Passive Rust Coatings

Carbon steel lines should be pickled, passivated, and coated with rust inhibitor. 

Passivating is done with anti-rust coatings and primer paints containing chromates, which are also found in yellow or green pigmented paints, galvanized sheets, welding rods, and in solvents (Cronin 1980 Burrows 1983). Chromate dermatitis in the aircraft industry was first reported in 1944 (Hall 1944). 

A number of methods have been devised to protect metals from corrosion. Most of these methods are aimed at preventing rust formation. The most obvious approach is to coat the metal surface with paint. However, if the paint is scratched, pitted, or dented to expose even the smallest area of bare metal, rust will form under the paint layer. The surface of iron metal can be made inactive by a process called passivation. A thin oxide layer is formed when the metal is treated with a strong oxidizing agent such as concentrated nitric acid. A solution of sodium chromate is often added to cooling systems and radiators to prevent rast formation. 

Polyaniline s metallic character, its position just below silver towards the noble-metal end of the electromotive force series, and its redox behaviour mentioned above, also make it possible to use a polyaniline coating to protect metals against corrosion. Our investigations show that the plastic removes electrons from the metal and is thereby reduced, while iron, for example, is oxidised to Fe203, This oxide layer is not rust (which consists of a loose mixture of various oxides and hydroxides), but functions as a protective passivating layer, in other words prevents further... 

It is suggested that the redox process prevents the corrosion (i.e., formation of rust or hydrated iron oxides) by catalyzing the formation of passivating coating of Fe Oj via chemical reactions shown in Figure 1.55. However, above mechanism remained doubtful and represent chemical unbalance. Therefore, more systematic studies are required to probe exact anti-corrosion mechanism. The efforts have been made to form CP-based copolymers (to improve solubility) and blends (to provide both... 

Because paint is such a common means of combating corrosion, many paint companies add chemicals to inhibit corrosion. A professional society of industrial painters lists 20 rust inhibitors that are environmentally approved additives for paints. Although the specific chemical content may vary, most of these corrosion inhibitors contain one of the following ions phosphate, borosilicate, chromate, or phosphosilicate. The precise mode of corrosion inhibition depends on several factors, but all of these ions, when part of a paint coating, form compounds with oxidized iron that inhibit fiirther formation of rust. Much like an oxide coating on the surface of aluminum prevents ongoing corrosion, these coatings stop the corrosion of the iron after only a small amount of material has reacted. This process is called passivation. 

Once chloride induced corrosion has started it is very unlikely that a coating of any sort will stop it. There is sufficient moisture and oxygen within the concrete to generate the small amount of rust needed to crack the concrete and then the water and oxygen will have a free path to the steel surface. Once the steel is depassivated a coating will not restore its passivity,... 

Carbon steels rust when they are in contact with humid air (Chap. 8) and therefore they are usually protected by a coating. In aqueous solutions, their corrosion rate depends on the pH (Figure 12.6). At low pH, proton reduction takes place and the corrosion rate becomes higher as the pH decreases. In neutral solution, oxygen transport controls the rate of corrosion, which therefore does not vary with pH. Finally, in an alkaline solution steel passivates and the corrosion rate decreases to very low values. This explains for example, why steel reinforcements in concrete do not deteriorate as long as the pH stays high (pH > 13) but may rust if the pH in the concrete drops to a lower value because of carbonation reactions of cement. 

Stainless steels (so-called because they do not become stained with rust) are examples of alloy steels, i.e. ones that contain a d-block metal in addition to carbon. Stainless steels have a significant content of the alloy metal and are of high commercial value because of their resistance to corrosion. All contain at least 10.5% (by mass) of chromium, the minimum that renders the steel corrosion-resistant under normal aqueous conditions (i.e. in the absence of acid, alkali or pollutants such as chloride ions). The resistance to corrosion arises from the formation of a thin layer of Cr203 (wl3nm thick) over the surface of the steel. The oxide layer passivates (see Section 10.4) the steel and is self-repairing, i.e. if some of the oxide coating is scratched off, further oxidation of the chromium in the steel necessarily repairs the wound . 

Thomas looked for lead (as a constituent of lead azelate) at the steel-rust interface in an attempt to confirm this theory. Samples coated with lead-based paint were exposed for three years and then cross-sections were examined in a LAMMS however, lead was not detected at the interface. As Thomas points out, this finding does not eliminate the mechanism as a possibility lead could still be present but in levels below the 100 ppm detection limit of the LAMMS [30,31]. Appleby and Mayne have shown that 5 to 20 ppm of lead azelate is enough to protect pure iron [25]. The levels needed to protect rusted steel would not be expected to be so low, because the critical concentration required for anodic inhibitors is higher when chloride or sulphate ions are present than when used on new or clean steel [35]. Possibly, a level between 20 and 100 ppm of lead azelate is sufficient to protect the steel. Another point worth considering is that the amounts of lead that would exist in the passive film formed by complex azelates, suggested by Appleby and Mayne, has not been determined. The lead soaps/lead azelate theory appears to be the most likely mechanism to explain how red-lead paints protect rusted steel. 

In general, the higher the oxidation potential the lesser the tendency to corrode. However, some metals corrode less than other metal with higher redox potential. For example, chromium (—0,74 V), zinc (—0,76 V), titanium (—0,89 V), aluminum (—1,71 V) etc. withstand corrosion much better than iron (—0,42 V). This is due to the fact that the surface of these metals coats with an insoluble very thin layer, just a veil, of hard-bitten oxide not reactive at all that, at variance with rust, passivizes the surface blocking the prosecution of corrosion. Table 13.2 provides a synoptic picture of the standard potentials, the so called electrode potential, relative to oxidation reactions of various metals. The standard electrode potential, abbreviated as , is given in volts and is the measure of the potential of any individual metal electrode which is with solute at an elfective concentration of 1 mol/dm at 1 atm of pressure. These potentials are referred to a hydrogen electrode whose reference potential is assumed equal to zero. This is because it is not possible to measure experimentally the value of the dilference of potential Ay between an electrode and its solution as, for example, in the case of zinc reaction (13.16), because any device used for making the measurement must be inserted in the circuit with two electrodes of which one is put in contact with the metal electrode of interest and the other with the solution. Now, this second electrode creates necessarily another interface metal-solution and the potential difference provided by the system is that between the two metals, without any possibility to infer the absolute value of each of them. This is why it is necessary to introduce a reference electrode, which any other potential can be referred to. To... 

Corrosion-resistant steels should be passivated. No further finish is required to provide corrosion resistance to 300 series steel where tarnish, rust, or surface stain would be objectionable, the 400 series and precipitation hardening steels should be given additional protection by a suitable plating or, after passivation, receive one coat of zinc chromate primer followed by a suitable top coat. 

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