Steels adherent rust

The addition of small amounts of alloying materials greatly improves corrosion resistance to atmospheric environments but does not have much effect against liquid corrosives. The alloying elements produce a tight, dense adherent rust film, but in acid or alkaline solutions corrosion is about equivalent to that of carbon steel. However, the greater strength permits thinner walls in process equipment made from low-alloy steel. 

Low-alloy steels, which contain about 2-3% of alloying elements, commonly copper, chromium and nickel. These steels still rust, but under certain conditions in the atmosphere, the rust formed becomes adherent and protective so that the corrosion rate becomes several times less rapid than with the ordinary steels mentioned above. These steels are often termed weathering steels. 

Fig. 3.3 Sulphate in adherent rust on steel exposed at Battersea in January (after Chandler and Stanners )...

Misawa et al. [32] proposed that an inner cohesive protective rust film is formed on low-alloy steels after long atmospheric exposure (industrial or urban). It consists of amorphous 5-FeOOH, the formation of which is catalyzed by copper and phosphorus on the steel surface alternate drying and wetting favors its protective qualities. Keiser et al. [33] confirmed that the typical inner adherent rust layer consists mostly of 5-FeOOH. 

We mentioned earlier that a careful quantitative characterization of the iron oxides and oxyhydroxides present in the rust layers allows (i) determining the protective ability of the rust layers, (ii) calculating the atomic fraction (a) of iron species coming from the steels that after the corrosion processes form part of the adherent rust, and (iii) understanding the mechanisms associated with the deterioration processes. 

Next, in order to calculate a, we need to measure the mass of the steel before the corrosion test, m (S), the mass of the steel after the corrosion test taking into consideration the mass of the adherent rust, mf(S +AR), and the mass of the steel after the corrosion test but without the rust, that is, the rust completely removed, mj(S). a can be calculated using the following formula [18] ... 

Exposure to most atmospheres results in a corrosion rate that becomes stabilized in 3-5 years. Over this period is formed a protective film or patina that is dark brown to violet. The patina is a tightly adhering rust formation on the surface of the steel that cannot be wiped off. Since the formation of this film is dependent on pollution in the air, in rural areas where there is little to no pollution a longer time may be required to form this film. In areas that have a high pollution level of SO., loose particles are formed with a much higher corrosion rate. This film of loose particles offers little or no protection against continued corrosion. 

In contrast to the thin, firmly adhering rust on the low-sulphur steel samples, the rust that forms on the sulphur-rich samples is loose and fiill of thick blisters. Apparently there is an increased formation of sulphate esters on these samples, whereby the sulphur is from the steel, not from the SO2 in the air as is the case with the sulphate esters observed in atmospheres containing SO2. 

Another way to protect a metal uses an impervious metal oxide layer. This process is known as passivation, hi some cases, passivation is a natural process. Aluminum oxidizes readily in air, but the result of oxidation is a thin protective layer of AI2 O3 through which O2 cannot readily penetrate. Aluminum oxide adheres to the surface of unoxidized aluminum, protecting the metal from further reaction with O2. Passivation is not effective for iron, because iron oxide is porous and does not adhere well to the metal. Rust continually flakes off the surface of the metal, exposing fresh iron to the atmosphere. Alloying iron with nickel or chromium, whose oxides adhere well to metal surfaces, can be used to prevent corrosion. For example, stainless steel contains as much as 17% chromium and 10% nickel, whose oxides adhere to the metal surface and prevent corrosion. 

Stainless steels are iron-based alloys that contain a minimum of approximately 11 % Cr, the amount needed to prevent rusting. Few stainless steels contain more than 30% Cr or less than 50% Fe. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface him. This oxide forms and heals itself in the presence of oxygen. (Krysiak)14... 

Once in dry dock, the outside of the ship was pressure-washed with mains water to remove marine growths attached to the steel plates as well as the loosely adherent corrosion products. This was repeated several times to assist in the removal of chloride ions from the rust layers. Approximately 11 tons of debris were removed from the external structure of the ship by this process. Several sections of the steel plates were found to have very thin areas less than 1 mm... 

---