Specifications for Titanium for industrial corrosion service are published by recognized industrial societies around the world.  The most common are ASTM, ASME, DIN, BS, SIS, and JIS standards.   In North America, and in many parts of the world, the ASTM, AWS, or ASME product specifications listed in Table 1 can be utilized.   Comparable specifications for most product forms and grades are published by the other standards organizations.  

ASME specifications are adapted from ASTM specifications for the same products, but include only those grades or alloys of Titanium than are permitted for pressure vessel construction.   ASME favors alloys which have higher ductility, usually with elongation of at least 10%. ASME requires more stringent material testing and documentation, often adopting as mandatory some of the optional requirements contained in the ASTM specifications. Otherwise, ASTM and ASME specifications are the same.   Note that the specification B-XXX-YY indicates an ASTM non-ferrous material specification and the year of the latest update.   The corresponding SB-XXX designations indicate ASME accepted specifications, for which only the current version is acceptable. While ASME materials always meet the ASTM Specification, ASTM material can not be substituted for ASME material.

ASTM stopped publication of B-337 in 1996 and replaced it with B 861 and B 862. It is suggested that engineering specifications reference the newer B 861 and B 862, but allow approved substitution of B 337 for a couple more years. ASME is in the process of adopting B 861 and B 862, but SB 337 is still current for Code use.

Under each of the product specifications in Table 1 are several grades which define chemical composition and mechanical properties. Tables 2, 3, and 5 list the more widely used ASTM grades and indicates which are accepted for ASME Code construction.  

In addition to properties, availability is a key consideration for industrial use of any of the Titanium grades.   It is best to stick with the "workhorse” grades for normal applications.   Grade 2 unalloyed Titanium is the preferred grade for most pressure vessels and other equipment for the process industries.

Unalloyed Grades

Grades 1, 2, and 3 are "un­alloyed" Titanium, referred to in the Titanium industry as commercially pure or “CP”.   Their relative strength is determined principally by the increasing content of interstitial elements, oxygen, nitrogen, hydrogen, carbon and iron.   These elements are impurities, much like carbon in steel, but their effect is different.    The metal consists of a regular three-dimensional matrix of Titanium atoms, with the interstitial elements occupying sites between the Titanium atoms.   Their presence causes lattice strain, which increases strength at the expense of ductility.

Grade 2 unalloyed Titanium is considered the workhorse industrial Titanium grade. The Grade 2 composition provides an optimum balance of strength and ductility and is suitable for most industrial corrosion applications.   Grade 2 is commonly specified for solid construction in corrosion resistant process equipment, including shafting, mechanical equipment, pipe, tanks, heat exchangers, and vessels.

The popularity of Grade 2 is also the result of an early Titanium industry campaign to standardize on a single industrial grade by making Grade 2 in standard sizes more available and less expensive per pound than Grade 3. This standardization was a key to growth in industrial applications, and helped the warehouse business to develop.

Except as discussed below, Grade 2 remains the best choice for general corrosion resistant process equipment construction.   All three unalloyed grades are accepted for ASME Division 1 and 2 construction.

Grade 1 unalloyed Titanium has the highest ductility of the unalloyed grades, but low strength.   Consequently, it is rarely specified for piping or vessels constructed of solid Titanium.   However, Grade 1 is used for production of explosive clad Titanium on steel plate because its ductility is needed in the bonding process (Grade 2 is sometimes used, but cracking at the bond line has been observed).   In clad construction, the strength of the cladding material is not normally considered in design, so the low strength is of no consequence.   Grade 1 is also used for plate type heat exchangers, loose linings, expanded metal and similar applications requiring high ductility. Grade 1 may have slightly better corrosion resistance than Grade 2, but the difference is hardly noticeable in most applications.

Grade 3 has the highest strength of the common unalloyed grades, but has had limited use in recent years.   Poor availability in the variety of product forms needed for a typical fabrication is the main limiting factor, although reduced fabricability due to lower ductility is also a consideration. Grade 3 has slightly worse corrosion resistance than Grade 2, but the difference is hardly noticeable in most applications.   Grade 3 could be considered as an alternate to Grade 2 in the construction of very large pressure vessels where schedule permits mill orders for material to be used, and the savings in material weight will offset the cost and fabricating difference.   Grade 3 might also be used in some shafting applications, but care in design to be sure that the highly stressed areas are in the wrought Grade 3 is desirable for optimum fatigue resistance.   In piping systems, where Grade 3 seamless pipe can be utilized to stay in a lighter pipe schedule, a savings can sometimes result.

Noble Metal Grades

Several Titanium alloys with small amounts of noble metals have industrial significance. While Titanium has excellent corrosion resistance to many corrosive media, it has limits in very hot neutral brine, and under acidic or reducing conditions as occur in crevices.   The noble metal addition improves the corrosion performance of the alloy under these conditions and extends the useful range of service temperatures in sea water and brine service.   The presence of a small amount of the noble metal has virtually no effect on the mechanical properties of the material but has a significant effect on corrosion resistance resulting from its presence at the Titanium surface in contact with the corrosive medium.   The protection mechanism is believed to be a small protective current from the more noble palladium or ruthenium which galvanically protects the adjacent Titanium surface.

Originally, an alloy containing a conservative amount of about 0.2 Pd was developed to take advantage of this effect.   The alloy development was based on early work which showed that small additions of most Platinum Group Metals enhanced Titanium’s corrosion performance.   Two Titanium-palladium alloys, Grade 7 (comparable mechanically to Grade 2) and Grade 11 (comparable to Grade 1) have had wide industrial use.    However, the presence of even a small amount of palladium dramatically increased the cost of the alloy, and as the cost of palladium increased, the industry sought to find a way to reduce the cost of this important alloy group.   The first step was to reduce the specified palladium content to 0.15 and hold it close to the lower specification limit of 0.12, which became possible as the industry learned how to better control alloy additions.  

Next, based on a review of the early work which showed that the effect of the palladium was retained down to 0.03 to 0.05%, a series of lean   palladium alloys were introduced.   Lower palladium Grade 16 is comparable mechanically to Grade 2, and Grade 17 to Grade 1.  

In addition, lean palladium alloys of Grade 9 (3Al-2.5V) and Grade 5 (6Al-4V) were introduced to allow greater use of these high strength alloys in developing non-pressure vessel applications.   The lean palladium still extends the range of application of these alloys and is effective for most applications, but reduces cost compared to the higher palladium grades.

To further reduce cost, alternate platinum group metals were reviewed.   The effect on corrosion resistance of ruthenium is similar to that of palladium, although it takes about twice as much ruthenium to get the same level of protection.   However, since the price of ruthenium is only 12 to 15% of the price of palladium, and is the lowest in cost of the PGM metals, the alloy has a cost advantage.

Cost today for the high palladium grades are about 1.9 times that of the corresponding unalloyed grade 2, 1.38 times for the lean palladium grades, and 1.12 times for the ruthenium grades. The availability of the ruthenium grades is still limited, although over 1 million pounds of Grade 29, the 6Al-4V-ELI 0.1 Ru grade, have been applied in tubular components for undersea oil and gas applications.   The ruthenium alloys are expected to become increasingly important in the years ahead, particularly once ASME Code approval allows more end users to take advantage of their economies compared to the palladium alloys.

The noble metal grades are sometimes used selectively for gasket faces and in similar situations where crevice corrosion of unalloyed Titanium might be a problem in particularly aggressive corrosion applications.  

Finally, a series of nickel-ruthenium grades were developed in Japan that have essentially the same properties as the grades which do not contain nickel.   These grades are not widely available, so applications to date are limited.

Table 4 lists the noble metal grades along with the corresponding standard grades and should help to understand the relationship of the noble metal grades to their standard counterparts.

Nickel Molybdenum Alloy

An important alloy is Grade 12, Ti 0.8 Ni-0.3 Mo.   This alloy was originally developed to provide higher temperature corrosion resistance in brines at a lower cost than Grade 7 (Ti-0.2 Pd alloy).   It was developed based on corrosion test results which showed that both nickel and molybdenum have a beneficial effect on Titanium corrosion resistance under crevice conditions in brine service.   Grade 12 has found additional applications where its higher allowable stresses at elevated temperatures under the ASME Code result in lower equipment cost.   Recent ASME approval of Grade 12 for Division 2 provides end users with even greater economy for pressure vessels.   Titanium Fabrication Corporation sponsored the original Code Case, was the first company to build equipment from this alloy, and has more experience with it than any other company.  

Aluminum Vanadium Alloys

Where an application requires greater strength, the workhorse Titanium alloys are Grade 5 (Ti 6Al-4V), or the low interstitial variant Grade 23 (Ti 6Al-4V ELI). These high strength alpha beta alloys have about the same corrosion resistance as unalloyed Titanium, but a yield strength of 120,000 psi or 110,000 for the ELI variant.   They have lower ductility, cost more, and are harder to machine and form than the unalloyed grades, although weldability is similar. It is good practice to specify extra low interstitial, ELI Grade 23 where toughness, fatigue and corrosion re­sistance are needed.   These grades are not accepted for ASME Code construction.  

Consequently, industrial application is generally limited to things like high strength bolts and keys, shafting, etc. (the 747 wings are supported by massive forgings and the Alvin submarine hull is fabricated from plate of this alloy).

Typical non-aerospace applications of Grade 5 include shafting for both chemical process and marine applications, jet­ pump components, research submarine pressure spheres and hardware for high performance toys, such as 12 meter sailboats and racing car components.   A helicopter transportable hyperbaric diver rescue system was fabricated by Titanium Fabrication Corporation from Grade 23 material.

Another alpha beta alloy, Grade 9   (Ti 3Al-2.5 V) is now permitted for Division 1 Code construction.   This higher strength alloy could be considered for solid construction of certain large, high temperature pressure vessels like autoclaves for pressure hydrometallurgy, or for higher pressure piping systems where its properties could lead to economy.  

One application where the Aluminum Vanadium alloys are gaining acceptance is in energy industry tubular products.   These have been applied in both geothermal brine and hydrocarbon wells.   As the production depths increase, temperatures and pressures increase, and often sour (H₂S containing) conditions exist, Titanium becomes the economical material of choice.   With the high cost of deep operations, particularly offshore, Titanium becomes the first choice for reliability and capability.

To meet the requirements of increased corrosion resistance, ruthenium enhanced Grade 28   (Ti 3Al-2.5V Ru), and Grade 29 (Ti 6Al-4V ELI Ru) have increased the practical range of application temperatures. There are also lean palladium alloys of the aluminum-vanadium compositions Grade 24 (Ti 6Al-4V-Pd) and Grade 18 (Ti 3Al-2.5V-Pd).   There is no lean palladium grade corresponding to Grade 23 (Ti 6Al-4V ELI), but specifications for this grade with palladium addition could be negotiated if it were needed.

These alloys are available in plate, sheet, pipe, tube, bar, billet and forgings, but small volume users should consult with their suppliers as they develop requirements.  

Ignition Resistant Grade (Titanium-Niobium Alloy)

A proprietary grade that does not as yet have an ASTM designation is the Ti 45 Nb alloy.   It was developed to provide Titanium corrosion resistance in inhibited mineral acids while minimizing susceptibility to ignition in pure oxygen environments.   This grade is finding application in pressure hydrometallurgy for leaching metal value form sulfide and refractory ores.   It is relatively expensive relative to the unalloyed grades, roughly comparable to Grade 7, but used selectively where its unique ignition resistance property is required solves a serious application problem.

Nitinol - the Metal with a Memory

Another interesting grade, which is sometimes forgotten, is a 55% Titanium, 45% nickel alloy which has unique shape memory properties.   This alloy can be formed to the desired shape and given a specific heat treatment.   If it is then formed to a different shape, for example for packing or to allow insertion in a confined space, then heated to a specific intermediate temperature, the metal returns to its original shape.   An early application was for a wire antenna for a spacecraft, where an electric current was used to reheat the wire to cause it to expand to a large loop.   The properties of the alloy are sometimes demonstrated by bending a short length of wire, then immersing it in a cup of hot coffee, causing it to return to its original straight condition.

One memorable application has been in underwire supports for machine washable brassieres.

Aerospace Alloys

It should be noted that the Titanium industry has grown from an aerospace base for which it has developed many high strength alloys, some of which are heat treatable, for application in jet engines and airframes.   These alloys are generally more difficult to fabricate, require more sophisticated design and nondestructive testing and often lack the corrosion re­sistance of the simpler grades commonly used in the industrial sector. These alloys are also priced 1 1/2 to 2 times higher than the industrial grades because of the rigid manufacturing requirements of the aerospace industry.

However, as industrial applications continue to increase in sophistication, the need for high performance and reliability increases, and the cost of downtime or replacement becomes prohibitive, designers should remember to look beyond the current ASTM Grades for special alloys with the properties they may need.   Reference to Table 4 and the ASTM Metals Handbook is a good starting point to locate these materials.

References

1.       “Effects of Metal Chemistry on Behavior of Titanium in Industrial Applications”, Forrest, A. L., Industrial Applications of Titanium and Zirconium, ASTM STP 728, E. W. Kleefisch, Editor.   ASTM 1980.

2.       “Developments in Titanium Alloy Environmental Behavior”, Schutz, R. W., Eighth World Conference on Titanium, Birmingham, England, October 22-26, 1995.

3.       Metals Handbook, Ninth Edition, Volume 13, Corrosion, Corrosion of Titanium and Titanium Alloys,   pp. 669-706.

4.       Process Industries Corrosion, The Theory and Practice, NACE International, Houston, Texas, Titanium Chapter, pp. 503-527.