Titanium protective oxide film

Wet chlorine is extremely corrosive, attacking most common metals except Hastel-loy C, titanium, and tantalum. Surface oxide films protect these metals from attack by the acids formed in chlorine hydrolysis. Tantalum is an ideal construction material for service with wet and dry chlorine. However it is expensive and normally used only in instruments such as transmitters, diaphragms, transducers, and thermowells. FRP is used for wet chlorine. Rubber-lined steel is also suitable for wet chlorine gas up to about 100°C. At low pressures and low temperatures, PVC, CPVC, and reinforced polyester resins are also used. Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene-hexafluoropropylene (FEP) are resistant at high temperatures. Other materials stable to moist chlorine include graphite and glass. 

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

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). 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

The corrosion resistance of unalloyed titanium in hydrochloric or sulfuric acids can be increased significantly by anodic protection, which maintains the oxide film so that the corrosion will be negligible even in severely reducing conditions. 

Contact with steel, though less harmful, may accelerate attack on aluminium, but in some natural waters and other special cases aluminium can be protected at the expense of ferrous materials. Stainless steels may increase attack on aluminium, notably in sea-water or marine atmospheres, but the high electrical resistance of the two surface oxide films minimises bimetallic effects in less aggressive environments. Titanium appears to behave in a similar manner to steel. Aluminium-zinc alloys are used as sacrificial anodes for steel structures, usually with trace additions of tin, indium or mercury to enhance dissolution characteristics and render the operating potential more electronegative. 

In general, however, for titanium immersed in acid solutions, potentials above zero on the saturated calomel scale are conducive to the formation of protective oxide, while at certain negative potentials hydride films, which also confer some protection, can be formed. Between the potential at which a continuous hydride film is formed and that at which protective oxide films appear, soluble titanium ions are produced and rapid corrosion ensues. 

As indicated above, when a positive direct current is impressed upon a piece of titanium immersed in an electrolyte, the consequent rise in potential induces the formation of a protective surface film, which is resistant to passage of any further appreciable quantity of current into the electrolyte. The upper potential limit that can be attained without breakdown of the surface film will depend upon the nature of the electrolyte. Thus, in strong sulphuric acid the metal/oxide system will sustain voltages of between 80 and 100 V before a spark-type dielectric rupture ensues, while in sodium chloride solutions or in sea water film rupture takes place when the voltage across the oxide film reaches a value of about 12 to 14 V. Above the critical voltage, anodic dissolution takes place at weak spots in the surface film and appreciable current passes into the electrolyte, presumably by an initial mechanism involving the formation of soluble titanium ions. 

Aqueous environments Neutral chloride solutions do not corrode titanium alloys at ambient temperatures, and smooth statically loaded specimens of susceptible alloys do not exhibit failure. In order to nucleate cracking it appears probable that the protective oxide film on alloys must be destroyed and its repair must not occur. If this breakdown occurs then cracking is observed in susceptible alloys. Consequently, the type of test and the type of specimen employed in any selected test are both important considerations, particularly in alloys exhibiting low susceptibility. 

Titanium, which was in commercial production in 1950, is thermodynamically a very reactive metal (machining swarf can be ignited in a similar fashion to that of magnesium ribbon) but this is offset by its strong tendency to passivate i.e. to form a highly stable protective oxide film. 

It is a valve metal and when made anodic in a chloride-containing solution it forms an anodic oxide film of TiOj (rutile form), that thickens with an increase in voltage up to 8-12 V, when localised film breakdown occurs with subsequent pitting. The TiOj film has a high electrical resistivity, and this coupled with the fact that breakdown can occur at the e.m.f. s produced by the transformer rectifiers used in cathodic protection makes it unsuitable for use as an anode material. Nevertheless, it forms a most valuable substrate for platinum, which may be applied to titanium in the form of a thin coating. The composite anode is characterised by the fact that the titanium exposed at discontinuities is protected by the anodically formed dielectric Ti02 film. Platinised titanium therefore provides an economical method of utilising the inertness and electronic conductivity of platinum on a relatively inexpensive, yet inert substrate. 

It has now gained acceptance as an impressed current anode for cathodic protection and has been in use for this purpose since 1971. The anode consists of a thin film of valve and precious metal oxides baked onto a titanium substrate and when first developed was given the proprietary name dimensionally stable anode , sometimes shortened to DSA. Developments on the composition of the oxide film have taken place since Beer s patent, and this type of anode is now marketed under a number of different trade names. 

At the other extreme, the oxide layers on aluminum, beryllium, titanium, vanadium, chromium, nickel, and tantalum are very insoluble in water at intermediate pH values and do not have easily accessible reduced states with higher solubility. The oxide films on those metals are therefore highly protective against aqueous corrosion. 

The high reactivity of titanium metal is disguised in normal use by its unusually corrosion-resistant protective oxide film (Chapter 16), but it must always be borne in mind by users of Ti process or laboratory equipment. In particular, titanium should not be used with pressurized oxygen pressures of oxygen of as little as 0.4 MPa have been reported to lead to ignition of Ti at ambient temperature if the oxide film is penetrated. 

Although titanium has a large positive E° for oxidation, and T dust will burn in air, the bulk metal is remarkably immune to corrosion because its surface becomes coated with a thin, protective oxide film. Titanium objects are inert to seawater, nitric acid, hot aqueous NaOH, and even to aqueous chlorine gas. Titanium is therefore used in chemical plants, in desalination equipment, and in numerous other industrial processes that demand inert, noncorrosive materials. Because it is nontoxic and inert to body fluids, titanium is even used for manufacturing artificial joints and dental implants. 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

With the more active metals, such as aluminum, titanium, and tantalum, oxide films form immediately on contact with air and behave as passive films in aqueous solutions. In the case of tantalum, the passive film is protective over the entire pH range in other cases, the films may... 

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