Chemical processing facilities are inherently corrosive places. Vast budgets are set aside to combat the effects of corrosion and depending on the size of the plant, can be a losing battle.
According to Wikipedia…Rust is an iron oxide, a usually red oxide formed by the redox reaction of iron and oxygen in the presence of water or air moisture. Several forms of rust are distinguishable both visually and by spectroscopy, and form under different circumstances. Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3).Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate. Surface rust is flaky and friable, and it provides no protection to the underlying iron, unlike the formation of patina on copper surfaces. Rusting is the common term for corrosion of iron and its alloys, such as steel.
No amount of painting will make up for the damage that corrosion does to the parent material. Painting can also give you a false sense of security when applied to tubular materials as the outside may look like new whereas the inner surface which is not visible can be nearing a point of failure.