Acid resistant Tantalum coatings have been developed in an effort get the extraordinary acid resistant properties of tantalum metal while being able to reduce the costs compared to solid tantalum. While
and inertness are extraordinary, historically tantalum's costs have been prohibitive for broad industry acceptance. As a result, industry has been developing and utilizing a number of acid resistant tantalum coatings technologies at an attempt to offer the right balance of performance and price.
Through the evolution of acid resistant tantalum coating technologies
Tantaline’s Tantalum Suface Alloy Technology has been developed. This technology has the capability of producing very repeatable and consistent pinhole free surface of pure tantalum metal at thicknesses between 0.002" - 0.008" (50um - 200um). Unlike spray tantalum coatings that are line of sight, the Tantaline process is geometry independent and even the most complex parts could be treated both internal and external surfaces. In addition because the tantalum metal is grown into the substrate and alloy bonded, typical coating modes of failure like delamination, chipping and spalling are virtually non-existent.
PVD Tantalum Coatings
PVD coatings are a common coating procedure for e.g. decorative coatings or hard surfacing (like e.g. Titanium Nitride) for tools. PVD is a physical deposition procedure, where the coating materials are evaporated under high vacuum and will precipitate on the substrate surface.
Traditional acid resistant tantalum coating procedures like PVD have been available commercially for years. PVD tantalum coatings which are line of sight, are not used widely for corrosion protection. The problem using PVD technologies for acid resistance is the tendency for the formation of pinholes in the tantalum coated surface.
Cold Spray Tantalum Coatings
Cold spray coatings can be used to create acid resistant coating utilizing tantalum metal. When utilizing cold spray techniques the tantalum metal layers must be made relatively thick (20 mils) and is a line of sight process limiting the possible geometries of substrates to coat. The key problems with this technique is the fact that the tantalum metal coating created has significant internal stress, is porous and is only physically adheared to the surface with no metalurgical or chemical interaction.
Thermal Spray Tantalum Coatings
Thermal sprays have typically been used to create various metal coatings including tantalum metal coatings. Thermal sprays differ from Tantaline in many ways:
- First thermal sprays are line sight and are highly dependant on the geometry of the substrate whereas Tantaline is geometry independent because it utilizes a gas phase process.
- Second, a thermal spray coatings create molten globules of the source that are and projected to the surface. Because of the relative size of the globules, voids, oxide inclusions and brittle oxides are common.
- Compared to thermal sprays, the Tantaline process occurs on an atomic level virtually eliminating the chance of creating voids and oxide inclusions which allows Tantaline to creating a 100% dense, dutile and stress free surface.
Tantalum Cladding
Tantalum cladding is often used as an alternative to tantalum coatings for fabricating parts out of solid tantalum. The technology is relative labor intensive and the material consumption is still high. As a result, acid resistant tantalum cladding is still cost prohibitive and reserved for only the most aggressive applications where specialty alloys will not survive.
When cladding a part in tantalum, tantalum foil/sheet (liner) is applied to the part’s surface. As tantalum cannot be welded to stainless steel Tantalum liners are always loose. Tantalum clad is applied in relatively thick sheets (1 mm and up) since there needs to be enough material for a stable design and fabrication. Where more than one sheet/foil is connected, the tantalum has to be welded. This requires a very high temperature (exceeding 3000C) and controlled atmosphere. The welding operation is difficult to handle and require special workshops. When tantalum clad is failing it is often due to material failures in the welded areas that may be porous, have reduced strengths or become brittle due to insufficient control of the welding atmosphere. Also there may be risk of alloying the base material into the welding zone, as the melting temperature of e.g. a stainless substrate is far below the welding temperature of tantalum metal.
Advantages
The main advantage by tantalum clad is that large structures (e.g. large vessels/tanks can be treated) and that heavy tantalum layers can be applied.
Disadvantages
The main disadvantages of cladding are the difficulty in fabricating and the very high costs. The costs of tantalum cladding are an order of magnitude higher than Tantaline’s Alloy Surface Bonding and is therefore only used as a replacement for solid tantalum or large equipment. Complex parts like valves and fittings are extremely difficult to produce and protect from corrosion by tantalum cladding.
Historically industry has tried to develop molten salt bath technologies to create acid resistant tantalum metal coatings on substrates but has been relatively unsuccessful. With the process being very difficult to control, consistent uniform tantalum coatings were difficult to produce and has limited applications today.
