CORROSION OF METALS
Weather Factors Affecting Corrosion
of Metals
Study of the atmospheric corrosion of metals has been an important activity of ASTM for most of its history
[1-4],2 and of many other private and public agencies [5,6]. Information has been obtained of corrosion loss
of numerous metal samples exposed at sites throughout the world. It is therefore ironic that in spite of this
volume of accumulated data and experience it is possible to predict the performance of any metal in a particular
application in only a very limited way. Unexpected failures resulting from corrosion continue to be a serious
problem. One of the main reasons why the results of atmospheric corrosion testing cannot be used directly for
design purposes may be seen from data collected by Larrabee and Ellis and compiled by ASTM Committee
G-1, Subcommittee IV [7]. Variation in the corrosion rate of steel for 46 sites is very great (Norman Wells to
Kure Beach 80-ft lot, 1: 1570); the ratio Head, Building Materials Section, Division of Building Research,
National Research Council of Canada, Ottawa, Ont., Canada. The italic numbers in brackets refer to the list
of references appended to this paper for corrosion rates of steel and zinc varies from 2 for Norman Wells to
118 for Kure Beach. It should be realized that the corrosion rate for a given metal at a given site will depend
upon the time of year or even upon the particular weather conditions when the exposure test is initiated [a].
Many studies measuring and correlating various weather factors with corrosion rates have attempted to
account for these variations and to improve prediction [9-121. It was realized, however, that if reasonable
correlations were found it would be somewhat fortuitous because other factors in the system were nct being
accounted for, especially the effect of corrosion products. Figure 1 attempts to represent the system involved
in the prediction of atmospheric corrosion and identifies the area to be dealt with in the present paper. It is
clear that complete predictability would have to be based on understanding of the total system, including
quantitative interrelations of the many factors. This ideal is far from being realized, but it does not follow that
the designer must wait for the final stage before he can use accumulated experience and data to assist him in
the selection of materials to be exposed to outdoor environment. This paper assesses to what extent
measurement of weather factors has advanced the prediction of the corrosion behavior of metal and what needs
to be done in the future.
Moisture:
It is over 40 years since Vernon [I31 found that only beyond a "critical humidity" will rapid acceleration of
corrosion occur. The significance of this fact was not fully appreciated until a method was developed for
measuring the percentage of time when this critical humidity is exceeded [9]. This period is called the time-
, of-wetness. Subsequent study has shown [lo-161 that it is the most important factor promoting atmospheric
corrosion of metals.
Because of this it is necessary to have a strict definition of time-of-wetness. It should be realized that what is
being attempted is a definition in terms of conditions of relative humidity that will result in an adequate film
of water on a metal surface to facilitate the highest rate of electrochemical reaction. This condition is obviously
influenced by surface contaminants (soluble ions which depress vapor pressure) and the nature of the corrosion
products that may render the surfaces hydroscopic or provide pores into which water may condense. Vernon
[I31 showed that in the presence of 0.01 percent sulfur dioxide (SOz) iron shows a sharp increase in corrosion
rate at 60 percent relative humidity (RH) for constant humidity conditions, but that for increasing humidity
the sharp increase in the rate begins in the range 70 to 80 percent RH. He also showed that the critical humidity
is different for different metals. Tomashov 1171 suggested classification of atmospheric corrosion by the
degree of dampness of the corroding surface. He postulated, first, that under visible moisture films or highly
wetted corrosion products the corrosion process proceeds with predominantly cathodic control, and second,
that under conditions of thin adsorbed films (below 100 percent RH) control is predominantly by the anodic
process. Although it must be accepted that the effect of humidity on the corrosion process is very complex, it
is reasonable to expect that levels of relative humidity can be designated to define the interval during which
metal corrodes at a high rate. The approach taken by the author to define this level is based on measurement
of the potential developed between platinum and zinc under normal atmospheric conditions [18,19]. Time-of-
wetness is the interval during which this potential exceeds 0.2 V. In the strictest sense it might be argued that
it should apply only to the atmospheric cor rosion of zinc, but adequate correlation of steel [lo] and steel
copper and zinc [11,12] indicates that this measurement can be used for other metals. Guttman [I21 used
measuring equipment developed by the author in studying corrosion of zinc and showed that the time-of-
wetness measured by this instrumentation corresponded to the time during which humidity exceeded 86.5
percent, based on 4-year averages. The author has measured time-of-wetness at a number of exposure sites
(Table 1). These results were compared with the results of RH measurements compiled to show the durations
of intervals of humidity. A computer analysis has been carried out of meteorological data col lected by the
Department of Environment, Atmospheric Environment Service, for the period 1957 to 1966 to provide the
percentage duration of the different levels of RH for 112 stations across Canada. Data for a selected number
Weather Factors Affecting Corrosion
of Metals
Study of the atmospheric corrosion of metals has been an important activity of ASTM for most of its history
[1-4],2 and of many other private and public agencies [5,6]. Information has been obtained of corrosion loss
of numerous metal samples exposed at sites throughout the world. It is therefore ironic that in spite of this
volume of accumulated data and experience it is possible to predict the performance of any metal in a particular
application in only a very limited way. Unexpected failures resulting from corrosion continue to be a serious
problem. One of the main reasons why the results of atmospheric corrosion testing cannot be used directly for
design purposes may be seen from data collected by Larrabee and Ellis and compiled by ASTM Committee
G-1, Subcommittee IV [7]. Variation in the corrosion rate of steel for 46 sites is very great (Norman Wells to
Kure Beach 80-ft lot, 1: 1570); the ratio Head, Building Materials Section, Division of Building Research,
National Research Council of Canada, Ottawa, Ont., Canada. The italic numbers in brackets refer to the list
of references appended to this paper for corrosion rates of steel and zinc varies from 2 for Norman Wells to
118 for Kure Beach. It should be realized that the corrosion rate for a given metal at a given site will depend
upon the time of year or even upon the particular weather conditions when the exposure test is initiated [a].
Many studies measuring and correlating various weather factors with corrosion rates have attempted to
account for these variations and to improve prediction [9-121. It was realized, however, that if reasonable
correlations were found it would be somewhat fortuitous because other factors in the system were nct being
accounted for, especially the effect of corrosion products. Figure 1 attempts to represent the system involved
in the prediction of atmospheric corrosion and identifies the area to be dealt with in the present paper. It is
clear that complete predictability would have to be based on understanding of the total system, including
quantitative interrelations of the many factors. This ideal is far from being realized, but it does not follow that
the designer must wait for the final stage before he can use accumulated experience and data to assist him in
the selection of materials to be exposed to outdoor environment. This paper assesses to what extent
measurement of weather factors has advanced the prediction of the corrosion behavior of metal and what needs
to be done in the future.
Moisture:
It is over 40 years since Vernon [I31 found that only beyond a "critical humidity" will rapid acceleration of
corrosion occur. The significance of this fact was not fully appreciated until a method was developed for
measuring the percentage of time when this critical humidity is exceeded [9]. This period is called the time-
, of-wetness. Subsequent study has shown [lo-161 that it is the most important factor promoting atmospheric
corrosion of metals.
Because of this it is necessary to have a strict definition of time-of-wetness. It should be realized that what is
being attempted is a definition in terms of conditions of relative humidity that will result in an adequate film
of water on a metal surface to facilitate the highest rate of electrochemical reaction. This condition is obviously
influenced by surface contaminants (soluble ions which depress vapor pressure) and the nature of the corrosion
products that may render the surfaces hydroscopic or provide pores into which water may condense. Vernon
[I31 showed that in the presence of 0.01 percent sulfur dioxide (SOz) iron shows a sharp increase in corrosion
rate at 60 percent relative humidity (RH) for constant humidity conditions, but that for increasing humidity
the sharp increase in the rate begins in the range 70 to 80 percent RH. He also showed that the critical humidity
is different for different metals. Tomashov 1171 suggested classification of atmospheric corrosion by the
degree of dampness of the corroding surface. He postulated, first, that under visible moisture films or highly
wetted corrosion products the corrosion process proceeds with predominantly cathodic control, and second,
that under conditions of thin adsorbed films (below 100 percent RH) control is predominantly by the anodic
process. Although it must be accepted that the effect of humidity on the corrosion process is very complex, it
is reasonable to expect that levels of relative humidity can be designated to define the interval during which
metal corrodes at a high rate. The approach taken by the author to define this level is based on measurement
of the potential developed between platinum and zinc under normal atmospheric conditions [18,19]. Time-of-
wetness is the interval during which this potential exceeds 0.2 V. In the strictest sense it might be argued that
it should apply only to the atmospheric cor rosion of zinc, but adequate correlation of steel [lo] and steel
copper and zinc [11,12] indicates that this measurement can be used for other metals. Guttman [I21 used
measuring equipment developed by the author in studying corrosion of zinc and showed that the time-of-
wetness measured by this instrumentation corresponded to the time during which humidity exceeded 86.5
percent, based on 4-year averages. The author has measured time-of-wetness at a number of exposure sites
(Table 1). These results were compared with the results of RH measurements compiled to show the durations
of intervals of humidity. A computer analysis has been carried out of meteorological data col lected by the
Department of Environment, Atmospheric Environment Service, for the period 1957 to 1966 to provide the
percentage duration of the different levels of RH for 112 stations across Canada. Data for a selected number
