Passivity metals

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

For example,copper has relatively good corrosion resistance under non-oxidizing conditions. It can be alloyed with zinc to yield a stronger material (brass), but with lowered corrosion resistance. Flowever, by alloying copper with a passivating metal such as nickel, both mechanical and corrosion properties are improved. Another important alloy is steel, which is an alloy between iron (>50%) and other alloying elements such as carbon. 

Metal passivation Metal pickling Metal powders Metal recovery... 

Reactions with Metals. Many common metals react with OF2, but the reaction stops after a passive metal fluoride coating is formed (3,4). 

AgF2 is prepared by the action of elemental fluorine on AgF or AgCl at 200°C. Both processes result in quantitative yields. Silver difluoride should be stored in Teflon, passivated metal containers, or in sealed quartz tubes. 

The Butler-Vohner equation can be appHed to many, but not all, systems. Moreover, many of the systems that do not foUow the Butler-Vokner model are of great practical importance, eg, in the corrosion of passivating metals (see Corrosion and corrosion control). 

There is often a period before corrosion starts in a crevice in passivating metals. This so-called incubation period corresponds to the time necessary to establish a crevice environment aggressive enough to dissolve the passive oxide layer. The incubation period is well known in stainless steels exposed to waters containing chloride. After a time period in which crevice corrosion is negligible, attack begins, and the rate of metal loss increases (Fig. 2.8). 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

Silt, sand, concrete chips, shells, and so on, foul many cooling water systems. These siliceous materials produce indirect attack by establishing oxygen concentration cells. Attack is usually general on steel, cast iron, and most copper alloys. Localized attack is almost always confined to strongly passivating metals such as stainless steels and aluminum alloys. 

Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adherence of these coatings relative to the thin films formed by the classical passive metals, such as stainless steel and titanium. Both stainless steel and titanium are relatively immune to erosion-corrosion in most cooling water environments. 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

Ox and Red are general symbols for oxidation and reduction media respectively, and n and (n-z) indicate their numerical charge (see Section 2.2.2). Where there is no electrochemical redox reaction [Eq. (2-9)], the corrosion rate according to Eq. (2-4) is zero because of Eq. (2-8). This is roughly the case with passive metals whose surface films are electrical insulators (e.g., A1 and Ti). Equation (2-8) does not take into account the possibility of electrons being diverted through a conductor. In this case the equilibrium... 

Two areas of passivity are located in Fig. 2-2 where Fe has a very low corrosion rate. In contrast to cathodically protected metals in groups I and II, the corrosion rate of anodically passivated metals in groups III and IV cannot in principle be zero. In most cases the systems belong to group IV where intensified weight loss corrosion or local corrosion occurs when U > U" There are only a few metals belonging to group III e.g., Ti, Zr [44] and A1 in neutral waters free of halides. 

Generally, pitting corrosion only occurs on passivated metals when the passive film is destroyed locally. In most cases chloride ions cause this local attack at potentials U > U q. Bromide ions also act in the same way [51], The critical potential for pitting corrosion UpQ is called the pitting potential. It has the same significance as in Eqs. (2-39) and (2-48). 

Weight loss corrosion of passive metals (e.g., CrNi stainless steels in acids) (cathodic protection in acids is not practicable see Fig. 2-14) ... 

Piping systems should be designed for an economic flow velocity. For relatively clean fluids, a recommended velocity range where minimum corrosion can be expected is 2 to 10 fps. If piping bores exist, maximum fluid velocities may have a mean velocity of 3 fps for a 3/8-in. bore to 10 fps for an 8-in.-diameter bore. Higher flow velocities are not uncommon in situations that require uniform, constant oxygen supply to form protective films on active/passive metals. 

Passivity—a condition of a metal or alloy in which the material is normally thermodynamically unstable in a given electrolytic solution but remains visibly unchanged for a prolonged period. The electrode potential of a passive metal is always appreciably more noble than its potential in the active state. 

The Cs structure and dimensions (Fig. 17.26b) were established by microwave spectroscopy which also yielded a value for the molecular dipole moment p. 1.72D. Other physical properties of this colourless gas are mp -115° (or -123°), bp -6°, A//f(g,298K) —34 10kJmol [or — 273kJmol when corrected for A//f(HF, g) ]. FCIO2 is thermally stable at room temperature in dry passivated metal containers and quartz. Thermal decomposition of the gas (first-order kinetics) only becomes measurable above 300° in quartz and above 200° in Monel metal ... 

Corrosion inhibitor/lubricity improvement additives are used panicularly in militai y fuel for the dual puiyiose of passivating metal surfaces and improving the lubricating properties of the fuel in equipment such as fuel pumps. The militai y also specifies the use of a fuel system icing inhibitor as an additive to prevent filter blocking by ice crystal formation, because militai y aircraft tend not to use fuel line filter heaters, which are standard equipment on civil aircraft. 

Figure 4-421. Corrosion characteristics of an active passive metal as a function of solution oxidizing power (eiectrode potential). (From Ref. [183].)...

Pitting of passive metals such as the stainless steels, aluminium alloys, etc., in the presence of specific ions, e.g. Ci ions Dezincification deaiuminification graphitisation... 

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