Protective oxidation

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. 

Saunders S R J, Evans FI E and Stringer J A (eds) Workshop on Mechanical Properties of Protective Oxide Scales. Materials at High Temperatures vo 12 (Teddington)... 

These alloys are of vital importance in the construction of modern aircraft and rockets. Aluminum, evaporated in a vacuum, forms a highly reflective coating for both visible light and radiant heat. These coatings soon form a thin layer of the protective oxide and do not deteriorate as do silver coatings. They are used to coat telescope mirrors and to make decorative paper, packages, toys. 

Hot Corrosion. Hot corrosion is an accelerated form of oxidation that arises from the presence not only of an oxidizing gas, but also of a molten salt on the component surface. The molten salt interacts with the protective oxide so as to render the oxide nonprotective. Most commonly, hot corrosion is associated with the condensation of a thin molten film of sodium sulfate [7757-82-6], Na2S04, on superaHoys commonly used in components for gas turbines, particularly first-stage turbine blades and vanes. Other examples of hot corrosion have been identified in energy conversion systems, particularly coal gasifiers and direct coal combustors. In these cases the salt originates from alkali impurities in the coal which condense on the internal... 

Lead—Calcium—Aluminum Alloys. Lead—calcium alloys can be protected against loss of calcium by addition of aluminum. Aluminum provides a protective oxide skin on molten lead—calcium alloys. Even when scrap is remelted, calcium content is maintained by the presence of 0.02 wt % aluminum. Alloys without aluminum rapidly lose calcium, whereas those that contain 0.03 wt % aluminum exhibit negligible calcium losses, as shown in Figure 8 (10). Even with less than optimum aluminum levels, the rate of oxidation is lower than that of aluminum-free alloys. 

There is also a two-step process of chromizing foUowed by aluminizing. Above 900°C the chromizing begins to rediffuse and the protective oxide changes to Al O from Cr202. Aluminum oxide is less volatile than chromium oxide and better for high temperature oxidation resistance above 1000°C. 

The metal dissolves readily in concentrated HCl, H PO, HI, or HCIO. Nitric acid (qv) forms a protective oxide skin on the metal and can be removed by ca 0.05 Af HF. Dissolution of Pu metal in HNO —HF mixtures is common practice in scrap-recovery plants. The metal does not dissolve readily in H2SO4 because passivation of the metal surface occurs. The reaction of water and Pu metal is slow compared to that in HCl, HI, or HCIO. ... 

Aluminum and aluminum alloys are employed in many appHcations because of the abiHty to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5 —10 nm thick when formed in air if dismpted it begins to form immediately in most environments. The weathering characteristics of several common aluminum alloy sheet products used for architectural appHcations are shown in Eigure 30. The loss in strength as a result of atmospheric weathering and corrosion is small, and the rate decreases with time. The amount of... 

Aluminide and sUicide cementation coatings such as TaAl on tantalum and MoSi2 on molybdenum oxidize at slow rates and possess some inherent self-repair characteristics. Fine cracks that appear and are common to these coatings can be tolerated because stable, protective oxides form within the cracks and seal them. Thermal cycling, however, accelerates faUure because of thermal expansion mismatch that ultimately dismpts the protective oxide coating. 

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. 

Although the pure metal has a silvery-white color, in the cast condition it may have a yellowish tinge caused by a thin film of protective oxide on the surface. When highly poHshed, it has high light reflectivity. It retains its brightness well during exposure, both outdoors and indoors. 

Even though titanium is an active metal, it resists decomposition because of a tenacious protective oxide film. 

Passivating (anodic) inhibitors form a protective oxide film on the metal surface they are the best inhibitors because they can be used in economical concentrations and their protective films are tenacious and tend to be rapidly repaired if damaged. 

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

In the presence of oxygen and water the oxides of most metals are more thermodynamically stable than the elemental form of the metal. Therefore, with the exception of gold, the only metal which is thermodynamically stable in the presence of oxygen, there is always a thermodynamic driving force for corrosion of metals. Most metals, however, exhibit some tendency to passivate, ie, to form a protective oxide film on the surface which retards further corrosion. 

Under cyclic or repeated stress conditions, rupture of protective oxide films that prevent corrosion takes place at a greater rate than that at which new protec tive films can be formed. Such a situation frequently resiilts in formation of anodic areas at the points of rupture these produce pits that serve as stress-concentration points for the origin or cracks that cause ultimate failure. 

Fretting Corrosion This attack occurs when metals shde over each other and cause mechanical damage to one or both. In such a case, frictional heat oxidizes the metal and this oxide then wears away or the mechanical removal of protective oxides results in exposure of fresh surface for corrosive attack. Fretting corrosion is minimized by using harder materials, minimiziug friction (via lubrication), or designing equipment so that no relative movement of parts takes place. 

Contaminants in fuels, especially alkali-metal ions, vanadium, and sulfur compounds, tend to react in the combustion zone to form molten fluxes which dissolve the protective oxide film on stainless steels, allowing oxidation to proceed at a rapid rate. This problem is becoming more common as the high cost and short supply of natural gas and distillate fuel oils force increased usage of residual fuel oils and coal. 

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. 

Acidic pH helps break down protective oxides on stainless steels. Corrosion usually develops faster and is more severe as pH decreases. At very low pH, however, attack inside crevices may be no more severe than on regions outside the crevice. 

Stainless steels contain 11% or more chromium. Table 5.1 lists common commercial grades and compositions of stainless steels. It is chromium that imparts the stainless character to steel. Oxygen combines with chromium and iron to form a highly adherent and protective oxide film. If the film is ruptured in certain oxidizing environments, it rapidly heals with no substantial corrosion. This film does not readily form until at least 11% chromium is dissolved in the alloy. Below 11% chromium, corrosion resistance to oxygenated water is almost the same as in unalloyed iron. 

Corrosion resistance of stainless steel is reduced in deaerated solutions. This behavior is opposite to the behavior of iron, low-alloy steel, and most nonferrous metals in oxygenated waters. Stainless steels exhibit very low corrosion rates in oxidizing media until the solution oxidizing power becomes great enough to breach the protective oxide locally. The solution pH alone does not control attack (see Chap. 4, Underdeposit Corrosion ). The presence of chloride and other strong depassivating chemicals deteriorates corrosion resistance. 

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-... 

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