Why not paint over rusty steel?


Writer:  Ron Knight (Chemist)

The very simple answer is that it just won’t work.  Understanding why it won’t work is a little more complicated.

Both steel and rust (iron oxide) are compounds of the element iron (Fe).  Steel is made from Fe and a variety of other elements including carbon, manganese and/or nickel. Iron oxide is the result of the reaction of iron and oxygen, therefore, both iron and steel will produce iron oxide when exposed to water and oxygen.

The term “density” refers to the physical weight of a specific volume of material.  Density is usually listed in grams per cubic centimeter.  The density of iron is 7.874, steel is 7.60 and that of rust is 5.242 (all in grams per cubic centimeter).  Please note that steel is less dense than iron and iron oxide is less dense than steel.  Keeping in mind that the volume (container size) remains constant and that all of the elements added to iron are less dense than iron, the new compounds (steel and/or iron oxide) have to be less dense than the iron alone.  The reason that steel in only 3.5% less dense than iron while iron oxide is 45% less dense than steel is that iron oxide is basically steel plus absorbed oxygen and the oxygen has very little weight.

Now comes the tricky part.  In nature absolute confined spaces do not exist so when all of the additional elements are added to a fixed amount of iron the volume of the compound has to increase (rust takes up more than three times the volume of the of the original iron). The result of this increase in volume can have several results.  If the layer of rust is not covered with a coating such as paint, the rust will create flakes of rust and when these flakes get large enough they will fall off (exfoliate) and the result will be a decrease in metal (usually thickness) and the strength of the metal will be reduced.  If the layer of rust is covered with a coating (especially a hard coating) such as paint, the metal will continue to rust and grow in volume.  This continued growth under the coating will cause the coating (paint) to blister and flake off and the corrosion of the metal will continue as before it was painted.  Painting over the rust creates another (and avoidable) problem.  The paint will have to be removed prior to treating the rusty metal surface. The paint will have to be removed by wire brushing and/or particle blasting which will create an added expense of labor for the cleanup and disposal.

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