Grade 7 Titanium and the Role of Palladium
Grade 7, UNS R52400, is unalloyed titanium with a deliberate palladium addition of 0.12 to 0.25 percent, classed under the German material number 3.7235. The palladium acts as a cathodic depolariser that stabilises the protective oxide film in environments where plain titanium would struggle to repassivate, particularly reducing acids such as hydrochloric and sulfuric acid and chloride solutions containing oxidising inhibitors. Mechanically the grade matches Grade 2, so it brings the corrosion benefit without changing the design allowables, and it remains fully weldable and highly ductile.
Specified Properties and Chemistry
ASTM B338 sets Grade 7 seamless tubing at 345 MPa (50 ksi) minimum tensile strength, 275 MPa (40 ksi) minimum 0.2% offset yield strength and 20 percent minimum elongation in 50 mm. Alloy chemistry is limited to 0.12 to 0.25 percent palladium, with maximum oxygen 0.25 percent, nitrogen 0.03 percent, carbon 0.08 percent, hydrogen 0.015 percent and iron 0.30 percent. Nominal density is 4.51 g/cm3 and the melting point is close to 1668 C, virtually the same as Grade 2, so the two grades behave almost identically on the shop floor apart from the corrosion performance.
| Property | ASTM B338 Grade 7 Requirement |
|---|---|
| Tensile strength, min | 345 MPa (50 ksi) |
| Yield strength, 0.2% offset, min | 275 MPa (40 ksi) |
| Elongation in 50 mm, min | 20% |
| Palladium | 0.12-0.25% |
| Iron, max | 0.30% |
| Density | 4.51 g/cm3 |
Why Coiled Tubing Is Supplied in This Grade
Coils remove field welds, and every eliminated weld is an eliminated leak path in a chemical injection or instrument line. Because Grade 7 has 20 percent minimum elongation and a single phase alpha structure, it can be coiled, uncoiled and straightened repeatedly without cracking, and it tolerates the tight bend radii needed on skids and inside cable trays.
Chemical injection and capillary lines in oil and gas production, where the injected fluid is acidic or contains chlorides.
Instrument and impulse lines in chlor-alkali, pulp and paper bleach plants and pickling houses.
Heat exchanger coils, immersion coils and heating or cooling serpentines.
Seawater and brine sampling lines where stainless steel would pit.
Pollution control circuits such as flue gas scrubber and chlorination systems.
Manufacturing, Coiling and Testing
Seamless coil is produced by extruding a hollow and cold reducing it to size, then vacuum annealing and pickling before spooling. Coils are wound onto wooden or plastic lined reels to protect the surface, and the bore is purged, dried and capped so that it arrives free of moisture and chloride contamination. Full length eddy current examination is standard, together with tensile and flattening tests from the finished coil and a hydrostatic test where the ordering specification requires it. Tolerances on outside diameter, wall thickness, ovality and wall eccentricity follow the tube requirements of ASTM B338, and additional hardness or flare testing can be added for tooling qualification. The standard carbon content limit is tight enough that field welding is straightforward, though any weld is still made under full argon purge. Each coil is released with an EN 10204 3.1 mill test certificate and heat number traceability.
Straightening, Bending and Installation
Field practice differs from pipe work. Coils are unrolled with a powered straightener rather than pulled by hand, because dragging the tube over steel causes iron contamination that can later act as a corrosion initiation site. Bending is performed with dedicated titanium or plastic lined tooling and generous radii, and tube ends are cut with a dedicated wheel or saw that has not been used on carbon steel. Tube supports use plastic or stainless sleeving so the titanium never touches plain carbon steel. Where the coil is used in an injection line, the flow velocity is normally kept below the level that causes erosion corrosion, and the system is flushed with clean water before the acid service begins. Titanium in this grade should not be used in dry chlorine gas or in methanol service without specific assessment.
Frequently Asked Questions
Q: What is the difference between Grade 7 and Grade 2 titanium tubing?
A: The chemistry and mechanical properties are almost identical. Grade 7 contains 0.12 to 0.25 percent palladium, which allows it to resist reducing acids and chloride environments that would attack Grade 2.
Q: How long can a seamless coil be supplied?
A: Coils are normally supplied between 30 m and 300 m per reel depending on diameter and wall thickness, and longer continuous lengths are quoted individually because weight and reel size become the limiting factors.
Q: Can Grade 7 coil be welded in the field?
A: Yes. It is welded with the same inert gas procedures as Grade 2, using a full internal purge and shielding until the joint cools below about 400 C.
Q: Why is Grade 7 preferred for chemical injection lines?
A: Because injection chemicals often create a reducing or mixed acid condition in which unalloyed titanium may not repassivate. The palladium addition keeps the protective film stable and dramatically lowers corrosion rates.
Q: How is the coil protected during shipment?
A: The bore is cleaned, dried and capped, the coil is wound on a lined reel, and the outside is wrapped. Contact with carbon steel is avoided to prevent iron contamination of the surface.
