HYDROGEN
The surface oxide film on titanium acts as an effective barrier to penetration by hydrogen, but 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 titanium) 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 176 degrees F (80 degrees 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 hybriding can be expected at elevated temperatures and pressures. Titanium is not recommended for use in pure hydrogen because of the possibility of hydriding should the oxide film be broken. Laboratory tests (Table 2) 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 is damaged by abrasion; or impressed potentials are more negative than -0.75V (vs. SCE).

2. The temperature is above 176 degrees F (80 degrees C). Below this temperature 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. One exception involves a situation where very heavy cathodic charging of titanium occurs by impressed current means. In this case, eventual hydride-related embrittlement may occur even at temperatures below 80 degrees C.

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.

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 sulfide, which acts as a recombination poison, apparently increases the absorption of hydrogen on titanium if it is coupled to actively corroding carbon steel or stainless steel.

Within the range of pH 3 to 12, the oxide film on titanium is stable and presents a good barrier to penetration by hydrogen. Efforts at cathodically charging hydrogen into this pH range have been unsuccessful in short term tests at voltages more positive than -0.75V (vs. SCE). If pH is below 3 or above 12, the oxide film is believed to be unstable and can breakdown, permitting easy access of available hydrogen to the underlying titanium metal. Mechanical disruption of the film (i.e., iron smeared into the surface) allows entry of hydrogen at any level provided the temperature is above 176 degrees F (80 degrees C).

Practically speaking, impressed cathodic potentials should remain more noble than -1.0V (vs. SCE) in near-ambient temperature seawater applications to avoid accelerated hydriding of titanium. Hydriding can be avoided if proper consideration is given to equipment design and service conditions to eliminate detrimental galvanic couples or other conditions that will promote hydriding.