Carbon steel pipes and vessels are often required to transport water or are submerged in water to some extent during service. This exposure can be under conditions varying temperature, flow rate, pH, and other factors, all of which can alter the rate of corrosion. The relative acidity of the solution is probably the most important factor to be considered. At low pH the evolution of hydrogen tends to eliminate the possibility of protective film formation so that steel continues to corrode but in alkaline solutions, the formation of protective films greatly reduces the corrosion rate. The greater alkalinity, the slower the rate of attack becomes. In neutral solutions, other factors such as aeration, became determining so that generalization becomes more difficult.

The corrosion of steels in aerated seawater is about the same overall as in aerated freshwater, but this is somewhat misleading because the improved electrical conductivity of seawater can lead to increased pitting. The concentration cells can operate over long distance, and this leads to a more nonuniform attack than in fresh water. Alternate cycling through immersion and exposure to air produces more pitting attack than continuous immersion. The effect of various alloying addition and exposure conditions on the corrosion behavior is shown in Table 1.

Table 1. Comparison of results under different type of exposure

Effects of alloy selection, chemical composition and alloy additions	Sea air	Freshwater	Alternately wet with seawater or Spray and dry	Continuously wet with seawater
Ferrous alloys	Pockmarked	Vermiform on cleaned bars	Pitting, particularly on bars with scale	Pitting, particularly on bars with scale
Wrought iron versus carbon steel	Steel superior to wrought and ingot irons	Iron and steel equal in low-moor areas	Low-moor iron superior to carbon steel	Low-moor iron superior to carbon steel
Sulfur and phosphorus content	Best results when S and P are low	Best results when S and P are low	Best results when S and P are low	Apparently little influence
Addition of copper	Beneficial: Effect increasing with copper content	Beneficial: 0.635% Cu almost as good as 2.185% Cu	Beneficial: 0.635% and 2.185% Cu much the same	0.635% Cu slightly beneficial: 2.185% Cu somewhat less so
Addition of nickel	3.75% Ni superior even to 2% Cu; 36% Ni almost perfect after 15-year exposure	3.75%Ni superior even to 2%Cu; 36%Ni excellent resistance	3.75%Ni beneficial usually more so than Cu: 36%Ni the best metal in the set	3.75% Ni slightly beneficial and slightly superior to Cu: 36% Ni the best metal in the set
Addition of 13.5% Cr	Excellent resistance to corrosion: cold blast metal perfect after 15-year exposure: equal to 36% Ni steel	Excellent resistance to corrosion: equal to 36% Ni steel	Subject to severe localized corrosion that virtually destroys the metal	Subject to severe localized corrosion that virtually destroys the metal
Behavior of cast irons	Excellent resistance to corrosion: cold blast metal superior to hot: no graphitic corrosion	Undergoes graphitic corrosion	Undergoes graphitic corrosion	Undergoes graphitic corrosion

Interestingly, the corrosion rates of specimens completely immersed in seawater do not appear to depend on the geographical location of the test site; therefore, by inference, the mean temperature does not appear to play an important role.

This constancy of the corrosion rate in seawater has been attributed to the more rapid fouling of the exposed steel by marine organisms, such as barnacles and algae, in warmer seas. It is further speculated that this fouling offsets that increases expected from the temperature rise.