Paint, against rust

The anchors, hangings, and other metallic parts must be protected against rust (galvanized or mst-preventing paint). 

Although zinc and paint are applied to steel for the same purpose—to protect the steel against rust—they behave in completely different ways. Zinc coatings corrode from the outer surface in toward the steel and provide cathodic protection, which ensures that no rust forms between the zinc coating and the steel (see Fig. 1.8). Conventional paint coatings are often destroyed by the formation of rust at the boundary layer between the paint coat and the... 

On offshore platforms, duplex systems (galvanizing plus a three-coat non-saponifiable paint system) have protected against rust for 10-20 years. A test site on Heligoland run by a German steel association provides more data (Schwenk and Friehe, 1972). 

Traditionally, paint has provided the most economical method for protecting steel against corrosion. However, as people who live in the Midwest know well, paint cannot prevent a car from rusting indefinitely. Eventually, flaws develop in the paint that allow the ravages of rusting to take place. 

Thin coatings not only are used for mechanical protection and lubrication, but are widely applied to protect against corrosion and chemical reaction [47-49]. The steel body of a car is first covered by a zinc layer, which is further treated in a phosphate bath, to promote the adhesion of the paint. Damaging this sandwich structure, by breaking one of these layers, results in quick corrosion of the car body rust. The improvement in the quality of this protective coating during the last 20 years is shown by the increase of the duration of the corrosion warranty proposed by car manufacturers (from 2 years less than 15 years ago, to 8 years at the present time). 

Chem. Descrip. Diethanolamine-free alkanolamine alkanoate Uses Corrosion inhibitor for aq. systems, metalworking and lubricant industries resist, against flash rusting in cleaning and metalworking applies, and in latex paint coatings... 

Built in 1959, the dike at Delfzijl was painted green (epoxy coating) to match the grass. The wall protects the town against flooding by the sea. An inspection made in 1991 showed the zinc coating to be still 80-120 xm thick. No rust had appeared after 32 years. 

In the previously described work, low levels of lead were found in the rust layer near the paint-rust interface, within 30 tm of the rust-paint interface. Thomas suggests that because lead salts do not appear to reach the metal substrate to inhibit the anodic reaction, it is possible that lead acts within the rust layer to slow down atmospheric corrosion by interfering with the cathodic reaction (i.e., by inhibiting the cathodic reduction of existing rust [principally FeOOH to magnetite]) [33], This presumably would suppress the anodic dissolution of iron because that reaction ought to be balanced by the cathodic reaction. No conclusive proof for or against this theory has been offered. 

Chilled iron shot or grit can be used for the removal of rust, mill scale, heat treatment scale, and old paint from forged, cast, and rolled steel. This abrasive breaks down gradually against steel substrates, so continual sieving to retain only the large particle sizes may be needed if a rough surface profile is desired in the cleaned surface. 

High-pressure water jetting with abrasive injection Pressurized water (up to 20,000 psi) is directed against the surface to be cleaned. Abrasive is metered into the water stream to facilitate the removal of rust and mill scale and to improve the efficiency of paint removal. Disposable abrasives are used. 

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