Comparing the two types of cathodic protection (CP), sacrificial anode cathodic protection (SACP) uses sacrificial anodes made of a material that is less noble than the structure being protected and are gradually consumed, while impressed current cathodic protection (ICCP) uses an external current source with a generally very low or non-consumable anode. Various materials have been used for ICCP systems, but new material combinations are showing promise for many applications.
#1: Combining Platinum and Titanium for Outstanding Corrosion Prevention
Platinized titanium anodes synergistically combine the favorable electrochemical features of platinum (Pt) with the corrosion resistance and other characteristics of titanium. They are anodes normally produced by the electrochemical deposition of a very thin layer of platinum metal or the oxides of platinum onto a titanium substrate. These anodes operate as inert anodes with high durability and are preferred because they remain insoluble in common electrolytes.
Platinum is a precious metal known for its unique favorable attributes, including:
- High resistance to corrosion
- Resistance to oxidation
- High electrical conductivity
- Ability to act as catalyst
- High chemical stability
- Capability to produce an excellent finish
The low consumption rate backed by high electrical conductivity makes platinum a preferred anode substance. But because of its high cost, only a thin layer of platinum is typically plated on different corrosion resistant materials such as tantalum (Ta), niobium (Nb) or titanium (Ti) to take advantage of these favorable features.
By electroplating the platinum metal on titanium, a composite metallic coating can also be produced on the substrate. (Learn more about this process in How Metallic Coatings Protect Metals from Corrosion.) This composite consists of titanium metal, platinum, oxides of titanium and metallic compounds of titanium and platinum. The process of heat treating the composite coating produces changes in chemical composition and morphology that improves its electrochemical properties.
The adoption of platinum plated and platinum cladded anodes has provided additional novel options and choices to designers of impressed current cathodic protection (ICCP) systems, because the additional benefits offered by anodes made of composites of platinum on titanium and platinum on tantalum are game changers in the corrosion protection industry, thus enabling their widespread adoption.
#2: Chemical Behavior of Platinum and Titanium
Platinum is preferred on an anode's outer surface because it is highly resistant to corrosion and can ensure current flow in most electrolyte media without leading to the formation of an insulating layer on itself. Because it doesn’t corrode, it doesn’t produce corrosion products and hence the consumption rate is very low.
Platinum is inert in fused salts and acids, whereas it is dissolved in aqua regia. There is no risk of hydrogen embrittlement. (You can learn about hydrogen embrittlement in the article An Introduction to Hydrogen Embrittlement.) It is one of the few rare metals that perfectly resist chlorides of seawater.
Titanium shows reasonably good resistance to a marine environment (seawater in particular). It does not react with concentrated (80%) solutions of metallic chlorides. However, it is susceptible to attack by hydrofluoric acid (HF)and hot hydrochloric acid (HCl) of higher concentrations. Even hydrogen peroxide and hot nitric acid can attack titanium. Oxidizing agents normally do not attack titanium because it readily forms a protective oxide coating. However, non-oxidizing substances such as sulfuric acid (above 5% concentration) and phosphoric acid (above 30%) can attack titanium. From a hydrogen embrittlement point of view, titanium fares better than tantalum as an anode material.
#3: Advantages of Platinized Titanium Anodes
Platinum has the advantages of electrochemical inertness, mechanical strength, workability and favorable electrical conductivity. However, it is prohibitively expensive. Development of platinum on titanium and platinum on tantalum (plated as well as cladded) materials has opened up the feasibility of using these for anode materials for metal finishing and cathodic protection systems in critical applications.
When used for anodes in aqueous media such as seawater, the titanium forms a stable layer of insulating oxide film on the surface that is stable below a certain breakdown voltage, thus preventing a current flow between the aqueous media and the anode. In the marine environment, the oxide formed on titanium is able to withstand 12 volts, beyond which the insulating barrier breaks down and current flow starts the corrosion process. As an example, the US submarine Seawolf has an automatic corrosion protection system based on platinum plated anode. The use of platinum on titanium (or platinum on tantalum) anodes has enabled a CP system with reasonable current density and low cost, which protects the nuclear-powered submarine from deterioration on a long-term basis.
New ways to produce titanium anodes at a commercial scale and thin films of platinum on titanium anodes by vapor depositing, rolling and plating have ensured superior and durable anodes at a reasonable cost.
These anodes allow moderate current densities without affecting the base metal. Platinum layers need not be free of pores to ensure effective performance. Low resistance maintained between the electrode and aqueous media (e.g., seawater) ensures the formation of a durable oxide film on titanium so as long as the voltage is maintained within a safe range. These anodes can be lightweight and a convenient size and shape, and ensure stability of operational voltage due to a low platinum consumption rate per ampere-hour.
In hard chrome plating applications, platinum on titanium anodes are environmentally friendly because they are lead-free. They maintain their geometrical shape for almost three years, ensure low downtime and pose a lower employee health risk because there is no lead chromate to be disposed of. Energy losses are lower with platinum-titanium anodes compared to lead anodes.
While lead anodes must be rods and sheets, platinum on titanium anodes can be made in T or U shapes, cylinders or plates, based upon the geometrical shapes of the parts to be plated.
The consumption rate of platinum on platinized titanium anodes is low and proportionate to the current flow. In the case of deep well groundbed applications (for land-based oil and gas wells) the platinized titanium anodes are an easily manageable, non-brittle alternative to magnetite or graphite anodes, because they come with small diameter hole, thus also saving the deep drilling expense.
Overall benefits of using platinized titanium anodes include:
- A low consumption rate that conserves precious platinum
- Favorable dimensional stability
- Corrosion resistance ensures durability with easy maintainability
- Light weight, as well as favorable current distribution in electroplating
#4: Disadvantages of Platinized Titanium Anodes
As a substrate for platinized anodes, titanium has the disadvantage of lower electrical conductivity compared to niobium or copper. A low breakdown voltage also is an important limitation for applications that involve a chloride medium. A lower operating voltage of 8 volts reduces the current density. Platinum on titanium substrate anodes are used in applications where lower electrical conductivity and breakdown potential are not a concern. For better electrical conductivity, copper-cored platinized titanium anodes are sometimes used.
Applications for platinized titanium anodes are limited to those electrolytes that do not react with titanium. They cannot be used in chromium baths that contain fluorides.
#5: Platinum Film Durability
Manufacturing of platinized titanium anodes has evolved and improved over the last two decades. Although the electrodeposition technique for coating platinum continues to be popular, the difficulty in achieving an adherent coating on titanium has been overcome by pre-roughening the titanium surface and pre-coating the etched substrate with a very thin film of a conductive primer.
According to some studies, platinum coat thickness generally varies from one to five microns, and in special applications of cathodic protection, (the thickness) could go up to 20 microns. For the cathodic protection of onshore bridge decks, a copper cored titanium with 2.5 micron platinum sheath has been developed. The studies have further concluded that in a concentrated NaCl solution, the platinum consumption could be less than 0.1 micrograms per ampere-hour, whereas in seawater (ten percent saturation) it could go up up to one microgram per ampere-hour.
For the cathodic protection systems of power station condensers using a mixture of river and sea water, the platinum consumption rate shoots up due to the simultaneous evolution of oxygen along with chlorine, and with brackish water, due to the presence of dissolved solids the consumption of platinum rose to tens of micrograms per ampere-hour.
In the case of nickel electroplating, the presence of brightening agents could affect platinum consumption rates, whereas sugar content in the brine feedstock in the steel vessels accentuated the rate of platinum consumption. (Get an Introduction to Electroplating here.)
#6: Applications for Platinized Titanium Anodes
The primary use of platinized titanium anodes is in the field of metal finishing and cathodic protection of ferrous metals that are used in structures buried in soil and the steel exposed to marine environments such as oil and gas producing platforms, ships, oil well casings and jetties. Platinum-titanium anodes successfully compete with cheaper graphite and lead electrodes in some of these applications.
Process plants that use platinized titanium include electro-chlorination plants, breweries, paper producers and producers of chemicals such as reagents, perchlorates and chlorates.
Platinized titanium anodes are extensively used in electrolytic processes. They have successfully replaced lead anodes in electroplating applications due to their lower consumption, dimensional accuracy, ability to form precise deposit thicknesses on desired geometric shapes, predictable plating chemistry and ease of maintenance. These anodes can be designed and formed with various geometries based upon the parts to be electroplated. Platinum on titanium anodes are highly preferred anodes for electrodeposition of copper, chromium, platinum, nickel, palladium and gold.
Platinized titanium is predominantly used as an anode material for the cathodic protection of seafaring ships, particularly for corrosion prevention of the hull and its components, including rudders, pumping systems, rotating parts, propellers, piping, submerged parts and structures, ballast tanks, dock system structures and cargo tanks.
Platinized titanium anodes are used in cathodic protection systems for underground storage tanks, pipes, tank bottoms, cable sheaths and structures buried under corrosive soil. (Learn more about the corrosive effects of soil in An Introduction to Soil Corrosion.)
Oil and gas applications
Platinized titanium anodes and tantalum anodes are being used in cathodic protection systems to protect piping, casings, sucker rods and aboveground storage tanks from corrosion.
Sewage systems, water supply systems and reinforced concrete structures
Platinum on titanium anodes are used in the cathodic protection systems of sewage treatment plants, water supply infrastructure and steel reinforced structures.
Platinized titanium anodes have successfully replaced lead anodes in hard chromium plating due to their advantages of lower maintenance, improved quality of deposition, higher productivity and consistency. They have achieved a position of dominance as a corrosion engineer's first choice for the impressed current type of cathodic protection of steel exposed to marine environments. Newer applications are being developed to take advantage of the superior attributes of platinized titanium.