Oxide protective layer

If a compact film growing at a parabolic rate breaks down in some way, which results in a non-protective oxide layer, then the rate of reaction dramatically increases to one which is linear. This combination of parabolic and linear oxidation can be tenned paralinear oxidation. If a non-protective, e.g. porous oxide, is fonned from the start of oxidation, then the rate of oxidation will again be linear, as rapid transport of oxygen tlirough the porous oxide layer to the metal surface occurs. Figure C2.8.7 shows the various growth laws. Parabolic behaviour is desirable whereas linear or breakaway oxidation is often catastrophic for high-temperature materials. 

Tantalum is not resistant to substances that can react with the protective oxide layer. The most aggressive chemicals are hydrofluoric acid and acidic solutions containing fluoride. Fuming sulfuric acid, concentrated sulfuric acid above 175°C, and hot concentrated aLkaU solutions destroy the oxide layer and, therefore, cause the metal to corrode. In these cases, the corrosion process occurs because the passivating oxide layer is destroyed and the underlying tantalum reacts with even mild oxidising agents present in the system. 

Metals that produce protective oxide layers (such as stainless steels) are especially susceptible to crevice attack. The reduced oxygen concentration in the crevice inhibits repair of the protective oxide film. This is especially true if acidic anions are present, which further retards oxide repair. Stainless steels containing molybdenum are usually less susceptible to attack. 

Stainless steels tend to pit in acid solutions. Pits form local areas of metal loss associated with breakdown of a protective oxide layer. Breakdown is stimulated by low pH as well as by the decrease of dissolved oxygen in occluded regions. Small, active pit sites form and remain stable because of the large ratio of cathodic surface area (unattacked metal surface) to the pit area. Active corrosion in the pit cathodically protects immediately adjacent areas. If conditions become very severe, pitting will give way to general attack as more and more of the surface becomes actively involved in corrosion. 

Alloys whose corrosion resistance depends on forming a protective oxide layer, such as stainless steel, are susceptible to severe localized attack when pH falls as a result of nonoxidizing acid excursions. How-... 

Thermodynamically, aluminum should be a highly reactive metal. However, reactivity is limited in most natural environments. When exposed to water or water and air, aluminum quickly forms a protective oxide layer. Once formed, the oxide slows further corrosion. This oxide layer may be as thin as about 5 x 10 m (50 A) when formed naturally in air, but it is thicker when formed in water and can be made up to about 3000 times thicker by anodizing. 

At a pH above about 9, in the presence of sodium carbonate or sodium hydroxide, for example, the protective oxide layer is rapidly dissolved and corrosion becomes severe (Fig. 8.1). Aluminum in the presence of sodium hydroxide corrodes as in Reaction 8.2 ... 

To fully understand the formation of the N13S2 scale under certain gas conditions, a brief description needs to be given on the chemical aspects of the protective (chromium oxide) Ci 203/(nickel oxide) NiO scales that form at elevated temperatures. Under ideal oxidizing conditions, the alloy Waspaloy preferentially forms a protective oxide layer of NiO and Ci 203 The partial pressure of oxygen is such that these scales are thermodynamically stable and a condition of equilibrium is observed between the oxidizing atmosphere and the scale. Even if the scale surface is damaged or removed, the oxidizing condition of the atmosphere would preferentially reform the oxide scales. 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

Corrosion is fought partly by developing alloys with a built-in proclivity to form protective oxide layers, such as stainless steels , and partly by designing protective coatings. A form of protection particularly closely linked to electrochemistry is... 

A protective oxide layer forms a continuous barrier between the reactants (oxygen and metal), which inhibits the reaction. The simplest assumption that can be made about the effectiveness of this barrier is that its protecting power is directly proportional to its thickness. Mathematically, AX/At = k2/X, which on integration gives the parabolic law,... 

This analysis shows that if the oxides of the two components of a binary alloy are mutually insoluble, and if one of the components has a much greater affinity for oxygen than the other, then the oxide of the baser metal will be formed exclusively even though it is present in the alloy in only a small amount. It seems that the importance of beryllium as an alloying constituent can be explained in this way. It has a high affinity for oxygen [p(BeO) = 10 atm at 1000°C] and also forms a highly protective oxide layer. The... 

Another factor that determines the long-term stability of the protective oxide layer is its ability to prevent sulphur penetration which would lead to the eventual formation of chromium sulphide beneath the external oxide layer. With most commercial nickel chromium alloys internal sulphidation... 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

Domestic heating coil internal corrosion. Where naturally soft or lean city water is supplied and the Langelier Saturation Index (LSI) is below -1.0, acid corrosion takes place as a result of the acidic nature of the water. This water often has a high dissolved gas content, which additionally leads to pinhole corrosion. Where water velocities are too high (say, over 6 ft/s 1.8 m/s) the protective oxide layer is stripped off and erosion corrosion takes place. 

Metals with protective oxide layers are not immune from attack, especially stainless steels and copper alloys, which may suffer aggressively from crevice corrosion. 

Tin finds widespread use because of its resistance to corrosion, or as foil or to provide protective coats/plates for other metals. Properties of lead which make industrial application attractive surround its soft, plastic nature permitting it to be rolled into sheets or extruded through dies. In the finely-divided state lead powder is pyrophoric in bulk form the rapidly-formed protective oxide layer inhibits further reaction. It dissolves slowly in mineral acids. Industrial uses include roofing material, piping, and vessel linings, e.g. for acid storage. 

Bluish, shimmering, brittle, relatively reactive metal. Is guite guickly covered with a protective oxide layer, which is why iron is treated with zinc With copper, forms the popular alloy brass, which was already known in antiquity. Used in batteries and as a stabilizer in plastics. Zinc oxide is used as a white pigment Zinc ions are essential to all life forms, e.g., as a component of alcohol dehydrogenase and many other enzymes. Hence human beings (70 kg) carry about 2.3 g (half as much as iron). 

Researchers have been developing SiC-CMCs in order to obtain an oxidation-resistant, tough thermostructural material. In general, a SiC-based material easily forms a protective oxide layer on its surface at high temperatures... 

Both silicon oxide and alumina slurries can be efficiently removed on PECVD TEOS oxide or silicon nitride substrates in a conventional SCI or in a SCI without any water peroxide in the case of outcropping tungsten (see Fig. 5). When water peroxide is not present to continuously regrow a protective oxide layer, OH species can etch the silicon. In the latter case, the backside of the wafer must therefore be protected with a nitride or oxide layer to avoid a severe silicon roughening effect. Nevertheless to achieve the same particle removal efficiency obtained with a scrubber, power mega-sonics also have to be used (see Fig. 18). 

The metal dissolves in dilute mineral acids, but concentrated sulfuric acid has little action on it. Its surface passivates when immersed in 2 to 5% solution of nitric acid in ethanol. A protective oxide layer forms over the metal surface, which prevents any further contact. 

Anodic inhibitors such as nitrites, chromates and molybdates are strong oxidizing passivators. They strengthen the protective oxide layer over the steel which otherwise would break down in the presence of chloride ions. The mechanism involves a redox reaction in which the chloride and nitrite ions engage in competing reactions the inhibitor is reduced and steel becomes oxidized to iron oxide as follows ... 

Couples with E values outside the practical limits of stability do not necessarily cause the destruction of the solvent. Some reactions, although they may be thermodynamically feasible, are kinetically very slow. The Co3J"/Co2+ couple has an E value of + 1.92 V, and Co3+ should not exist in aqueous solution. However, its oxidation of water to oxygen is very slow, and solutions containing [Co(H20)6]3+ evolve dioxygen slowly. The value for the A13 + /A1 couple is -1.66 V, and indicates that aluminium should dissolve in acidic aqueous solutions. A stable protective oxide layer on the metal s surface normally retards the reaction, a circumstance gratefully acknowledged by aeroplane manufacturers. 

An interesting property of HNO3 is its ability to passivate some metals, such as iron and aluminum. This property is of significant industrial importance, since modem processes for producing the acid depend on it. Modern suitability formulated stainless sleel alloys are usefully resistant to nitric add through a wide range of conditions. The acid s passivity or the metal s resistance to attack is attributed to the formation of a protective oxide layer on the surface of the metal. 

Chloride is a depassivating ion. It reduces the corrosion resistance of many metals, especially those that naturally and strongly passivate to form a protective oxide layer, such as stainless steels and aluminum, by attacking the passivated film. 

---