Corrosion passive iron oxide layer

The passive iron oxide layer is destroyed when pH is reduced to about 11.0 or below, causing the porous oxide layer (rust) to form during corrosion. 

It was only in 1992-1993 that we finally found out after an in-depth evaluation of the interactions between various metal surfaces and coatings of polyaniline (applied as pure dispersion or as dispersion paints) that together with a remarkable corrosion potential shift (ermobling) and an iron oxide layer formation (passivation) lead to a significant anticorrosion effect [71]. In a study together with Elsenbaumer et al. [72], we discovered that the corrosion rate was reduced by a factor of up to 10,000. The iron oxide that formed between the metal surface and the polyaniline primer coating was determined to be Fe203, later confirmed with even clearer x-ray photoelectron spectra (XPS) [23b]. 

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. 

The corrosion of iron is one of the most widespread and technologically important examples of metallic corrosion. In the presence of water and oxygen, the corrosion of iron proceeds to form a complicated mixture of hydrated iron oxides and related species a complete description is beyond the scope of the present discussion, and the interested reader is referred to the previously cited general references on corrosion as well as to the well known descriptions of electrochemical equilibria in aqueous solution given by Pourbaix (8, 9,). Iron is a base metal, subject to corrosion in aqueous solutions. In the presence of oxidizing species, iron surfaces can be passivated by the formation of an oxide layer if the oxide layer formed is Imperfect, rapid corrosion may occur. In simplest form, the reaction of iron to form iron oxide can be written as ... 

Hatva, T. (1989) Iron and manganese in ground-water in Finland Occurrence of glacifluvial aquifers and removal by biofiltration. Publ. Water environmental research institute. Nat. Board Waters Envir., Finland, No. 4, 99 p. Haupt, S. Strehlow, H.H. (1987) Corrosion layer formation and oxide reduction of passive iron in alkaline solution A combined electrochemical and surface analytical study. Langmuir 3 837-885... 

Sometimes it happens that incursion of oxoanion reduction in place of hydrogen evolution as the cathodic reaction in the corrosion of iron leads, not to an increased rate of corrosion, but to a drastic retardation. This is because strongly oxidizing conditions (e.g., in concentrated nitric acid) can force the immediate oxidation of iron to iron(III), rather than via the persistent iron(II) intermediate (as described in Sections 16.1 and 16.2), so that an insoluble iron (III) oxide layer forms at once on the anodic and cathodic surfaces alike and the iron becomes passivated (Section 16.3). Michael Faraday s demonstration of this phenomenon is instructive ... 

Corrosion-inhibitive properties of the compound Na2P03F have been tested by Andrade et al., either by incorporating it in a mortar or as a penetrant[68]. This compound, which is currently available as a proprietary product, is reported to act as an anodic inhibitor, possibly with some cathodic action. The minimum required ratio of phosphate to chloride was suggested as 1 1. The mechanism of action of this admixture is to stabilize the passive layer of iron oxide on the steel and also increase the density of concrete, thus decreasing the permeability... 

For passivation treatments other than scale removal following thermal treatment, less aggressive acid solutions are usually employed. The primary purpose of these treatments is to remove contaminants that may be on the component s surface and could prevent the formation of the oxide layer locally. The most common contaminant is imbedded or free iron particle from forming or machining tools. Mechanical polishing can be employed to provide a uniform surface finish and to remove these contaminants. The polishing materials should be used for stainless only as they can carry over small particulates from one part to the next. In addition, the work-hardened state of this fine particulate, even from a stainless vessel, can have a lower threshold for corrosion and act as an initiation site if not removed. A dilute (10%) solution of nitric acid is... 

After a first sweep towards the positive which is not shown in the diagram and which is dominated by the dissolution of the airfoimed oxide layer, a sweep in the positive direction starts at the negative potential end of the cathodic part of the curve. In the first part, from A to the corrosion potential B where the curve becomes anodic, Hj evolution is the most important process. In this region both samples are very similar. The corrosion potential at B is nearly the same for unimplanted, with Cr implanted and with Ar bombarded iron. From B to C the anodic dissolution of the metal takes place and at C the active to passive transition starts. Here one observes the most significant difference between the two samples. The critical current density for passivation of implanted iron is more than one order of... 

Etching of iron by the feedback mode of the SECM has recently been reported by Still and Wipf (26). They produced localized corrosion at passivating iron surfaces by generating chloride ions at an SECM tip. Here is a case in which the metal is covered with an oxide layer that needs to be removed in order to facilitate metal dissolution. These experiments are described in great detail in Chapter 12. 

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. 

Because there is no overlap of the domains of stability for water and iron, iron is normally susceptible to corrosion in the presence of water and oxygen. At sufficiently positive potential and high pH, passivation can be achieved by formation of a protective oxide layer (anodic protection). Immunity from corrosion can be achieved... 

It is possible for the passivation (oxide) layer on the surface of a metal to be continuously removed or not allowed to develop, by erosion from particulate matter or gas bubbles. Not only is the surface eroded but the removal of the protective oxide layer allows corrosion to take place. The problem is accentuated by the presence of an obstruction or debris, on the metal surface that diverts and accelerates the flow near the surface along a defined path. Sato et al [1977] report experimental data on erosion-corrosion resistance of condenser tubes fabricated from various cupro nickel alloys. They suggest that high iron bearing cupro nickels are superior in respect of erosion corrosion by clean sea water. 

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... 

Other metals also can be "passivated." For example the presence of 12 percent chromium in steel renders the steel passive in an oxygenated environment because it promotes the formation of a thin but tightly-bound oxide layer. The presence of chromates in the electrolyte will encourage the formation of y-FeaOa on iron surfaces, isolating the surfaces from corrosion. 

Other microorganisms promote corrosion of iron and its alloys through dissimilatory iron reduction reactions that lead to the dissolution of protective iron oxide/hy dr oxide films on the metal surface. Passive layers are either lost or replaced by less stable films that allow further corrosion. Obuekwe and coworkers [60] evaluated corrosion of mild steel under conditions of simultaneous production of ferrous and sulfide ions by an iron-reducing bacterium. They reported extensive pitting when both processes were active. When only sulfide was produced, initial corrosion... 

---