TYPES OF CORROSION
Titanium, like any other metal, is subject to corrosion in some environments. The types of corrosion
that have been observed on titanium may be classified under the general headings: general
corrosion, crevice corrosion, stress corrosion cracking, anodic breakdown pitting, hydriding and
galvanic corrosion.
In any contemplated application of titanium, its susceptibility to corrosion by any of these modes
should be considered. In order to understand the advantages and limitations of titanium, each of
these types of corrosion will be explained.
GENERAL CORROSION
General corrosion is characterized by a uniform attack over the entire exposed surface of the metal.
The severity of this type of attack can be expressed by a corrosion rate. This type of corrosion is
most frequently encountered in hot reducing acid solutions.
Oxidizing agents and certain multi-valent metal ions have the ability to passivate titanium in
environments where the metal may be subject to general corrosion. Many process streams,
particularly H2SO4 and HCl solutions, contain enough impurities in the form of ferric, cupric ions,
etc., to passivate titanium and give trouble-free service. In some cases, it may be possible to inhibit
corrosion by the addition of suitable passivating agents.
Anodic protection has proven to be quite effective in suppressing corrosion of titanium in many acid
solutions. Almost complete passivity can be maintained at almost any acid concentration by the
proper application of a small anodic potential. Table 29^(47) gives data showing the passivation
achieved in some typical environments.
This procedure is most often employed in acid solutions having a high breakdown potential such as
sulfates and phosphates. In halides and some other media, there is a danger of exceeding the
breakdown potential which can result in severe pitting. The method is only effective in the area
immersed in the solution. It will not prevent attack in the vapor phase.
If the use of passivating agents or anodic protection is not feasible, TIMETAL Code-12 or 50A
Pd may solve the problem since these alloys are much more corrosion resistant than the
commercially pure grades.
CREVICE CORROSION
This is a localized type of attack that occurs only in tight crevices. The crevice may be the result of
a structural feature such as a flange or gasket, or it may be caused by the buildup of scales or
deposits. Figure 15 shows a typical example of crevice corrosion under a deposit.