Corrosion protection natural passivation

It is known that thin (-20 A) passive films form on iron, nickel, chromium, and other metals. In s ressive environments, these films provide excellent corrosion protection to the underlying metal. The structure and composition of passive films on iron have been investigated through iron K-edge EXAFS obtained under a variety of conditions, yet there is still some controversy about the exact nature of... 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

Muster, T.H. and Cole, I.S., The protective nature of passivation films on zinc Surface charge. Corrosion Sci., 46, 2319, 2004. 

Nickel and its alloys spontaneously form passive oxide films upon exposure to the atmosphere at ambient temperature. This film helps to provide very useful corrosion resistance in a variety of environments. The protective nature of this film can be enhanced by specific alloying additions. 

The free corrosion potentials of these molybdenum-free CrNi steels in natural and aerated seawater are within the range Uh = 0.2 V to 0.4 V. These steels are sufficiently passive to avoid serious general corrosion, but they must be integrated in the structural corrosion protection measures due to their sensitivity to pitting and crevice corrosion. This is often realised automatically since the steels have electrically conductive connections to low-alloyed steels or other less noble materials. 

The use of SEM-EDS techniques provide good insight into the surface corrosion products grown on Al-Mg-Si alloy during the immersion test in seawater with and without the natural products as corrosion inhibitors. The SEM results indicate that the natural products (NH, VL and TS) absolutely minimized the corrosion products on the specimen surfaces. They also protect the passive film from dissolution in aggressive solution like seawater. 

Provide a passive film of Fe304 on the metal surface or a chemical film. The corrosion of water wall and economizers also takes place depending on the protective nature of Fe304 film (3Fe + 4H2O Fe304 + 4H2 t)- The chemical films are formed by application of film forming inhibitors, like amines and morpholine. 

When protective films are easily formed on the metal surface by exposure to air, the metal is naturally in the passive state and corrosion resistant. Many metals, however, have to be protected by alloying with one of the naturally passive metals, e.g. chromium or aluminium. In other words, the naturally passive metals have very low passivation current they form their protective coating rather quickly when current is applied to them. When they are alloyed with corroding metals, they lower the passivation current of the latter and help to form a pore free inhibitive oxide layer. 

The primary role of coatings in corrosion protection is to retard or prevent the diffusion of corrosive solution to the metal beneath the coating. Sol-gel coatings have the ability to imdeigo chemical reaction with natural oxide/hydroxide films on the metal substrate (Fig. 12.5 and Table 12.1) and passivate the metal surface. The hydroxyl group of hydrolyzed metal alkoxides (M(OH)x (OR)n-x or M-OH M = Si, Ti, Zr) in coating sol forms a covalent bond with the hydroxyl group of natural oxide/hydroxide layers on the metal surface. The chemical... 

Chlorides, which are ubiquitous in nature, play an important role in the corrosion of metals. Chlorides and other anions also play an important role in locali2ed corrosion, ie, the breakdown of the insoluble protective reaction product films, eg, passive films, that prevent corrosion of the underlying metal. A variety of mechanisms attempting to explain the role of chloride in general and in locali2ed corrosion have been proposed (23—25). 

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

Corrosion products formed as thin layers on metal surfaces in either aqueous or gaseous environments, and the nature and stability of passive and protective films on metals and alloys, have also been major areas of XPS application. XPS has been used in two ways, one in which materials corroded or passivated in the natural environment are analyzed, and another in which well-characterized, usually pure metal surfaces are studied after exposure to controlled conditions. 

Many passive metals suffer pitting attack when aggressive ions (usually chloride) enter the system. It is possible to forestall pitting, or to stop it once started, using cathodic protection. It is not necessary to polarise to the protection potential of the metal a negative shift of 100 mV from the natural corrosion potential in the environment will often be sufficient. This technique has been applied to various stainless steels and to aluminium . The philosophy is not unlike that applied to rebar in concrete. 

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. 

If dissimilar metals are placed in contact, in an electrolyte, the corrosion rate of the anodic metal will be increased, as the metal lower in the electrochemical series will readily act as a cathode. The galvanic series in sea water for some of the more commonly used metals is shown in Table 7.4. Some metals under certain conditions form a natural protective film for example, stainless steel in oxidising environments. This state is denoted by passive in the series shown in Table 7.4 active indicates the absence of the protective film. Minor... 

---