Corrosion of Titanium and Titanium Alloys

Titanium alloys were originally developed in the early 1950s for aerospace applications, in which their high strength-to-density ratios were especially attractive. Although titanium alloys are still vital to the aerospace industry for these properties, recognition of the excellent resistance of titanium to many highly corrosive environments, particularly oxidizing and chloride-containing process streams, has led to widespread non-aerospace (industrial) applications.

Because of decreasing cost and the increasing availability of titanium alloy products, many titanium alloys have become standard engineering materials for a host of common industrial applications. In fact, a growing trend involves the use of high-strength aerospace-founded titanium alloys for industrial service in which the combination of strength to density and corrosion resistance properties is critical and desirable.

The excellent corrosion resistance of titanium alloys results from the formation of very stable, continuous, highly adherent, and protective oxide films on metal surfaces. Because titanium metal is highly reactive and has an extremely high affinity for oxygen, these beneficial surface oxide films form spontaneously and instantly when fresh metal surfaces are exposed to air and/or moisture. In fact, a damaged oxide film can generally reheal itself instantaneously if at least traces of oxygen or water are present in the environment. However, anhydrous conditions in the absence of a source of oxygen may result in titanium corrosion, because the protective film may not be regenerated if damaged.

The nature, composition, and thickness of the protective surface oxides that form on titanium alloys depend on environmental conditions. In most aqueous environments, the oxide is typically TiO₂, but may consist of mixtures of other titanium oxides, including TiO₂, Ti₂O₃, and TiO. High-temperature oxidation tends to promote the formation of the chemically resistant, highly crystalline form of TiO, known as rutile, whereas lower temperatures often generate the more amorphous form of TiO, anatase, or a mixture of rutile and anatase.

Although these naturally formed films are typically less than 10 nm thick and are invisible to the eye, the TiO; oxide is highly chemically resistant and is attacked by very few substances, including hot, concentrated HCl, H₂SO₄, NaOH, and (most notably) HF. This thin surface oxide is also a highly effective barrier to hydrogen.

In any contemplated application of titanium, its susceptibility to degradation by any of these forms of corrosion should be considered. In order to understand the advantages and limitations of titanium alloys, each of these forms of corrosion will be explained. Although they are not common limitations to titanium alloy performance, galvanic corrosion, corrosion fatigue, and erosion-corrosion are included in the interest of completeness.

General corrosion is characterized by a relatively uniform attack over the exposed surface of the metal. At times, general corrosion in aqueous media may take the form of mottled, severely roughened metal surfaces that resemble localized attack. This often results from variations in the corrosion rates of localized surface patches due to localized masking of metal surfaces by process scales, corrosion products, or gas bubbles; such localized masking can prevent true uniform surface attack.

Titanium alloys may be subject to localized attack in tight crevices exposed to hot (>70 ^oC) chloride, bromide, iodide, fluoride, or sulfate-containing solutions. Crevices can stem from adhering process stream deposits or scales, metal-to-metal joints (for example, poor weld joint design or tube-to-tubesheet joints), and gasket-to-metal flange and other seal joints.

Pitting is defined as localized corrosion attack occurring on openly exposed metal surfaces in the absence of any apparent crevices. This pitting occurs when the potential of the metal exceeds the anodic breakdown potential of the metal oxide film in a given environment. When the anodic breakdown potential of the metal is equal to or less than the corrosion potential under a given set of conditions, spontaneous pitting can be expected.

Titanium alloys are widely used in hydrogen containing environments and under conditions in which galvanic couples or cathodic charging causes hydrogen to be evolved on metal surfaces. Although excellent performance is revealed for these alloys in most cases, hydrogen embrittlement has been observed.

The surface oxide film of titanium is a highly effective barrier to hydrogen penetration. Traces of moisture or oxygen in hydrogen-containing environments very effectively maintain this protective film, thus avoiding or limiting hydrogen uptake. On the other hand, anhydrous hydrogen gas atmospheres may lead to absorption, particularly as temperatures and pressures increase.

Stress-corrosion cracking (SCC) is a fracture, or cracking, phenomenon caused by the combined action of tensile stress, a susceptible alloy, and a corrosive environment. The metal normally shows no evidence of general corrosion attack, although slight localized attack in the form of pitting may be visible. Usually, only specific combinations of metallurgical and environmental conditions cause SCC. This is important because it is often possible to eliminate or reduce SCC sensitivity by modifying either the metallurgical characteristics of the metal or the makeup of the environment.

Another important characteristic of SCC is the requirement that tensile stress is present. These stresses may be provided by cold work, residual stresses from fabrication, or externally applied loads.

The key to understanding SCC of titanium alloys is the observation that no apparent corrosion, either uniform or localized, usually precedes the cracking process. As a result, it can sometimes be difficult to initiate cracking in laboratory tests by using conventional test techniques.

It is also important to distinguish between the two classes of titanium alloys. The first class, which includes ASTM grades 1, 2, 7, 11 and 12, is immune to SCC except in a few specific environments. These specific environments include anhydrous methanol/halide solutions, nitrogen tetroxide (N₂O₄), and liquid or solid cadmium. The second class of titanium alloys, including the aerospace titanium alloys, has been found to be susceptible to several additional environments, most notably aqueous chloride solutions.

The coupling of titanium with dissimilar metals usually does not accelerate the corrosion of titanium. The exception is in strongly reducing environments in which titanium is severely corroding and not readily passivated. In this uncommon situation, accelerated corrosion may occur when titanium is coupled to more noble metals. In its normal passive condition, materials that exhibit more noble corrosion potentials beneficially influence titanium.

These beneficial alloying additions have been incorporated into several commercially available titanium alloys, including the titanium-palladium alloys (grades 7 and 11), Ti-0.3Mo-0.8Ni (grade 12), Ti-3Al-8V-6Cr-4Zr-4Mo, Ti-15Mo-5Zr, and Ti-6Al-2Sn-4Zr-6Mo. These alloys all offer expanded application into hotter and/or stronger HCl, H₂SO₄, H₃PO₄, and other reducing acids as compared to unalloyed titanium. The high-molybdenum alloys offer a unique combination of high strength, low density, and superior corrosion resistance.

Fig 1. Corrosion of dissimilar metals coupled to titanium in flowing ambient-temperature seawater

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