Page 171 - CW E-Magazine (24-6-2025)

Special Report

Organic protective coatings – failures & protection

IntroductIon mance is, in most cases, due to poor Dr. S.K. ChaKravorty
rganic coatings provide a rela- application and surface preparation. Consultant (Plant Engineering)
tively thin barrier film between Email: chakravorty4410@gmail.com
Osubstrate material and the Surface preparation
environment. Paints, varnishes, lacquers, Surface preparation involves sur- very little extra cost by selecting a very
and similar coatings undoubtably pro- face roughening to obtain mechanical good top-coat material.
tect more metal from corrosion on a bonding (teeth) as well as removal of
tonnage basis than any other method. dirt, rust, mill scale, oil, grease, welding The coating thickness must be such
Exterior surfaces are most familiar, flux, crayon marks, wax, and other that no bare metal is exposed. It is al-
but interior coatings or linings are also impurities. In other words, a clean, rough most impossible to apply one coat of
widely utilised. Approximately $2-bn surface is needed. The best method is to paint and have it completely free of
is spent in the US each year on organic grit blast or sand blast the steel surface. pinholes or other defects. Multiple
coatings. A myriad of types and pro- Other methods are pickling and other coats are needed so a pinhole in one
ducts are involved, and some are accom- types of chemical treatments, scraping, coat is covered by a complete film of
panied by outlandish claims. Substan- wire brushing, flame cleaning, chisel- another. Thickness is of importance also
tial knowledge of this complex field is ling, and chipping. In addition to eco- because paint deteriorates or weathers
required for successful performance. nomic considerations, the selection of with time.
The best procedure for the uninitiated surface preparation method depends
is to consult with a reputable producer upon the metal to be painted; the shape, Many companies let all of their
of organic coatings. As a general rule, size and accessibility of the structure; paint jobs to outside contractors on a
these coatings should not be used where the coating system; and the service bid basis. This may result in low first
the environment would rapidly attack conditions. cost, but it is usually more costly in
the substrate material. For example, a the long run. If a plant has sufficient
paint would not be used to line the Selection of primer work, it should hire and train its own
inside of a tank car for shipping hydro- Primers can contain rust inhibited painters(2).
chloric acid. One defect or a small area pigments such as zinc chromate and
of exposed metal would result in rapid zinc dust and thereby provide another Failures and defects
perforation. An evaluation test program function in addition to acting as barriers. Coatings are a complex combi-
is also recommended(1). Wettability is needed so that crevices nation of raw materials that must be
and other surface defects will be mixed, applied to a prepared substrate,
Factors for organic coatings filled rather than bridged. Short dry- and dried and cured correctly to per-
Aside from proper application, ing times are advantageous to preclude form to their maximum capability.
the three main factors to consider for contamination before the top-coats are They must protect the substrate from
organic coatings, listed in order of applied, particularly in field applica- the damaging effects of rain, sunlight,
importance, are tions. wind, heat, cold, humidity, and oxygen
Surface preparation; in various combinations and cycles
Selection of proper primer or priming Selection of top-coat and still retain their integrity and often
coat; and Top-coat selection is important. their aesthetic qualities. Some coat-
Selection of top-coat or coats. Use of cheap paints is false economy ings, such as those applied to ships,
because the majority of the cost of a paint also must be resistant to abrasion, in
If the metal surface is not properly system is in application. Many times, the case of cargo hold coatings, and
prepared, the paint may peel off paint is applied primarily for appea- cyclic changes of chemicals and tank
because of poor bonding. If the primer rance – it might be cheaper to provide cleaning, in the case of tank linings.
does not have good adherence or is not corrosion allowance by making the Therefore, it is not surprising that those
compatible with the top-coat, early fail- steel thicker in the first place. An impor- coatings can suffer from premature
ure occurs. If the first two factors are tant point here is that good appearance failure and/or exhibit defects that may
wrong, the system will fail regardless and good corrosion protection, even in or may not result in failure. It is not
of the top-coat used. Poor paint perfor- severe atmospheres, can be obtained at generally straightforward to establish

Chemical Weekly June 24, 2025 171

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