CORROSION PROPERTIES

GENERAL

Titanium and its alloys provide excellent resistance to general and localized attack under most oxidizing, neutral and inhibited reducing conditions in aqueous environments. They also remain passive under mildly reducing conditions, although they may be attacked by strongly reducing or complexing media. Titanium is especially known for its outstanding resistance to chlorides and other halides generally present in most process streams.

Titanium's corrosion resistance is due to a stable, protective, strongly adherent oxide film which forms instantly when a fresh surface is exposed to air or moisture. This passive film is typically less than 250 A. (A, an angstrom, is 4 x 10^-9 in.) Film growth is accelerated under strongly oxidizing conditions such as in HNO3 and CrO3 (nitric acid, chromic acid), etc. media.

The composition of this film varies from TiO2 at the surface to Ti2O3 to TiO at the metal interface. Oxidizing conditions promote the formation of TiO2. This film is transparent in its normal thin configuration and not detectable by visual means.

A study of the corrosion resistance of titanium is basically a study of the properties of the oxide film. The oxide film on titanium is very stable and is attacked only by a few substances including hot concentrated reducing acids, most notably, hydrofluoric acid. Titanium is capable of healing this film almost instantaneously in every environment where a trace of moisture or oxygen is present because of titanium's strong affinity for oxygen.

Anhydrous conditions in the absence of a source of oxygen should be avoided since the protective film may not be regenerated if damaged.

RESISTANCE TO WATERS

FRESH WATER - STEAM
Titanium resists all forms of corrosive attack by fresh water and steam to temperatures as high as 600 degrees F (316 degrees C). The corrosion rate is very low and a slight weight gain is generally experienced. Titanium surfaces are likely to acquire a tarnished appearance in hot water or steam but will be free of corrosion.

Some natural river waters contain manganese which deposits as manganese dioxide on heat exchanger surfaces. This is harmful and promotes pitting in both austenitic stainless steels and copper alloys. Chlorination treatments used to control sliming result in severe pitting and crevice corrosion on stainless steel surfaces. Titanium is immune to these forms of corrosion and is an ideal material for handling all natural waters.

SEAWATER - GENERAL CORROSION
Titanium resists corrosion by seawater to temperatures as high as 500 degrees F (260 degrees C). Titanium tubing which has been exposed to seawater for many years at depths of over a mile shows no measurable corrosion. It has provided over twenty five years of trouble-free seawater service for the chemical, oil refining and desalination industries. Pitting and crevice corrosion are totally absent, even when marine deposits form. The presence of sulfides in seawater does not affect the resistance of titanium to corrosion. Exposure of titanium to marine atmospheres or splash or tidal zones does not cause corrosion.

EROSION
Titanium has the ability to resist erosion by high velocity seawater. Velocities as high as 120 ft./sec. cause only minimal rise in the erosion rate. The presence of abrasive particles, such as sand, has only a small effect on the corrosion resistance of titanium under conditions that are extremely detrimental to copper and aluminum base alloys. Titanium is considered one of the best cavitation-resistant materials available for seawater service.

STRESS-CORROSION CRACKING
TIMETAL 35A and TIMETAL 50A are essentially immune to stress- corrosion cracking (SCC) in seawater. This has been confirmed many times. Other unalloyed titanium grades with an oxygen content greater than 0.25 wt.% may be susceptible to SCC under some conditions.

CORROSION FATIGUE
Titanium, unlike many other materials, does not suffer a significant loss of fatigue properties in seawater. In fatigue- limited applications, Boiler Code criteria or actual in situ fatigue testing should be considered.

CREVICE CORROSION
Crevice corrosion of unalloyed titanium may occur in seawater at temperatures above the boiling point. TIMETAL Code-12 (Grade 12) and TIMETAL 50A Pd (Grades 7 and 16) and 35A Pd (Grades 11 and 17) offer resistance to crevice corrosion in seawater at temperatures up to 500 degrees F (260 degrees C).

GALVANIC CORROSION
The Coupling of titanium with dissimilar metals does not usually accelerate the corrosion of the titanium. The exception is in highly reducing acidic environments where titanium may not passivate. Under these conditions, it has a potential similar to aluminum and will undergo accelerated corrosion when coupled to other more noble metals.

Table 1 (see page 10) gives the galvanic series in seawater. In this environment titanium is passive and exhibits a potential of about 0.0 V versus a saturated calomel reference cell (SCE) which places it high on the passive or noble end of the series.

For most environments, titanium will be the cathodic member of any galvanic couple. It may accelerate the corrosion of the other member of the couple, but in most cases, the titanium will generally remain unaffected. Figure 2 shows the accelerating effect that titanium has on the corrosion rate of various metals when they are galvanically coupled in seawater. If the area of the titanium exposed is small in relation to the area of the other metal, the effect on the corrosion rate is negligible. However, if the area of the titanium (cathode) greatly exceeds the area of the other metal (anode), severe corrosion of the other metal may result.

Because titanium is the cathodic member, hydrogen may be evolved on its surface proportional to the galvanic current flow. This may result in the formation of surface hydride films that are generally stable and cause no problems, If the temperature is above 176 degrees F (80 degrees C), however, hydrogen may diffuse into the metal and cause hydride-related embrittlement.

In order to avoid problems with galvanic corrosion, it is best to construct equipment of a single metal. If this is not practical, use two metals that are close together in the galvanic series, insulate the joint or cathodically protect the less noble metal. If dissimilar metals are necessary, and since titanium is usually not attacked, construct the critical parts from titanium, and use large areas of the less noble metal and heavy sections to allow for increased corrosion.