Passivated barrier film

The corrosion resistance of high-silicon cast iron is attributed to the development of a thin passive barrier film of hydrated oxides of silicon on the metal surface. This film develops with time due to the dissolution of iron from the metal matrix, which leaves behind silicon that hydrates due to the presence of moisture. Any flaws in the barrier film will reduce its effectiveness. 

Passivity the state of a metal in which a low corrosion rate is brought about by reaction with its environment under a high anodic driving force through formation of a surface barrier film, usually an oxide. 

Although orthophosphates are themselves passivating, anodic inhibitors (and also cathodic inhibitors, forming a calcium phosphate barrier film), the film strength is weak, even in simple HW systems and they are not used for this purpose. Nevertheless, despite the thermal instability of sodium hexametaphosphate and other polyphosphates, phosphates in general have several important properties that make them useful in boiler plant operations. These properties include ... 

Since one major motivation behind the use of plasma BPSG was to provide an improved passivation barrier, the better crack resistance is an advantage, but the greater sodium penetration is a negative. Therefore, it is not clear if it would be advantageous to make this replacement11 for a final passivation film. Its use as an inter-metallic dielectric may be more useful. 

Many metals are resistant to corrosion due to a compact adherent oxide film which acts as a barrier between the metal and its environment. Aluminium and stainless steels are examples of such metals. Often, the properties of such a barrier film will depend on the environment in which the metal is situated. For example iron in seawater does not produce corrosion products which are protective but in dilute carbonate solutions with no or very low chloride concentrations, a passive complex Fe(0H)2/FeC03 layer is formed [19, 20]. 

The electrochemical oxidation of the nickel is of special interest since it is a typical passivation metal in which very thin passive oxide films of a few nm thickness on the surface can cover the substrate metals efficiently. The passive oxide layer on the nickel was studied by Sikora and Mac Donald [118] who claimed that the passive film consisted of the inner nickel oxide of a barrier layer and an outer Ni(OH)2 porous or hydrated layer, in which the inner layer behaves as a p-type oxide with a cation vacancy. Oblonsky and Devine measured the surface enhanced Raman spectra of the nickel passivized in a neutral borate solution and estimated the amorphous Ni(OH)2 in the passive potential region and the NiOOH in the higher transpassive region [119]. Further, the passive films formed in the acidic and neutral solutions were assumed as partially hydrated nickel oxide [120,121]. The anodic film formed in the alkaline solution was assumed to be Ni(OH)2 in the... 

The enhanced corrosion resistance of phosphated steel can be attributed to two phenomena the increased paint adherence and the chemical passivation of the metal surface by an insulating barrier film of phosphates. A difference between phosphate and chromate conversion coatings is that the later are thought to function as... 

Cathodic precipitates increase cathodic site passivity with precipitation of insoluble compounds. Frequendy used cathodic precipitation inhibitors are CaCOs, MgCOs, or zinc sulfates that precipitate as Zn(OH)2. The efficiency of these inhibitors is only controlled by pH adjustment. Calcium carbonate (limestone) dissolves in water as calcium bicarbonate Ca(HC03)2- Carefid pH control forms smooth and hard calcium carbonate barrier films. Once the precipitate is formed, pH must be carefully controlled to avoid film dissolution at lower pH values ... 

Nitrites are environmentally fiiendly anodic inhibitors. They form a passive film with ferric oxide and inhibit the corrosion of copper, nickel, and tin alloys in alkaline environments (pH levels 9-10), but aggressive ions such as chloride and sulfate ions attack and destroy the barrier film. They reduce the rate of anodic dissolution on steel as shown in Fig. 14.10 [61]. Nitrites are used only in closed systems because they oxidize to nitrates in the presence of oxygen. They are not as efficient inhibitors as chromates. 

There is no question on either viewpoint that a diffusion-barrier film is the basis of passivity of many metals that are passive only by Definition 2. Examples of protective films that isolate the metal from its environment are (a) a visible lead sulfate film on lead immersed in H2SO4 and (b) an iron fluoride film on steel immersed in aqueous HF. 

In line with the theory of passivators just described, transition metals are those expected and found to be inhibited best by passivators their anodic polarization curves have the shape shown in Fig. 17.1, allowing passivity to be established and then maintained at low current densities. A lesser degree of inhibition can be obtained with the nontransition metals, such as Mg, Cu, Zn, and Pb, using, for example, chromates. Protection of these metals apparently results largely from formation of relatively thick diffusion-barrier films of insoluble metal... 

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