Cathodic protection is often seen as a black-box technology – something that only experts can understand. However, the history of cathodic protection dates back over 188 years, and the science behind it is well understood.
Pipelines, ships, offshore platforms, reinforced concrete, tanks, piling and just about anything metallic in contact with an electrolyte can all benefit from cathodic protection (CP). Even our own bodies harbour microscopic examples of CP. This article will briefly look at the history of CP, with a focus on its application to pipelines.
The origins of CP
Sir Humphry Davy (1778-1829), a scientist and great experimentalist of the early 19th century, generated electricity for many of his experiments using galvanic cells similar to those originally created by Alessandro Volta in the 1790s. These so-called “˜voltaic piles’ were batteries that typically consisted of brine-soaked card sandwiched between copper and zinc and other combinations of dissimilar metals to produce a steady, reproducible supply of electricity. The portrait of Sir Davy in Figure 2 includes a typical voltaic pile – a common feature in portraits of electrochemical scientists.
Sir Davy’s observations of the corrosion processes within these galvanic cells paved the way for him to develop the first CP system. In January 1824, he postulated that it would be possible to prevent corrosion of copper sheathing on ships by connecting it to zinc, tin or iron. The copper sheathing on the ships was needed to reduce fouling and prevent attack on the timber by worms.
Sir Davy recognised the importance of a metal’s position on the electrochemical scale when he wrote “…if [copper] could be rendered slightly negative, the corroding action of seawater upon it would be null.” He went on to ask “But how was this to be affected? I at first thought of using a voltaic battery.”
His first thought was to use an impressed current system. He was ahead of his time.
Practical considerations dictated Sir Davy use sacrificial anodes of zinc, tin and iron – all metals that were more electronegative relative to copper – and hence, when connected, would drag or “˜polarise’ the copper potential to more negative values.
Sir Davy reported on full-scale trials in June 1824 and showed the complete effectiveness of zinc and iron in protecting copper. In June 1825, he also published the influence of ship movements on the efficiency of the protection action and the effects on fouling.
This work defined the two ways in which we apply CP to this very day – by using sacrificial anodes or by imposing an impressed current where current is forced from an anode through the electrolyte onto a structure (cathode) using a DC power supply. Both methods of applying CP act to shift the potential of the structure to be protected in the negative direction. The anode is consumed while the cathode is protected if there is sufficient current to provide the requisite polarisation.
In the decades following Sir Davy’s work, extraordinary advances were made in the understanding of the science that underpins CP amongst other fields of science and engineering. This included Michael Faraday’s experimental proof of the electrochemical equivalence between electric current and corrosion; Josiah Gibbs’ development of the thermodynamics that lets us determine whether an electrochemical reaction can occur; Julius Tafel’s investigations during the 1890s and early 1900s into how changes in the metal potential can regulate the anodic and cathodic reaction rates; and Walther Nernst, who showed how the potential of a metal could be calculated if the concentrations of reactants and products were known, and in doing so, also showed how the stability of chemical species could be predicted if the potential and pH were known. In 1945, Marcel Pourbaix summarised all of these features into his first Pourbaix diagrams. R.B. Mears and R.H. Brown also provided a clear kinetic description of CP in 1938 that is still valid today.
Applications to pipelines
CP using sacrificial zinc anodes had become widely used by the 1920s, particularly in the pipeline and shipping industries. A key pioneer in the United States was Robert J. Kuhn who began using the first impressed current transformer rectifiers in 1928 to protect a long-distance gas pipeline in New Orleans.
Mr Kuhn continued with wide-ranging field tests and determined that shifting the potential to -0.85 volts relative to a copper/copper-sulphate reference electrode gave the optimum corrosion protection for ferrous structures in soil. This criterion was shown to be suitable for steel, not only in soils but also in seawater.
Australians feature strongly in the development of practical applications of CP to pipelines. Brian Hatfield published a biography of one of the early major contributors in Australia, William Alexander Johnson, who was appointed Electrolysis Officer for the Melbourne and Metropolitan Board of Works (MMBW) in 1928.
Melbourne had enormous problems with the effects of an extensive DC traction system and a substantial effort was made to mitigate the electrolysis caused by trains and trams. Figure 1 (page 70) shows a photograph of MMBW stray current testing in Bell Street, Coburg, dated 30 April 1932. Note the corrosion testing officers in their lab coats in the background.
Mr Johnson installed the first impressed current CP system in Australia to protect a combination of a 762 mm and a 1,371 mm diameter mild-steel water mains, a large gas pipeline and a lead sheathed telephone cable in September 1935 at the southern end of St Georges Road in Northcote, Melbourne. Figure 3 shows the transformer rectifier used for the task. The system was in operation up until 1994, with several renovations along the way.
Australia now has some of the largest and most extensive CP systems in the world. Our hydrocarbon, water, liquids and slurry pipeline networks today consists of tens of thousands of protected kilometres. Our CP systems protect reinforcing steel in hundreds of thousands of square metres of concrete. Wharves, jetties, offshore platforms, shipping, well casings, tanks and countless other structures have CP successfully applied to them in Australia and around the world.
The Australian industry has a comprehensive set of standards covering CP, including Parts 1-5 of the AS 2832 – Cathodic protection of metals series, with Part 1 covering pipes and cables, and
AS 2239 – Galvanic (sacrificial) anodes for CP, as well as other CP-related standards. Most states have legislative electrical safety regulations that specifically include CP and regulate the effect it might have on other structures. It is a mature and well-regulated industry.
A number of Australians have also been involved in possibly the largest CP system in the world: Libya’s Great Man Made River Project. About 3,500 km of 4 m diameter pre-stressed concrete cylinder pipe brings fresh water from the great sub-Saharan artesian basin to the towns and industries in Libya. The system consists entirely of zinc anodes, with Australian companies being principal suppliers, providing advice and onsite supervision. Figure 4 shows zinc anodes being lowered into a drilled hole to be filled with a slurry of gypsum-bentonite backfill.
CP has a long history and is now applied successfully to a vast number of structures. Our present knowledge allows us to appreciate the almost limitless range of possible applications, although, as with all technology, our understanding of the science will always continue to grow.