Titanium is very resistant to alkaline media including solutions of sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide. Regardless of concentration, titanium generally exhibits corrosion rates of less than or equal to 5 milliinches per year - mpy (0.127 mm/yr). Near nil corrosion rates are exhibited in boiling calcium hydroxide, magnesium hydroxide, and ammonium hydroxide solutions up to saturation. Despite low corrosion rates in alkaline solutions, hydrogen pickup and possible embrittlement of titanium can occur at temperatures above 170°F (77°C) when solution pH is greater than or equal to 12. Successful application can be achieved where this guideline is observed.
Inorganic Salt Media
Titanium is highly resistant to corrosion by inorganic salt solutions. Corrosion rates are generally very low at all temperatures to the boiling point. The resistance of titanium to chloride solutions is excellent. However, crevice corrosion is a concern. Other acidic salt solutions, particularly those formed from reducing acids, may also cause crevice corrosion of unalloyed titanium at elevated temperatures. For instance, a boiling solution of 10 percent sodium sulfate, pH 2.0, causes crevice corrosion on grade 2 titanium. The grade 12 and grade 7 alloys, on the other hand, are resistant to this environment.
Titanium generally shows good corrosion resistance to organic media and is steadily finding increasing application in equipment for handling organic compounds. Kane points out that titanium is a standard construction material in the Wacker Process for the production of acetaldehyde by oxidation of ethylene in an aqueous solution of metal chlorides. Successful application has also been established in critical areas of terephthalic and adipic acid production. Generally, the presence of moisture (even trace amounts) and oxygen is very beneficial to the passivity of titanium in organic media.
In certain anhydrous organic media, titanium passivity can be difficult to maintain. For example, methyl alcohol can cause stress corrosion cracking in unalloyed titanium when the water content is below 1.5%. At high temperatures in anhydrous environments where dissociation of the organic compound can occur, hydrogen embrittlement of the titanium may be possible. Since many organic processes contain either trace amounts of water and/or oxygen, titanium has found successful application in organic process streams.
Titanium is generally quite resistant to organic acids. Its behaviour is dependent on whether the environment is reducing or oxidizing. Only a few organic acids are known to attack titanium. Among these are hot non-aerated formic acid, hot oxalic acid, concentrated trichloroacetic acid and solutions of sulphamic acid. Aeration improves the resistance of titanium in most of these nonoxidizing acid solutions. In the case of formic acid, it reduces the corrosion rates to very low values. Unalloyed titanium corrodes at a very low rate in boiling 0.3 percent sulphamic acid and at a rate of over 100 mpy (2.54 mm/y) in 0.7 percent boiling sulphamic acid. Addition of ferric chloride (0.375 g/l) to the 0.7 percent solution reduces the corrosion rate to 1.2 mpy (0.031 mm/y).
Boiling solutions containing more than 3.5 g/l of sulphamic acid can rapidly attack unalloyed titanium. For this reason, extreme care should be exercised when titanium heat exchangers are descaled with sulphamic acid. The pH of the acid should not be allowed to go below 1.0 to avoid corrosion of titanium. Consideration should also be given to inhibiting the acid with ferric chloride. Titanium is resistant to acetic acid over a wide range of concentrations and temperatures well beyond the boiling point. It is being used in terephthalic acid and adipic acid up to 400°F (204°C) and at 67% concentration.
Good resistance is observed in citric, tartaric, stearic, lactic and tannic acids. Grades 12 and 7 may offer considerably improved corrosion resistance to organic acids which attack unalloyed titanium. Similarly, the presence of multi-valent metal ions in solution may result in substantially reduced corrosion rates.
Titanium has excellent resistance to gaseous oxygen and air at temperatures up to about 700°F (371°C). At 700°F it acquires a light straw color. Further heating to 800°F (426°C) in air may result in a heavy oxide layer because of increased diffusion of oxygen through the titanium lattice. Above 1200°F (649°C), titanium lacks oxidation resistance and will become brittle. Scale forms rapidly at 1700°F (927°C). Titanium resists atmospheric corrosion. Twenty year ambient temperature tests produced a maximum corrosion rate of 0.0010 mpy (2.54 x 10-5 mm/y) in a marine atmosphere and a similar rate in industrial and rural atmospheres.
Caution should be exercised in using titanium in high oxygen atmospheres. Under some conditions, it may ignite and burn. J.D. Jackson and Associates reported that ignition cannot be induced even at very high pressure when the oxygen content of the environment was less than 35%. However, once the reaction has started, it will propagate in atmospheres with much lower oxygen levels than are needed to start it. Steam as a diluent allowed the reaction to proceed at even lower O2 levels. When a fresh titanium surface is exposed to an oxygen atmosphere, it oxidizes rapidly and exothermically. Rate of oxidation depends on O2 pressure and concentration. When the rate is high enough so that heat is given off faster than it can be conducted away, the surface may begin to melt. The reaction becomes self-sustaining because, above the melting point, the oxides diffuse rapidly into the titanium interior, allowing highly reactive fresh molten titanium to react at the surface.
The surface oxide film on titanium acts as an effective barrier to penetration by hydrogen. Disruption of the oxide film allows easy penetration by hydrogen. When the solubility limit of hydrogen in titanium (about 100-150 ppm for grade 2) is exceeded, hydrides begin to precipitate. Absorption of several hundred ppm of hydrogen results in embrittlement and the possibility of cracking under conditions of stress.
Titanium can absorb hydrogen from environments containing hydrogen gas. At temperatures below 170°F (77°C) hydrogen pickup occurs so slowly that it has no practical significance, except in cases where severe tensile stresses are present. In the presence of pure hydrogen gas under anhydrous conditions, severe hydriding can be expected at elevated temperatures and pressures. Surface condition is also important to hydrogen penetration.
Titanium is not recommended for use in pure hydrogen because of the possibility of hydriding if the oxide film is broken. Laboratory tests have shown that the presence of as little as 2% moisture in hydrogen gas effectively passivates titanium so that hydrogen absorption does not occur. This probably accounts for the fact that titanium is being used successfully in many process streams containing hydrogen with very few instances of hydriding being reported.
A more serious situation exists when cathodically impressed or galvanically induced currents generate nascent hydrogen directly on the surface of titanium. The presence of moisture does not inhibit hydrogen absorption of this type.
Laboratory experiments have shown that three conditions usually exist simultaneously for hydriding to occur:
• 1. The pH of the solution is less than 3 or greater than 12; the metal surface must be damaged by abrasion; or impressed potentials are more negative than -0.70V.
• 2. The temperature is above 170°F (77°C) or only surface hydride films will form which, experience indicates, do not seriously affect the properties of the metal. Failures due to hydriding are rarely encountered below this temperature. (There is some evidence that severe tensile stresses may promote hydriding at low temperatures.)
• 3. There must be some mechanism for generating hydrogen. This may be a galvanic couple, cathodic protection by impressed current, corrosion of titanium, or dynamic abrasion of the surface with sufficient intensity to depress the metal potential below that required for spontaneous evolution of hydrogen.
Sulphur Dioxide and Hydrogen Sulphide
Most of the hydriding failures of titanium that have occurred in service can be explained on this basis. In seawater, hydrogen can be produced on titanium as the cathode by galvanic coupling to a dissimilar metal such as zinc or aluminum which are very active (low) in the galvanic series. Coupling to carbon steel or other metals higher in the galvanic series generally does not generate hydrogen in neutral solutions, even though corrosion is progressing on the dissimilar metal. The presence of hydrogen sulphide, which dissociates readily and lowers pH, apparently allows generation of hydrogen on titanium if it is coupled to actively corroding carbon steel or stainless steel.
Within the range pH 3 to 12, the oxide film on titanium is stable and presents a barrier to penetration by hydrogen. Efforts at cathodically charging hydrogen into titanium in this pH range have been unsuccessful in short-term tests. If pH is below 3 or above 12, the oxide film is believed to be unstable and less protective. Breakdown of the oxide film facilitates access of available hydrogen to the underlying titanium metal. Mechanical disruption of the film (i.e. iron is smeared into the surface) permits entry of hydrogen at any pH level. Impressed currents involving cathodic potentials more negative than -0.7V in near neutral brines can result in hydrogen pickup in long-term exposures. Furthermore, very high cathodic current densities (more negative than -1.0V SCE) may accelerate hydrogen absorption and eventual embrittlement of titanium in seawater even at ambient temperatures.
Hydriding can be avoided if proper consideration is given to equipment design and service conditions in order to eliminate detrimental galvanic couples or other conditions that will promote hydriding.
Titanium is resistant to corrosion by gaseous sulphur dioxide and water saturated with sulphur dioxide. Sulphurous acid solutions also have little effect on titanium. Titanium has demonstrated superior performance in wet SO2 scrubber environments of power plant FGD systems.
Titanium is not corroded by moist or dry hydrogen sulphide gas. It is also highly resistant to aqueous solutions containing hydrogen sulphide. The only known detrimental effect is the hydriding problem discussed in the previous section. In galvanic couples with certain metals such as iron, the presence of H2S will promote hydriding. Hydriding, however, does not occur in aqueous solutions containing H2S if unfavourable galvanic couples are avoided. For example, titanium is fully resistant to corrosion and stress cracking in the National Association of Corrosion Engineers (NACE) test solution which consists of oxygen-free water containing about 3,000 ppm dissolved H2S, 5 percent NaCl, and 0.5 percent acetic acid (pH 3.5). Tensile specimens of titanium alloy grades 2,4,7 and 12 stressed to 98 percent of yield strength in this environment survived a 30-day room temperature exposure.
In addition, C-ring specimens of these same grades of titanium were subjected to a stress corrosion cracking test as specified in ASTM G38-73 Standard Recommended Practice. Two series of tests were run: one with the specimens stressed to 75% of yield, and the other stressed to 100% of yield. The specimens were exposed in an ASTM synthetic seawater solution saturated with H2S and CO2 at 400°F (204°C). Solution pH was 3.5 and specimens were exposed for 30 days. There were no failures and no evidence of any corrosion.
Titanium is highly resistant to general corrosion and pitting in the sulphide environment to temperatures as high as 500°F (260°C). Sulphide scales do not form on titanium, thereby maintaining good heat transfer.
Nitrogen and Ammonia
Titanium reacts with pure nitrogen to form surface films having a gold colour above 1000°F (538°C). Above 1500°F (816°C), diffusion of the nitride into titanium may cause embrittlement. Jones et al. (1977) have shown that titanium is not corroded by liquid anhydrous ammonia at room temperature. Low corrosion rates are obtained at 104°F (40°C). Titanium also resists gaseous ammonia. However, at temperatures above 302°F (150°C), ammonia will decompose and form hydrogen and nitrogen. Under these circumstances, titanium could absorb hydrogen and become embrittled. The high corrosion rate experienced by titanium in the ammonia-steam environment at 428°F (220°C) is believed to be associated with hydriding.
Titanium is also resistant to ammonium hydroxide. Excellent resistance is offered by titanium to concentrated solutions (up to 70% NH4OH) to the boiling point.
The formation of ammonium chloride scale could result in crevice corrosion of grade 2 titanium at boiling temperatures. Grades 12 and 7 are totally resistant under these conditions. This crevice corrosion behaviour is similar to that for sodium chloride.
Titanium has good resistance to many liquid metals at moderate temperatures. In some cases at higher temperatures it dissolves rapidly. It is used successfully in some applications up to 1650°F (899°C). Kane cites the use of titanium in molten aluminium for pouring nozzles, skimmer rakes and casting ladles. However, rapidly flowing molten aluminium can erode titanium and some metals such as cadmium can cause stress corrosion cracking.
Anodising and Oxidation Treatments
Anodising has been recommended for many years as a method of improving the corrosion resistance of titanium and removing surface impurities such as embedded iron particles. It was reasoned that since titanium’s corrosion resistance is due to the oxide film that forms on its surface, any treatment, such as anodising, which thickens this film will serve to increase the corrosion resistance of titanium.
Careful laboratory tests have shown this may not be true. The films formed on titanium at elevated temperatures in air have been found to have a rutile structure which is quite resistant to acids and can, therefore, improve the corrosion resistance. Anodising, on the other hand, forms a hydrated structure which is much less resistant to acids. Tests in boiling HCl solution have shown no significant difference in corrosion resistance between anodised and freshly pickled specimens. Anodising has been shown to give a marginal improvement in resistance to hydrogen absorption but not nearly as much as thermal oxidation. It is true that anodising helps to remove surface impurities such as embedded iron particles. However, excessively long anodising times may be required to completely remove these particles. Examination with a scanning electron microscope has proven that surface iron contamination still persists, although diminished, even after 20 minutes anodising. A more effective method is to pickle in 12% HNO3/1% HF at ambient temperature for 5 minutes followed by a water rinse. Specimens known to have embedded iron particles were found to be completely free of any surface iron contamination by the scanning electron microscope following this procedure.