Principles of Marine Corrosion 6.7 Some Other Important Materials 121
Part A | 6.7
cur also under corrosion products, under washers and
fasteners, and under marine growth. While the general
concepts are well understood, quantitative estimation of
the depth and extent of pitting under these conditions
awaits model development similar to that for general
corrosion. This includes developing relationships between seawater nutrient content and the likely depth of
the extreme pits.
6.7 Some Other Important Materials
6.7.1 Stainless Steel
Stainless steels often are considered the ideal solution
to corrosion problems involving seawater conditions.
This is correct for exposures in the atmosphere, particularly for marine grade stainless steel such as grade
316. In the atmosphere, the high chromium content of
316 ensures that the exterior of the steel is always covered with a highly resistant oxide film. Any steel that
is exposed to the atmosphere as a result of scratches
or damage to the oxide film quickly oxidizes to form
more protective oxide film. Under aggressive conditions, such as near the coast, or inside swimming pool
halls, a small amount of corrosion may occur, leaving
the so-called tea-staining – small areas of brown rust
stains. In the tidal zone, the oxide film is somewhat
less effective and under immersion conditions, it can be
quite ineffective, with long-term corrosion approaching
that of mild steel. The reason is that marine biofilms
and marine biological growth can inhibit the recovery
of oxide film, even though the water may be highly oxygenated. Particularly, under fasteners such as bolt heads
or nuts, or under marine growth (Fig. 6.10), localized
or pitting corrosion can be severe [6.45]. Also, other
grades of stainless steel, such as 304, contain lower
amounts of chromium and usually are much less resistant to atmospheric marine corrosion than is 316.
Conversely, a range of highly alloyed steels is available, ostensibly with very good corrosion resistance.
In many cases, longer term experience and quantitative
data are lacking, although behavior similar to Fig. 6.5
has been proposed [6.46]. At this time, the use of stainless steel under conditions where rapid oxidation may
not be possible requires expert advice. Stainless steels
are considerably more expensive than mild and low alloy steels.
6.7.2 Aluminum
A variety of aluminum alloys are available and these
can have quite different corrosion characteristics [6.47].
In general, however, they can be summarized by noting that in air aluminum alloys form a protective oxide
that, similar to that for stainless steel, is self-healing
when damaged. As a result, aluminum tends to be
very resistant to atmospheric corrosion. In seawater,
it forms a noticeable gray oxide and is known to be
subject to pitting. The latter is not always superficially
obvious since the pits tend to undercut the metal surface [6.48]. The corrosion resistance of aluminum in
freshwater environments is complicated by the fact that
it is pH dependent, with lower and higher pH waters being very aggressive. Unlike steels, including stainless
steel, the available evidence indicates that aluminum
is not subject to microbiological attack under seawater
conditions [6.49]. Again, the application of aluminum
alloys requires expert advice.
6.7.3 Copper-Nickels
There is a long tradition of using copper-nickels and
various brasses for fittings on naval vessels and for
smaller bore pipework. The common grades are 90 W 10
and 70 W 30 copper-nickels, with the former being both
less resistant to corrosion and cheaper than the latter, reflecting the cost of the nickel component. Both
general corrosion and pitting tend to be relatively low,
but copper-nickels are prone to microbiologically influenced corrosion, despite the fact that copper is used
in antifouling treatments as a biocide. A considerable
body of literature exist that deal with the practical
aspects of corrosion of copper–nickel alloys for infrastructure applications [6.14, 15, 50] and with their
resistance to microbiologically influenced corrosion.
6.7.4 Reinforced Concrete
Reinforced concrete structures are used in many marine locations. Local economics of material availability
and the easy of construction compared to steel structures as well as maintenance issues usually are decisive
in the choice. Reinforced concrete structures are generally considered to have good resistance to marine
exposure conditions, largely because the steel reinforcing bars are protected to some degree by the concrete.
Typically, the steel bars are located 3050 mm or more
inside the concrete. However, not all modern reinforced
concrete structures exposed to marine conditions perform well, and there are many cases where the concrete
has cracked along the steel bars, exposing them to sea-
Part A | 6.7
cur also under corrosion products, under washers and
fasteners, and under marine growth. While the general
concepts are well understood, quantitative estimation of
the depth and extent of pitting under these conditions
awaits model development similar to that for general
corrosion. This includes developing relationships between seawater nutrient content and the likely depth of
the extreme pits.
6.7 Some Other Important Materials
6.7.1 Stainless Steel
Stainless steels often are considered the ideal solution
to corrosion problems involving seawater conditions.
This is correct for exposures in the atmosphere, particularly for marine grade stainless steel such as grade
316. In the atmosphere, the high chromium content of
316 ensures that the exterior of the steel is always covered with a highly resistant oxide film. Any steel that
is exposed to the atmosphere as a result of scratches
or damage to the oxide film quickly oxidizes to form
more protective oxide film. Under aggressive conditions, such as near the coast, or inside swimming pool
halls, a small amount of corrosion may occur, leaving
the so-called tea-staining – small areas of brown rust
stains. In the tidal zone, the oxide film is somewhat
less effective and under immersion conditions, it can be
quite ineffective, with long-term corrosion approaching
that of mild steel. The reason is that marine biofilms
and marine biological growth can inhibit the recovery
of oxide film, even though the water may be highly oxygenated. Particularly, under fasteners such as bolt heads
or nuts, or under marine growth (Fig. 6.10), localized
or pitting corrosion can be severe [6.45]. Also, other
grades of stainless steel, such as 304, contain lower
amounts of chromium and usually are much less resistant to atmospheric marine corrosion than is 316.
Conversely, a range of highly alloyed steels is available, ostensibly with very good corrosion resistance.
In many cases, longer term experience and quantitative
data are lacking, although behavior similar to Fig. 6.5
has been proposed [6.46]. At this time, the use of stainless steel under conditions where rapid oxidation may
not be possible requires expert advice. Stainless steels
are considerably more expensive than mild and low alloy steels.
6.7.2 Aluminum
A variety of aluminum alloys are available and these
can have quite different corrosion characteristics [6.47].
In general, however, they can be summarized by noting that in air aluminum alloys form a protective oxide
that, similar to that for stainless steel, is self-healing
when damaged. As a result, aluminum tends to be
very resistant to atmospheric corrosion. In seawater,
it forms a noticeable gray oxide and is known to be
subject to pitting. The latter is not always superficially
obvious since the pits tend to undercut the metal surface [6.48]. The corrosion resistance of aluminum in
freshwater environments is complicated by the fact that
it is pH dependent, with lower and higher pH waters being very aggressive. Unlike steels, including stainless
steel, the available evidence indicates that aluminum
is not subject to microbiological attack under seawater
conditions [6.49]. Again, the application of aluminum
alloys requires expert advice.
6.7.3 Copper-Nickels
There is a long tradition of using copper-nickels and
various brasses for fittings on naval vessels and for
smaller bore pipework. The common grades are 90 W 10
and 70 W 30 copper-nickels, with the former being both
less resistant to corrosion and cheaper than the latter, reflecting the cost of the nickel component. Both
general corrosion and pitting tend to be relatively low,
but copper-nickels are prone to microbiologically influenced corrosion, despite the fact that copper is used
in antifouling treatments as a biocide. A considerable
body of literature exist that deal with the practical
aspects of corrosion of copper–nickel alloys for infrastructure applications [6.14, 15, 50] and with their
resistance to microbiologically influenced corrosion.
6.7.4 Reinforced Concrete
Reinforced concrete structures are used in many marine locations. Local economics of material availability
and the easy of construction compared to steel structures as well as maintenance issues usually are decisive
in the choice. Reinforced concrete structures are generally considered to have good resistance to marine
exposure conditions, largely because the steel reinforcing bars are protected to some degree by the concrete.
Typically, the steel bars are located 3050 mm or more
inside the concrete. However, not all modern reinforced
concrete structures exposed to marine conditions perform well, and there are many cases where the concrete
has cracked along the steel bars, exposing them to sea-