Corrosion: Effects of Temperature and Humidity on Metals

This article discusses the effects of temperature and humidity on the corrosion of metals. It explains how moisture, high relative humidity, and high temperatures are essential factors in corrosion. The article also provides solutions to prevent corrosion and protect metals from damage.

CORROSION
This a natural process that leads to the destruction of materials, especially metals by a
chemical or electrochemical reaction with the environment. The chemical reaction involves
metals reacting with atmospheric moisture and oxygen to form a coating called metal oxide.
Effect of Temperature and humidity on the corrosion of metals.
Essential factors in corrosion are moisture, high relative humidity, or high level of
temperatures. In the absence of these factors, metal surfaces would have little or no corrosion.
In humid areas, condensation takes place during the night. The moisture remains on the
surface, it reacts with the metal surfaces and becomes basic, or it reacts with carbon dioxide
forming a dilute acid.
Temperature
Temperature affects the rate of corrosion in two ways. The first increase in temperature
increases the rate of corrosion. Second, the visible effect is always the lag of metal surfaces
and objects caused by large heat capacities, behind changes in the temperature.
At night when the temperature is low, the metals become warmer than the surrounding, and
they do not condense until after the saturation point has. The temperature of the surrounding
starts rising, the remaining heat of the metal acts as a coolant; hence, some moisture remains
on the surfaces.
The moistures stay longer than the time air takes to saturate hence causing variation with the
width of the metal surfaces, atmospheric humidity, relative humidity, and radiation.
The temperature has resulted in severe corrosion on metal surfaces in the tropics. This is
because the dew point of an atmosphere shows the equilibrium conditions of evaporation and
condensation from a metal surface.
Solution
It’s highly advisable to maintain the temperatures at 10 to 15 degrees above the dew point to
always ensure that no corrosion will occur by condensation on a metal surface that would be
colder than the surrounding environment.
Relative Humidity.
Humidity is the amount of moisture present in the atmosphere. The conditions are necessary
for corrosion to take place in the presence of an electrolyte that can be able to react when

exposed to high levels of humidity, forming a coating on the surface of the metal. The coating
layers can be easily noticed; however, the particles present reach high concentrations,
especially when there is a fluctuation in humidity levels.
Humidity level always varies depending on the nature and reactivity of the metal surface.
Example, if the surrounding is free of contaminants, the humidity level for iron is 60 %.
When electrolytes are present corrosion occurs at the same levels of anode and cathode
reactions. The anodic reaction leads to the corrosion of metals surfaces, while the cathode
reaction always acts as the oxygen reducing response, as shown in the figure below.
Marine environments always have high levels of humidity. Studies have also shown that the
size of zinc surfaces absorbed by water increases with relative humidity and also that
corrosion increases with the thickness of the absorbing layer. Presence of magnesium
chloride on metal can make a surface wet at 34% humidity while sodium chloride on the
same surface requires 77% relative humidity to create the same effect.

Conclusion
Temperature plays a vital role in the corrosion of metals. The higher the temperature, the
higher rates of corrosion on metal. This is possible because electromagnetic reactions
generally occur and react faster under high temperatures. So at high levels of temperatures,
metals react with oxygen to form oxides. Oxides are always chemical compounds that contain
at least some component of oxygen accelerated by temperature.
Under high humidity levels, corrosions occur at a faster rate to metal than under dry
conditions. This is because the moisture available in the air reacts with the oxygen present at
the cathode and takes up the electrons produced by a corroding metal or surface at the anode.
If moisture is not present, then this reaction cannot occur; therefore, corrosion ceases.
Example in 1930 Vernom was the first to investigate the effects of humidity on the corrosion
of metals, and his results are shown in the figure below.
From the above graph, it can be seen that corrosion increases with humidity. In most
conditions, when humidity is below the figure above, corrosion is always at minimal levels.
Clean steel surfaces and materials should not still be exposed to the high level of moisture to
avoid corrosion levels.

Solutions.
From our study, the reinforcement in the glider of a bridge which was constructed in
1970 is exposed to high humidity levels that range from 47.5 at the lowest to 79.9 most
top in just a month of May. It can also be noticed that in July, the humidity levels range
from the most moderate 51.9 to 90.4 as the highest. With these high levels of humidity,
rates of corrosion can increase rapidly because moisture acts as acceleration for a reaction to
occur at the anode and the cathode. To prevent corrosion, it's highly advisable to use
protective coatings such as paint. Paint always inhibits the transfer of chemical reaction
charges from the corrosive liquid to the metals present. It's a cost-effective method to use.
Also, Applying coatings that involve covering the metal with another metal that can oxidize
due to its low reaction levels when exposed to high temperatures and humidity can help
reduce corrosion in places with high humidity. They are two methods of sacrificial coating,
cathode protection, and anodic protection. Cathode protection involves coating iron with zinc
through a process called galvanizing while anodic protection consists of coating iron steel
with a less reactive metal such as Tin that cannot corrode easily.
The bridge metals from our case were corroded because of high temperatures. From the
table given in the month, May its shows temperature ranged from as low as 27.1 to as
high as 38.5 under such conditions corrosion occurs at a very high rate than normal
temperature levels. The same case happens in July the temperature levels range from
26.0 to 38.3; this causes high rates of corrosion at fast speeds. A 10 degrees rise of
temperature or more can cause high rates of corrosion. Metals in high-temperature places can
have design Modification. This can help in reducing the levels of corrosion and also in
improving the durability of the existing protective coatings. The designs are always meant to
reduce the amount of dust and water trapped, also to encourage the movement of air and
avoid open holes or surface. Metals such as aluminum can also be used because they can
always reduce the levels of corrosion.

Reference.
[1] Vernom, W H J, 'Trans Faraday Soc' 31 (1), 668, 1935, pp. 34-40
[2] R.A. Rapp, in Encyclopaedia of Materials: Science and Technology, 2001, pp. 89-100
[3] P.Y. Hou, in Reference Module in Materials Science and Materials Engineering,
2016, pp.29-35
[4] A.S. Khanna, in Handbook of Environmental Degradation of Materials (Second Edition),
2012, pp. 100-121
[5] W. GAO, Z. Li, in Developments in High-Temperature Corrosion and Protection of
Materials, 2014, pp. 56-65
[6] A Popova, E Sokolova, S Raicheva, M Christov - Corrosion science,2003 Elsevier,
PP, 100-110
[7] M Esmaily, M Shahabi-Navid, JE Svensson… - Corrosion …, 2015 – Elsevier, pp.102
116
[8] XX Ye, H Ai, Z Guo, H Huang, L Jiang, J Wang, Z Li… - Corrosion …, 2016 – Elsevier
pp. 156-166
[9] OØ Knudsen, A Forsgren - 2017 - content.taylorfrancis.com, pp. 157-164