Editorial Feature

Titanium - Corrosion by Water

Since titanium metal first became a commercial reality in 1950, corrosion resistance has been an important consideration in its selection as an engineering structural material. Titanium has gained acceptance in many media where its corrosion resistance and engineering properties have provided the corrosion and design engineer with a reliable and economic material.

Here we examine the corrosive and erosive effects of water on titanium.

Fresh Water Corrosion - Steam

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

Some natural river waters contain manganese which deposits as manganese dioxide on heat exchanger surfaces. Chlorination treatments used to control sliming results in severe pitting and crevice corrosion on stainless steel surfaces. Titanium is immune to this form 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°F (260°C). Titanium tubing, exposed for 16 years to polluted seawater in a surface condenser, was slightly discolored but showed no evidence of corrosion. Titanium has provided over thirty years of trouble-free seawater service for the chemical, oil refining and desalination industries.

Exposure of titanium for many years to depths of over a mile below the ocean surface has not produced any measurable corrosion. Pitting and crevice corrosion are totally absent, even if 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 tide zone 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 a minimal rise in 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

ASTM Grades 1 and 2 are essentially immune to stress-corrosion cracking (SCC) in seawater. This has been confirmed many times as reviewed by Blackburn et al. (1973). Other unalloyed titanium grades with oxygen levels greater than 0.2% may be susceptible to SCC under some conditions. Some titanium alloys may be susceptible to SCC in seawater if highly-stressed, pre-existing cracks are present. ASTM Grade 5 with a low oxygen content is considered one of the best of the high strength titanium-base alloys for seawater service.

Corrosion Fatigue

Titanium, unlike many other materials, does not suffer a significant loss of fatigue properties in seawater.

Biofouling

Titanium does not display any toxicity toward marine organisms. Biofouling can occur on surfaces immersed in seawater. Cotton et al. (1957) reported extensive biofouling on titanium after 800 hours immersion in shallow seawater. The integrity of the corrosion resistant oxide film, however, is fully maintained under marine deposits and no pitting or crevice corrosion has been observed.

It has been pointed out that marine fouling of titanium heat exchanger surfaces can be minimized by maintaining water velocities in excess of 2 m/sec. Chlorination is recommended for protection of titanium heat exchanger surfaces from biofouling where seawater velocities less than 2 m/sec are anticipated.

Microbiologically Influenced Corrosion

Titanium, uniquely among the common engineering metals, appears to be immune to Microbiologically Influenced Corrosion (MIC). Laboratory studies confirm that titanium is resistant to the most aggressive aerobic and anaerobic organisms. Also, there has never been a reported case of MIC attack on titanium.

Crevice Corrosion

Localized pitting or crevice corrosion is a possibility on unalloyed titanium in seawater at temperatures above 180°F (82°C). ASTM Grades 7 and 12 offer resistance to crevice corrosion in seawater at temperatures as high as 500°F (260°C).

Galvanic Corrosion

Titanium is not subject to galvanic corrosion in seawater, however, it may accelerate the corrosion of the other member of the galvanic couple.

Source: Timet

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