Titanium-coated conductor rails

No formation of lead chromate sludge and virtually maintenance-free anodes are the requirements that make insoluble, dimensionally stable anodes an issue. Dimensionally stable anodes are, for example, anodes made of titanium to which a platinum layer in the micrometer range is applied. Titanium-coated copper bars are used here as the current supply for the titanium base material.

Titanium-coated copper bars are used wherever high corrosion protection and high current carrying capacity are crucial, for example in electrochemistry, chlor-alkali electrolysis, galvanizing or chrome-plating systems. Increasingly stringent environmental regulations and expensive disposal require the use of environmentally friendly technologies. Titanium-coated copper bars can be used in fluoride-free electrolytes for hard and bright chrome plating instead of lead anodes. The undesirable lead chromate that occurs during chrome plating with lead anodes is not produced in the first place. Other areas of application are dimensionally stable anodes in chlor-alkali electrolysis or as a current supply for various electroplating processes.

Two different qualities are available on the market. On the one hand, there are titanium sheaths mounted on a copper core, which have no metallurgical connection between the core and the skin. In this case, the structures of copper and titanium remain separate. Air inclusions between the skin and core cannot be avoided. The result is high contact resistance and the resulting high electrical power loss between the titanium skin and the copper core.

An optimum solution is titanium-clad copper rods, which are manufactured using composite extrusion. This process, which requires a great deal of experience, guarantees optimum mixing through diffusion of the copper core and titanium cladding at the boundary layers. This is referred to as a metallurgically bonded structure or metallurgical bond. If a voltage drop calculation is carried out on metallurgically bonded copper-titanium, only the layer thickness of the titanium (0.5 to 2 mm) is included in the calculation as the length. The result is a very low electrical resistance and consequently a very low voltage drop. With a layer thickness of 0.5 mm, the voltage drop is often only in the mV range. High quality also pays off with composite materials. There are no unscheduled impairments and the service life is many times longer than with non-metallurgically bonded quality. But how can customers tell whether the quality of the composite is optimal and of high quality?

Various test methods provide information about the actual quality of the bond. The so-called splitting test is simple but effective. This test quickly provides an initial overview of the quality of the bond. An attempt is made to drive a chisel between the titanium layer and the copper core. If the layers can be separated, the bond is of inferior quality. High-quality metallurgical joints deform to the maximum without being damaged.

Another test is the torsion test. Strips of the composite material are twisted around their own axis several times. There must be no separation of the materials. If the composite material is of good quality, the layers of copper and titanium cannot be separated from each other, not even by torsion. This means that metallurgically bonded titanium-coated copper rails can also be used with relatively tight bending radii.

More complex test methods, which are offered by a few good manufacturers, are thermal shock testing and ultrasonic testing. In the thermal shock test, the composite material to be tested is annealed at 520 °C for one hour and then quenched in a water bath. This process is repeated up to 40 times. The titanium sheath must not detach from the copper core.

Due to the heat resistance of the composite, metallurgically bonded rods can be coated directly with iridium, platinum or mixed oxides. This process requires heat treatment with many cycles at high temperatures. In recent years, ultrasonic testing has become established as a quality control method. The greater effectiveness of the testing technology means that certain types of defects, such as cracks or bonding defects, can be detected. Ultrasonic testing immediately provides a statement on the properties of bonds in the form of a yes/no statement, but no statement on the intensity of the bond. Due to its properties, the sound beam of the ultrasound propagates in the same direction as light. Diffraction phenomena only occur if the obstacles causing them are of the same order of magnitude as the wavelength. Ultrasonic waves are hardly absorbed by metals. The property of reflection of sound waves is utilized, which occurs at interfaces, but also at defects. If no reflections occur, an optimum metallurgical bond can be assumed.

Titanium-coated metallurgically bonded materials are exceptionally resilient. This means that other titanium components can be welded to the coated titanium without any problems and without damaging the bond. Welding, laser or spot welding are suitable methods. The professional connection of titanium-coated copper rods repeatedly raises questions. The common view is to remove poorly conducting titanium from contact surfaces. This is done by milling or turning. On the face of it, this approach seems logical. The conductivity of copper is 58 m/(Ohm - ^mm2), while that of titanium is only 2.2 m/(Ohm - ^mm2). This means that a copper wire with a cross-section of 1 mm2 and a length of 58 m has a resistance of 1 ohm. With titanium, a comparable resistance is already achieved at a length of 2.2 m.

However, the reality is different; for example, environmental influences must be evaluated. The copper contact surfaces stripped of titanium corrode through contact with aggressive acids, vapors and condensates in the electroplating application. The copper on the contact surfaces is eaten away by corrosion. The resulting point contacts on the contact surfaces result in poor current transmission. The more complex and more expensive titanium sheath is therefore no longer removed today, but used instead as high-quality corrosion protection.