376
Fundamentals of Corrosion
may be done if needed but avoid using abrasive discs or other nonmetallic
scouring pads that have been used to remove paints that contained copper or
mercury anti-fouling compounds. Wood or plastic scrapers are preferred.
The most common causes of galvanic corrosion problems of aluminum in
ships are placing an aluminum deckhouse on a steel support and attaching
steel or copper-based alloys to the hull or the piping, or both. These more noble
metal fittings should be electrically insulated from the aluminum; even so,
waster-plate practice is recommended. Cathodic protection is recommended
where copper alloys are involved, even though the copper alloy is electrically
insulated from the aluminum, because there is a degrading effect from the
simple proximity of the copper alloy to the aluminum alloy.
Unlike steel, aluminum cannot be made thermodynamically stable in seawater by cathodic protection methods. Nevertheless, cathodic protection is
highly effective because it counteracts any tendency toward localized hydrolytic acidification with its attendant breakdown and passivity. Only zinc(MIL A-18001) and certain aluminum-based galvanic anodes are permitted;
magnesium anodes and mercury-bearing aluminum anodes are prohibited
for aluminum hulls.
Cleanliness is always important, as both solid particles and grease marks
invite localized hydrolytic acidification and consequent local film breakdown.
Regular freshwater washdown of decks and bilges is helpful in removing
solid debris and in keeping down the concentration of chloride, which plays
an essential role in hydrolytic acidification.
Paint coatings play an important role in controlling hull corrosion. It is
standard practice to have a 2-mil primer, followed by a 2-mil antifouling
paint containing either copper or mercury. An organotin compound is presently the preferred toxicant in antifouling paint for aluminum hulls.
Where there are threaded bolts or fittings of aluminum exposed to seawater, 50% zinc dust in petrolatum can be used on the threads as an antiseize lubricant. Graphite-, lead-, or tin-bearing lubricants must be avoided.
Stainless steel lock nuts and washers are usually acceptable on such aluminum components.
Standard stipulations for aluminum alloys for marine atmosphere service
call for “stress-corrosion-resistant alloys,” without, however, specifying the
test. Where cadmium-plated bolts or nuts are used with aluminum alloy
parts, they are preferably isolated from the aluminum with aluminum alloy
washers under the bolt head and nut, but cadmium-plated washers may
be used for bolts under high tension. The usual precautions against stress
concentration (i.e., shot preening and stress relieving) are recommended
where feasible to minimize stress corrosion. In highly textured alloys, a rule
of thumb is to keep working stress in the longitudinal direction less than
50% of the yield, 35% in the long transverse direction, and 25% in the short
transverse direction. Aluminum-hulled boats should be provided with nonconducting mooring lines when next to steel ships and piers, and insulating
camels should be used.
Fundamentals of Corrosion
may be done if needed but avoid using abrasive discs or other nonmetallic
scouring pads that have been used to remove paints that contained copper or
mercury anti-fouling compounds. Wood or plastic scrapers are preferred.
The most common causes of galvanic corrosion problems of aluminum in
ships are placing an aluminum deckhouse on a steel support and attaching
steel or copper-based alloys to the hull or the piping, or both. These more noble
metal fittings should be electrically insulated from the aluminum; even so,
waster-plate practice is recommended. Cathodic protection is recommended
where copper alloys are involved, even though the copper alloy is electrically
insulated from the aluminum, because there is a degrading effect from the
simple proximity of the copper alloy to the aluminum alloy.
Unlike steel, aluminum cannot be made thermodynamically stable in seawater by cathodic protection methods. Nevertheless, cathodic protection is
highly effective because it counteracts any tendency toward localized hydrolytic acidification with its attendant breakdown and passivity. Only zinc(MIL A-18001) and certain aluminum-based galvanic anodes are permitted;
magnesium anodes and mercury-bearing aluminum anodes are prohibited
for aluminum hulls.
Cleanliness is always important, as both solid particles and grease marks
invite localized hydrolytic acidification and consequent local film breakdown.
Regular freshwater washdown of decks and bilges is helpful in removing
solid debris and in keeping down the concentration of chloride, which plays
an essential role in hydrolytic acidification.
Paint coatings play an important role in controlling hull corrosion. It is
standard practice to have a 2-mil primer, followed by a 2-mil antifouling
paint containing either copper or mercury. An organotin compound is presently the preferred toxicant in antifouling paint for aluminum hulls.
Where there are threaded bolts or fittings of aluminum exposed to seawater, 50% zinc dust in petrolatum can be used on the threads as an antiseize lubricant. Graphite-, lead-, or tin-bearing lubricants must be avoided.
Stainless steel lock nuts and washers are usually acceptable on such aluminum components.
Standard stipulations for aluminum alloys for marine atmosphere service
call for “stress-corrosion-resistant alloys,” without, however, specifying the
test. Where cadmium-plated bolts or nuts are used with aluminum alloy
parts, they are preferably isolated from the aluminum with aluminum alloy
washers under the bolt head and nut, but cadmium-plated washers may
be used for bolts under high tension. The usual precautions against stress
concentration (i.e., shot preening and stress relieving) are recommended
where feasible to minimize stress corrosion. In highly textured alloys, a rule
of thumb is to keep working stress in the longitudinal direction less than
50% of the yield, 35% in the long transverse direction, and 25% in the short
transverse direction. Aluminum-hulled boats should be provided with nonconducting mooring lines when next to steel ships and piers, and insulating
camels should be used.