Penn Ma, beiyutitanium.com, discusses how titanium grade should be based on actual working conditions, not general reputation.
Titanium has a reputation for being corrosion resistant, and in the right conditions it earns it.
The difficulty is that relying on reputation alone does not mean it will match specific conditions. A specifier reads that titanium resists seawater and assumes it will resist a hot acid stream, or that it is used in bleaching plant and assumes it will hold in a fluoride-bearing one.
Titanium can move from effectively immune to rapidly attacked across a surprisingly narrow change in chemistry. For chemical service, the question is not if titanium is corrosion resistant, but if it is resistant to a specific, fully described environment.
Passivity has an envelope
Titanium owes its corrosion resistance to a thin, tenacious oxide film. That film is stable and self-healing across a broad span of oxidising and neutral conditions, which is why titanium performs so well in oxidising acids, chlorides and seawater. It is not stable everywhere.
The film can be undermined by hot reducing acids, by acidic fluoride environments, and by the local chemistry that develops inside a crevice. So, passivity is best thought of as an envelope defined by the process conditions rather than a fixed property of the metal.
The variables that set the boundary of that envelope are chemistry and concentration, temperature, the degree of aeration, the presence of crevices, and specific contaminants.
Oxidising versus reducing is the first fork
A first-order distinction in chemical service is whether the environment is oxidising or reducing. In oxidising and neutral conditions, the oxide film is reinforced and titanium is in its comfort zone.
In uninhibited, non-oxidising reducing acids such as hydrochloric or sulphuric acid, the film is harder to maintain, and commercially pure titanium can corrode as temperature or concentration rise, with published data reporting a high corrosion rate for commercially pure titanium in boiling dilute sulphuric acid.
Whether titanium is excluded depends on the full chemistry and the grade rather than on the acid alone, since palladium-bearing grades and oxidising inhibitors extend the useful range. This is a boundary to be respected rather than hidden, and it is also where alloying earns its place, which is why the grade cannot be chosen without first placing the service on the oxidising to reducing scale.
Temperature and chloride move the boundary
Within the chloride envelope, temperature is a principal variable. Titanium resists chloride pitting and crevice attack well at ambient temperature, but its crevice resistance falls as temperature rises.
Published titanium corrosion data for saturated chloride indicate that crevice corrosion of commercially pure titanium in sharp crevices occurs at about 200 degrees Fahrenheit, roughly 93 degrees Celsius, and above, and is not likely below about 158 degrees Fahrenheit, roughly 70 degrees Celsius, with the onset moved to lower temperatures by reduced pH and by high concentrations of calcium or magnesium ions.
The practical conclusion is that there is no single critical temperature that can be quoted independently of the environment, and a crevice, once present, sets a stricter requirement than the flat surface next to it.
Two conditions to name explicitly: fluorides and reducing acids
One contaminant deserves to be flagged because it can remove titanium from contention rather than merely shifting the boundary.
Titanium is attacked by hydrofluoric acid, and free fluorides in an acidic solution can form hydrofluoric acid in situ and break down the protective oxide. Fully complexed fluoride is less aggressive, so the risk depends on speciation rather than on the mere presence of fluorine, but any process that can present free fluoride at low pH should be identified before titanium is selected.
Hot reducing acids sit alongside this as a condition to be named and quantified rather than assumed away, though there the usual response is a change of grade rather than the exclusion of titanium.
Matching the grade to the condition
Once the environment is described, the grade becomes a considered response rather than a default. Commercially pure Grade 2 is the workhorse for oxidising and neutral chloride service and is specified across a large part of the chemical industry.
Where the service turns reducing or the pH drops, Grade 7, essentially Grade 2 with about 0.15 per cent palladium, extends resistance because the palladium stabilises the passive film in low pH and reducing acids.
Grade 12 improves crevice resistance in hot brines and mildly reducing conditions, but it should not be treated as interchangeable with Grade 7 in strongly reducing, low pH acid. The grades are not a ladder of quality but a set of responses to different conditions, and choosing between them is only possible once the condition is defined.
Define the envelope first
The discipline that helps avoid titanium corrosion failures in chemical plant is unglamorous. Before selecting a grade, the environment should be described in full: the chemistry and its concentration, the operating and upset temperatures, whether the stream is oxidising or reducing, whether it can carry free fluoride at low pH, and whether the design introduces crevices at gaskets, deposits or supports.
With that envelope fixed, the grade follows, and the material can be verified on receipt with an inspection certificate 3.1 to EN 10204, which records the specific inspection results and confirms conformity with the order. Identity of the installed part still rests on matching heat-batch marking and a traceability chain through cutting, fabrication and installation. Titanium is a capable material for chemical service. It simply asks to be selected against a defined environment rather than a reputation.
References
TIMET, Corrosion Resistance of Titanium (technical manual), for oxide-film stability, crevice corrosion in chloride, reducing-acid behaviour and fluoride attack. https://www.timet.com/assets/local/documents/technicalmanuals/corrosion.pdf
EN 10204:2004, Metallic Products, Types of Inspection Documents (defines the inspection certificate 3.1). https://knowledge.bsigroup.com/products/metallic-products-types-of-inspection-documents
ASTM B348, B265 and B338 are product-form specifications for titanium bar, plate and heat-exchanger tube; cited for grade and form, not for corrosion limits. https://www.astm.org/
About the author
Penn Ma is export sales manager for Jiaxing Beiyu New Materials Technology Co., Ltd. (Beiyu Titanium), a China-based titanium materials supplier and exporter.
