Classification of Titanium Corrosion Patterns
Mar 12, 2024
Titanium is described as highly corrosion resistant, and for most duties that description is accurate. The failures that do occur are almost always local rather than general, and recognising which local mechanism is at work is the difference between solving a corrosion problem and repeatedly replacing the same component.
General Versus Localised Attack
General corrosion is uniform and comprehensive: the whole wetted surface thins at a similar rate. Unalloyed titanium is passive across a very wide range of conditions, so general corrosion is rarely the limiting mechanism and is usually easy to design for by allowing a corrosion allowance. Localised corrosion is different. It attacks a small area at a rate far above the general rate, and because it concentrates, it produces perforation, cracking or sudden failure long before the wall has measurably thinned. Localised attack is where almost all practical titanium corrosion problems sit, and it divides into a small number of recognisable patterns.
Crevice Corrosion
Crevice corrosion occurs in narrow gaps where solution cannot circulate: under gaskets, beneath pipe supports, in thread roots, at rolled tube-to-tubesheet joints and where deposits rest on the surface. In a tight gap the oxygen in the trapped liquid is consumed faster than it can be replenished, the environment acidifies and chloride concentrates, and the passive film can break down. The gap then prevents repassivation because fresh oxygen never reaches the site. The key variable is temperature: titanium is generally regarded as safe in chlorides up to roughly 70 °C, and crevice corrosion becomes a realistic risk above that, which is why Grade 7 with its palladium addition is specified for hot chloride service.
Pitting Corrosion
Pitting is attack in an open, exposed area, and it is associated with halide ions. Titanium has very high resistance to pitting in chloride solutions and is essentially immune over the normal range of seawater and process temperatures, which is the property that first made it attractive for marine and chemical service. Where halide concentrations and temperatures rise, or where the surface has been contaminated with iron from fabrication tooling, pit initiation becomes possible. The practical lesson is that clean, iron-free surfaces matter as much as the base grade when pitting is the concern.
Stress Corrosion Cracking
Stress corrosion cracking is brittle failure that occurs when a susceptible material is exposed to a specific environment while a tensile stress is present. For titanium the classic environments are anhydrous methanol and other alcohols containing halides, and certain nitrogen tetroxide mixtures. The combination matters: methanol alone is not the problem, and neither is the halide alone, but the two together at a stress level above the threshold will crack titanium, and dry methanol is more aggressive than methanol containing a small amount of water. Fabrication practice therefore pays attention to the residual stress left by cold working and by welding, and a stress relief anneal is used where the environment demands it.
Erosion and Flow-Assisted Attack
Erosion corrosion occurs where flowing fluid applies mechanical action that removes the protective film faster than it can reform. The result is metal loss that follows the flow pattern rather than the general surface, showing up at bends, downstream of restrictions, at impingement points and in pump casings. Titanium's film is fast to reform, which gives it good resistance, but particulates in the stream, high local velocities and impingement angles will still initiate loss. Slowing the local velocity, removing solids and avoiding sharp changes of direction are the design answers.
Galvanic Corrosion With Dissimilar Metals
Galvanic corrosion, also called dissimilar metal contact corrosion, is driven by the potential difference between two metals in electrical contact in an electrolyte. Titanium sits at the noble end of the series in seawater, so in most couplings it is the cathode and the other metal is the one that is consumed. The classic case is titanium tube in a carbon steel tube box or a titanium component fastened with steel bolting. The attack is not limited to the base metal side either: a large coupled steel area can cathodically polarise the titanium, generating hydrogen on its surface and leading to hydrogen uptake. The countermeasures are standard: use compatible materials, provide electrical insulation at joints where possible, and avoid the combination altogether in aggressive electrolytes.
Hydrogen Absorption and Hydride Embrittlement
Hydrogen can be absorbed into titanium from cathodic protection, from pickling, from welding without adequate shielding, or from galvanic coupling, and once the amount absorbed exceeds the solid solubility of the metal, brittle hydride forms and cracks can initiate. This is why the material specifications cap hydrogen at 0.015 % for most grades, and why acid cleaning has to be controlled and followed by thorough rinsing. Hydrogen damage is a processing and fabrication discipline issue more than a service selection issue, and it is the one mechanism entirely under the fabricator's control.
Matching the Mechanism to the Fix
| Mechanism | Where it appears | Design or selection response |
|---|---|---|
| Crevice | Gaskets, supports, thread roots, rolled joints | Eliminate gaps, exclude oxygen ingress, or select Grade 7 above about 70 °C |
| Pitting | Open surfaces exposed to concentrated halides | Keep surfaces clean and iron free through pickling and passivation |
| Stress corrosion cracking | Methanol or alcohol with halides, certain oxidisers | Avoid the environment, control residual stress with a stress relief anneal |
| Erosion | Bends, restrictions, impingement, pump casings | Reduce local velocity and remove entrained solids |
| Galvanic | Contact with steel, aluminium or copper alloys | Isolate joints or match materials to avoid a strong couple |
| Hydrogen embrittlement | Pickling, welding, cathodic protection, coupling | Control hydrogen to specification and shield all hot titanium work |
Reading the table in reverse is a useful inspection technique. Given a failed part, the location of the damage usually identifies the mechanism: damage confined under a gasket is crevice corrosion, damage along a weld or a sharp bend suggests residual stress, and damage downstream of a valve suggests flow assisted attack.
Frequently Asked Questions
Q: Is titanium corroded by seawater?
A: Not in general. Titanium is effectively immune to uniform corrosion and pitting in ambient seawater. The risk is crevice corrosion in tight gaps at elevated temperature, generally above about 70 °C.
Q: When does crevice corrosion become a real risk for titanium?
A: In chloride bearing solutions it becomes a practical concern above roughly 70 °C, particularly where a gap restricts circulation so the trapped liquid cannot be replenished with oxygen.
Q: What causes stress corrosion cracking in titanium?
A: The combination of tensile stress and a specific environment, most commonly anhydrous methanol or other alcohols containing halides. Water content in the alcohol and residual fabrication stress both influence the outcome.
Q: Why does coupling titanium to steel cause problems?
A: Titanium is noble in seawater, so the steel becomes the anode and corrodes quickly as the area of the couple increases. The titanium can simultaneously absorb hydrogen from the cathodic reaction.
Q: How is hydrogen embrittlement of titanium prevented?
A: Keep hydrogen within the 0.015 % maximum set by the material specifications, shield all hot welding and cutting work with inert gas, and control acid pickling so that hydrogen uptake is minimised.
