Principles of
111
Part A | 6.1
6. Principles of Marine Corrosion
Robert E. Melchers
Seawater is an aggressive environment for materials particularly in warmer and polluted waters. In
this environment, steel infrastructure, such as offshore platforms, pipelines, and tanks may corrode
quickly, and for engineering and management
purposes, the rate at which this occurs is of interest. Here, recently developed understanding
of the corrosion processes over extended exposure periods is reviewed and recently developed
science-based mathematical models for the prediction of corrosion and pitting of structural steels
are described. The effect of various influencing
factors is reviewed, including that of seawater
quality and the effect of microorganisms on corrosion. Also, short discussions of the corrosion of
stainless steels, aluminum, and copper-nickels,
all materials that are used under marine exposure
conditions, are given. This is followed by a description of recent observations of the corrosion of
steel reinforcing bars in reinforced concrete, another widely used construction material in marine
environments. It shows that reinforced concrete
6.1 Chemical and Physical Composition
of Seawater .......................................... 111
6.2 Materials Used in Marine Environments.. 113
6.3 Marine Corrosion of Steel....................... 113
6.4 Modeling Longer Term Corrosion of Steel 116
6.5 Other Influences on Steel Corrosion........ 118
6.6 Pitting Corrosion of Steel ....................... 119
6.7 Some Other Important Materials ............ 121
6.7.1 Stainless Steel.............................. 121
6.7.2 Aluminum ................................... 121
6.7.3 Copper-Nickels ............................ 121
6.7.4 Reinforced Concrete ..................... 121
6.8 Conclusion ............................................ 123
References................................................... 123
can be a highly successful construction material,
even in long term and under marine conditions,
but that severe loss of reinforcement may occur
without obvious external signs detectable by visual
inspection.
Seawater is a hostile environment – not only for people.
Often it is considered to be the most severe environment to which materials can be exposed. Warmer waters
accelerate the rate of corrosion, and warmer climate
seawater is particularly aggressive. This was found to
be a major problem during the later part of the Pacific campaigns in the Second World War when military
equipment was found to corrode unexpectedly quickly
in the Tropics [6.1]. However, even in colder waters,
corrosion can be severe, as described in the following.
6.1 Chemical and Physical Composition of Seawater
On average, both the chemical and physical compositions of seawater are surprisingly uniform in the oceans
around the world (Table 6.1). There are also similar
small variations with depth, even for very deep waters [6.3]. The annual variation in water temperature
(which typically lags about 3 months behind the seasons) is largely responsible for cyclic changes in many
parameters, such as dissolved oxygen (DO) in seawater.
It is also responsible in the variation in biological components, including the microbiological components,
such as algae, archaea, and bacteria [6.4].
In enclosed seas and estuaries, and generally closer
to shore, there are usually deviations in average water quality from those in open sea conditions. This
applies particularly to nutrients, such as organic carbon, nitrogenous materials, phosphates, and sulfates.
Typically, these variations are caused, in the natural environment, by natural, usually cyclic, annual processes
111
Part A | 6.1
6. Principles of Marine Corrosion
Robert E. Melchers
Seawater is an aggressive environment for materials particularly in warmer and polluted waters. In
this environment, steel infrastructure, such as offshore platforms, pipelines, and tanks may corrode
quickly, and for engineering and management
purposes, the rate at which this occurs is of interest. Here, recently developed understanding
of the corrosion processes over extended exposure periods is reviewed and recently developed
science-based mathematical models for the prediction of corrosion and pitting of structural steels
are described. The effect of various influencing
factors is reviewed, including that of seawater
quality and the effect of microorganisms on corrosion. Also, short discussions of the corrosion of
stainless steels, aluminum, and copper-nickels,
all materials that are used under marine exposure
conditions, are given. This is followed by a description of recent observations of the corrosion of
steel reinforcing bars in reinforced concrete, another widely used construction material in marine
environments. It shows that reinforced concrete
6.1 Chemical and Physical Composition
of Seawater .......................................... 111
6.2 Materials Used in Marine Environments.. 113
6.3 Marine Corrosion of Steel....................... 113
6.4 Modeling Longer Term Corrosion of Steel 116
6.5 Other Influences on Steel Corrosion........ 118
6.6 Pitting Corrosion of Steel ....................... 119
6.7 Some Other Important Materials ............ 121
6.7.1 Stainless Steel.............................. 121
6.7.2 Aluminum ................................... 121
6.7.3 Copper-Nickels ............................ 121
6.7.4 Reinforced Concrete ..................... 121
6.8 Conclusion ............................................ 123
References................................................... 123
can be a highly successful construction material,
even in long term and under marine conditions,
but that severe loss of reinforcement may occur
without obvious external signs detectable by visual
inspection.
Seawater is a hostile environment – not only for people.
Often it is considered to be the most severe environment to which materials can be exposed. Warmer waters
accelerate the rate of corrosion, and warmer climate
seawater is particularly aggressive. This was found to
be a major problem during the later part of the Pacific campaigns in the Second World War when military
equipment was found to corrode unexpectedly quickly
in the Tropics [6.1]. However, even in colder waters,
corrosion can be severe, as described in the following.
6.1 Chemical and Physical Composition of Seawater
On average, both the chemical and physical compositions of seawater are surprisingly uniform in the oceans
around the world (Table 6.1). There are also similar
small variations with depth, even for very deep waters [6.3]. The annual variation in water temperature
(which typically lags about 3 months behind the seasons) is largely responsible for cyclic changes in many
parameters, such as dissolved oxygen (DO) in seawater.
It is also responsible in the variation in biological components, including the microbiological components,
such as algae, archaea, and bacteria [6.4].
In enclosed seas and estuaries, and generally closer
to shore, there are usually deviations in average water quality from those in open sea conditions. This
applies particularly to nutrients, such as organic carbon, nitrogenous materials, phosphates, and sulfates.
Typically, these variations are caused, in the natural environment, by natural, usually cyclic, annual processes