Selecting the right titanium grade for a chemical process unit often comes down to a deceptively simple question: commercially pure (CP) or alloyed? Both families offer the passivity and light weight that make titanium indispensable in aggressive environments, but their corrosion behavior diverges where it matters most \u2014 in hot reducing acids, in chloride stress-corrosion scenarios, and under the precise temperatures and pressures that keep a plant running.<\/p>\n
A purchasing engineer who misreads that divergence can end up with a maintenance nightmare. The correct choice, grounded in data rather than habit, extends service life, reduces unplanned downtime, and protects the capital investment locked into vessels, piping, and heat exchangers. This article draws on published corrosion test programs, ASTM standard methodologies, and real-world fabrication experience to draw a clear line between CP titanium grades (typically Grade 2) and the workhorse alloy Ti-6Al-4V (Grade 5). It pays special attention to the media and process conditions encountered in chlor-alkali, nitric acid, sulfuric acid, and organic acid service \u2014 exactly where the decision has the largest financial consequence.<\/p>\n
CP titanium contains no deliberate alloying additions beyond iron and oxygen residuals that control strength. Grade 1 through Grade 4 differ mainly in these interstitial elements; the crystal structure remains a single \u03b1\u2011phase up to the beta\u2011transus temperature (~882\u202f\u00b0C). That single-phase structure, combined with a highly adherent rutile\u2011type oxide film, yields outstanding resistance to oxidizing environments and to chloride\u2011induced pitting and crevice corrosion.<\/p>\n
Ti\u20116Al\u20114V is a two\u2011phase \u03b1+\u03b2 alloy. The aluminium stabilises the \u03b1 phase, while vanadium, a strong \u03b2 stabiliser, introduces a body\u2011centred cubic phase at room temperature. This microstructural duality triples the room\u2011temperature yield strength compared to CP Grade 2 (typically \u2265\u202f825\u202fMPa versus \u2265\u202f275\u202fMPa) but also makes the alloy susceptible to selective phase attack and stress\u2011corrosion cracking under conditions that would not affect CP grades. In practical terms, the same vanadium\u2011rich beta phase that gives Grade 5 its strength becomes a corrosion liability when the environment turns reducing.<\/p>\n
Understanding this fundamental trade-off lets engineers look past the generic \u201ctitanium = corrosion proof\u201d assumption and begin to match the material to the specific chemistry and temperature cycle of the plant.<\/p>\n
Corrosion data cited below draws from the ASTM G31 mass\u2011loss immersion test protocol and from electrochemical potential scans reported in open technical literature. All rates are expressed in mm\/year or, for pitting, as a critical pitting potential (Epit) measured versus a saturated calomel electrode (SCE).<\/p>\n
Nitric acid, chromic acid, and wet chlorine gas create conditions that reinforce the protective oxide film. Published G31 data show that CP Grade 2 in boiling 65\u202f% nitric acid corrodes at a rate below 0.05\u202fmm\/year \u2014 often measured as low as 0.01\u202fmm\/year in modern vacuum\u2011annealed product. Ti\u20116Al\u20114V under the same conditions typically exhibits a rate below 0.1\u202fmm\/year, which is still excellent but roughly three to five times higher than CP. The difference is rarely large enough to eliminate Grade 5 from the candidate list, but it makes CP titanium the default for nitric\u2011acid heater tubes and storage tanks where even trace metal ion pickup is unacceptable.<\/p>\n
In moist chlorine gas at temperatures up to about 80\u202f\u00b0C, both materials stay passive and show corrosion rates well below 0.001\u202fmm\/year. Here the choice tends to hinge on mechanical requirements: a thin\u2011walled duct, for instance, may need the higher stiffness of Ti\u20116Al\u20114V to resist buckling under vacuum, while a flange face sees no such demand and can be constructed from CP plate.<\/p>\n
Sulfuric and hydrochloric acids in the reducing regime attack titanium because the oxide film is thermodynamically unstable. CP grades resist dilute, aerated sulfuric acid up to roughly 10\u202f% concentration at room temperature, but the corrosion rate climbs once the temperature exceeds 50\u202f\u00b0C. Boiling 10\u202f% H\u2082SO\u2084, for example, yields a typical CP Grade 2 corrosion rate around 0.3\u202fmm\/year. Ti\u20116Al\u20114V, conversely, can exceed 1.0\u202fmm\/year in the same bath \u2014 the vanadium\u2011rich beta phase dissolves preferentially, creating microscopic galvanic couples. In 5\u202f% hydrochloric acid at 60\u202f\u00b0C, the gap can be even wider, with CP titanium registering 0.1\u20110.3\u202fmm\/year while Grade 5 reaches 2\u202fmm\/year or more, depending on oxygen content.<\/p>\n
A useful reference is the iso\u2011corrosion diagram published by the Titanium Information Group, which shows that the safe temperature\u2011concentration envelope for Ti\u20116Al\u20114V in H\u2082SO\u2084 is about 20\u202f\u00b0C narrower than that of CP Grade 2. For most chemical plants, the lesson is unambiguous: if the process stream swings acidic and reducing, steer toward CP grades and keep the operating temperature within the proven envelope.<\/p>\n
One of the strongest value propositions for any titanium material is immunity to chloride pitting in all but the most extreme conditions. In aerated 3.5\u202f% NaCl at 25\u202f\u00b0C, the Epit for CP Grade 2 sits above +1.2\u202fV SCE \u2014 far beyond any potential encountered in service. Ti\u20116Al\u20114V shows a slightly lower Epit, around +1.0\u202fV SCE, but the difference is academic in most neutral brines. The real operational risk is crevice corrosion at elevated temperatures. Standard ASTM G48 tests demonstrate that CP Grade 2 can withstand 3\u202f% NaCl at 70\u202f\u00b0C without crevice attack, whereas Ti\u20116Al\u20114V may show minor weight loss under similar conditions if crevice former pressure is high. When designing gasketed joints for hot brine service, many process licensors explicitly specify CP titanium for the seating faces.<\/p>\n
Hot chloride environments can crack many stainless steels, but they rarely threaten titanium \u2014 unless the alloy contains vanadium. Ti\u20116Al\u20114V has been documented to suffer stress\u2011corrosion cracking in chloride solutions at temperatures above approximately 230\u202f\u00b0C, particularly when the crack tip is exposed to low\u2011pH conditions promoted by hydrolysis. CP titanium, in contrast, remains essentially immune to chloride SCC at all practical service temperatures, a fact recorded in the NACE MR0175 \/ ISO 15156 guidance for sour service materials. For a chemical plant processing hot acidic chlorides, this gives CP titanium a clear safety\u2011critical advantage.<\/p>\n
The table below consolidates representative corrosion rates drawn from multiple published G31 test campaigns. The values are typical and should be treated as \u201cexpect order\u2011of\u2011magnitude\u201d figures; site\u2011specific coupon testing is always recommended.<\/p>\n
| Corrosive medium | Temperature | CP Titanium (Grade 2) rate (mm\/y) | Ti-6Al-4V (Grade 5) rate (mm\/y) | Key reference | |——————|————-|———————————–|———————————|—————| | 65\u202f% HNO\u2083, boiling | 121\u202f\u00b0C | \u2264\u202f0.05 | \u2264\u202f0.10 | ASTM G31 data compilation | | 10\u202f% H\u2082SO\u2084, boiling | 102\u202f\u00b0C | ~0.30 | ~1.20 | ASM Handbook Vol.\u202f13B | | 5\u202f% HCl, 60\u202f\u00b0C | 60\u202f\u00b0C | 0.10\u20110.30 | 1.5\u20112.5 | K. W. J. Barnard, Titanium: A Technical Guide<\/em> | | 3.5\u202f% NaCl, 25\u202f\u00b0C | 25\u202f\u00b0C | <\u202f0.001 | <\u202f0.001 | Electrochemical Epit data, SCE ref. | | Seawater, 70\u202f\u00b0C, crevice | 70\u202f\u00b0C | No attack | Minor crevice loss | ASTM G48 modified method | | Wet Cl\u2082, 80\u202f\u00b0C | 80\u202f\u00b0C | <\u202f0.001 | <\u202f0.001 | Industry field reports |<\/p>\n Analysis of the table reinforces a key procurement principle: the differential between the two materials is negligible in oxidizing, near\u2011ambient\u2011temperature services and grows to a factor of five or more in hot reducing acids. The cost difference \u2014 Grade 5 is typically 15\u201130\u202f% more expensive than Grade 2 depending on mill form \u2014 must therefore be justified by a genuine need for the alloy\u2019s mechanical strength, not by a perceived \u201cbetter corrosion resistance.\u201d<\/p>\n A plant\u2019s bill of materials rarely stays at the level of a generic grade call\u2011out. The final selection also depends on available mill forms, fabrication constraints, and downstream joining processes.<\/p>\n For solid round stock used in pump shafts, valve stems, and fastener blanks, Titanium Bar<\/a><\/strong><\/em> can be sourced in both CP and Grade 5 conditions from one supplier, allowing the maintenance engineer to standardize procurement channels. Grade 5 bar is often the choice for high\u2011cycling rotating components, but the wetted end that contacts process fluid may still be clad or sleeved with CP titanium if the medium is aggressive.<\/p>\n Thin\u2011gauge components such as heat\u2011exchanger baffle sheets, tank liners, and gaskets are most economically produced from Titanium Sheet<\/a><\/strong><\/em>. CP Grade 2 sheet exhibits excellent deep\u2011draw formability, which matters when a chemical vessel lining must follow complex contours without fracturing. Ti\u20116Al\u20114V sheet can be formed but requires stress\u2011relief annealing and careful springback compensation \u2014 adding cost that must be weighed against a corrosion advantage that may not exist.<\/p>\nHow the decision changes with product form<\/h2>\n