to sea water potential is likely to vary for each alloy, not as an indication that alloys close to each other in the series are likely to change position. This rarely happens. The second more anodic potential band for stainless steels should be interpreted as the potential that develops in a shielded area where crevice corrosion has initiated.
Lee and Tuthill have developed quantitative guidelines for the amount of carbon steel or Ni-Resist required suppressing crevice corrosion of types 304 and 316 stainless steels in seawater.
These data indicate that carbon steel is very effective in suppressing crevice corrosion of 304 and 316 stainless steel in up to 100:1 SS to CS area ratios at 14°C (57°F) in seawater. At 28°C there is complete protection at 10:1 and a tenfold reduction in the percentage of sites where initiation occurs at 50:1 SS:CS area ratios. Ni-Resist (NR) is found to provide full protection for 316 and substantial protection for 304 at 50:1 SS: NR area ratios at temperatures up to 28°C.
The position of the copper alloys in the galvanic series suggests that copper alloys will not suppress crevice corrosion in stainless steels and, in fact, may accelerate crevice attack once it has started. Experience indicates copper alloys provide no useful galvanic protection for stainless steels.
Type 304 is the least noble of the nickel stainless steels and alloy 825 the most noble, being separated by about 0.05 volt. The various nickel stainless steels are generally coupled mechanically to each other and to nickel-base alloys without serious galvanic effects. There are two major qualifications:
- Should type 316L inadvertently be welded with type 308L filler metal instead of 316L, the weld metal will suffer severe localized corrosion. Hard facing overlays for rotating seal faces and weld overlay of tail shafts are other applications where close attention must be given to the position of individual alloys with respect to each other in the galvanic series in order to avoid costly failures.
- Both type 303 and 303Se suffer extraordinarily severe corrosion in seawater. The high density of manganese sulfide or selenide inclusion in these free-machining alloys create a surface with numerous built in austenite-to-inclusion galvanic cells.
Carbon filled rubber O-rings and gaskets are widely used in contact with stainless steels in seawater. The corrosion that occurs under O-rings and black rubber gaskets is normally crevice corrosion. However, in some instances acids used for chemical cleaning have softened these carbon-filled rubbers sufficiently to release carbon and set up adverse galvanic cell action, greatly accelerating the crevice attack that occurs in these rubber-to-stainless crevices.
Galvanic corrosion is a two way street and the effect on the other material coupled to stainless steels must always be considered. Investigating why copper alloy tube sheets were being so severely corroded when copper alloy condensers were retubed with stainless steel and titanium tubes, Gehring, Kuester and Maurer found that the whole inside surface area of the tube of the more noble alloys became effective as a cathode in copper alloy to stainless steel or titanium couples.
The more noble materials are so easily polarized that the cathodic area available for the reduction reaction (the rate controlling process) is multiplied far beyond the old two or four diameters rule of thumb, which was based on copper alloy behavior. Later work by Gehring and Kyle indicated that the intensity of the galvanic effect decreased with salinity. The increasing resistance of the lower salinity waters limits the effective cathodic area.
Galvanic corrosion can occur between different constituents of the same metal as well as between different metals. Iron embedded in the surface of stainless steel, manganese sulfide stringers and less highly alloyed weld metal are common examples.
Summary of galvanic corrosion of nickel stainless steels
- Galvanic corrosion occurs between weld metal and base metal, between different areas of the same metal and between different metals in water.
- The intensity of galvanic corrosion is determined by the conductivity, oxygen content and the effective anode-to-cathode area ratio.
- Galvanic effects are spread over a large area in brackish and seawater; are confined to the immediate area of the junction in fresh water; and are often negligible in deaerated brines.
- Steel, Ni-Resist, zinc and aluminum are very effective in suppressing crevice corrosion on stainless steels except types 303 and 303Se.
- Carbon, graphite-lubricated gaskets, packing, greases etc. are very effective in initiating severe corrosion of stainless steels.
- Galvanic effects can be significantly reduced by removing coatings from the anode and by coating the more noble (cathodic) material.
- Stainless steel (or titanium) tubing increases copper alloy tube sheet attack to the point where impressed current cathodic protection is normally required to control tube sheet corrosion.
- Galvanic corrosion between different grades of nickel stainless steels mechanically joined is rare, but can be severe when welded. Caution and exposure tests are suggested.
- Avoid use of types 303 and 303Se.
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