200 Years On: Sir Humphry Davy and Cathodic Protection Free

The year 2024 marked the 200th anniversary of the first two of three remarkable papers^1-3  by Sir Humphry Davy describing the earliest scientific investigations into what we now call cathodic protection (CP). Although the word “cathode” was not in use until proposed by Michael Faraday in 1834,⁴  this paper continues to use the modern abbreviation “CP” for convenience. Similarly, Faraday also proposed using the word “anode” in his 1834 paper and we will call the anodic metals used by Davy “anodes,“ rather than “protectors.”

Davy presented three seminal papers to the Royal Society in London, beginning with his first on January 22, 1824¹  giving the scientific reasons, background, and laboratory research results for the application of CP to prevent corrosion of copper sheeting on timber ships in seawater. His second paper describing additional experiments and observations for copper sheeting on vessels in Chatham and Portsmouth Navy dockyards was presented on June 17, 1824²  and his third paper detailing full-scale research and results for ships on the high seas was read on June 9, 1825.³ 

There was controversy then, as there is now, about some aspects of Davy’s work.^5-6  This paper discusses some of these issues and demonstrates that Davy was well aware of any shortcomings but importantly, he also suggested possible solutions to overcome the occasional problem of an increase in fouling of the copper sheet under specific circumstances.^3,7  Notably, the application worked unambiguously to prevent corrosion of the copper in all cases, with fouling only occurring on some vessels and Davy was in the process of understanding the differences in operational circumstances. It will be shown that it was primarily a loss in funding and a wish by the British Admiralty (after pressure from some ship’s captains and the media) for quick solutions that cut short further investigatory work after just 2 y (i.e., 1823 to 1825). It was left to future scientists and engineers to show how CP could be used effectively and efficiently to protect any metal or alloy immersed in an electrolyte, including even reinforced concrete.⁸ 

When reviewing his work, it must be seen in the context of the scientific understanding of electrochemical processes at the time. For instance, Davy was able to formulate the principles and application of CP decades before the electron was discovered by J.J. Thomson in 1897 and long before chemical and electrochemical reactions were written in the format used today. Judging his discoveries against modern principles is unfair at best and anyway, he was spot-on about plenty of technical points, as will be seen.

The basic principles leading to Davy’s work on CP received a kick start when Luigi Galvani⁹  discovered what he believed to be animal electricity, Figure 1. Plenty of researchers before Galvani had made frogs’ legs and other animal parts twitch and convulse by connecting them to the available sources of electricity, such as Leyden jars or spark generators. The marvelous achievement by Galvani, for us at least, was that he made the legs twitch by connecting two dissimilar metals to the creature’s limbs. The usual (but not exclusive) metals he used were copper and zinc, Figure 2. He was able to stimulate the frog legs into animation without any external source of electricity. Galvani concluded the electricity originated from within the animal. It was supposed that this electric fluid was secreted from the brain.

Alessandro Volta did not accept Galvani’s animal electricity theory. Volta based his conclusions on experiments with dissimilar metals and mineral-based (nonanimal) liquids. Volta recognized that electricity was produced by the dissimilar metals in contact with the conductive tissue and produced his first batteries in the 1790s consisting of brine-soaked cards sandwiched between pieces of copper and zinc and other combinations of dissimilar metals. Volta produced for the first time a steady, reproducible supply of electricity. Volta published a version of his work in 1800¹⁰  including a description of how to make one of his electricity supplies, soon called a Voltaic pile, Figure 3.

An immediate explosion of work emerged using Volta’s pile, Figure 4. Nicholson and Carlisle (1800)¹¹  soon afterward observed that hydrogen and oxygen evolved when the two electrode leads were immersed in water and thereby discovered electrolysis by the decomposition of water.

Humphry Davy quickly adopted the new technology and conducted an enormous amount of work using huge Voltaic piles. Davy used the large voltages and currents available from up to hundreds of galvanic couples connected in series to separate elements in molten substances by electrolysis and discovered potassium, sodium, magnesium, barium, calcium, strontium, and aluminum, among many other scientific feats. Sir Humphrey Davy (he was knighted in 1812) wrote in 1826⁷  that the discovery of electrolysis by Nicholson and Carlisle was a “…capital fact” and clearly made the most of it. An example of the terrific electrical power he was able to produce can be seen in Figure 5 which shows the battery banks below a lecture theatre where he was demonstrating electric arc lighting. The portrait of Davy in Figure 6 includes a typical Voltaic pile, a common feature in portraits of electrochemical scientists.

This background placed Davy in a perfect position to investigate a costly corrosion issue for the British Navy. In 1822, Davy was first approached by the British Navy Board to provide advice regarding the corrosion of copper sheeting on the Royal Navy’s timber ships.⁵  The copper sheeting was effective at protecting the ships’ timber from worms and preventing the growth of “weeds” which otherwise had the effect of slowing the ships’ movement through the water. Davy read the board’s letter to the Council of the Royal Society of which Davy was President. The Council formed a committee (with Davy as President) to investigate the matter and decided to test copper specimens supplied by the Navy. Davy eventually dispensed with the committee and began working personally on the problem in 1823, reported directly to the Admiralty on January 17, 1824, and then read a groundbreaking paper to the Royal Society on January 22, 1824.

This, Davy’s first paper on CP, proves the ability of CP to prevent corrosion. There may well be other consequences, but the beneficial effect on corrosion was unambiguously proven and is not in dispute.

Let’s look at some of the key issues raised in this first paper.

Davy, along with his assistant Michael Faraday, proved incorrect the general supposition at the time that the “rapid decay” (i.e., corrosion) of copper was due to impurities, surmising that “pure” copper was acted upon more rapidly than the specimens which contained alloy (although the type of alloying was not provided) and that “changes” (corrosion) in various specimens of ships copper collected by the Navy Board “…must have depended upon other causes than the absolute quality of the metal”. The quality of the metal is important but other factors were also believed by Davy to play a critical role including “temperature, the relative saltness (sic.) of the sea, and perhaps the rapidity of the motion of the ship; circumstances in relation to which I am about to make decisive experiments”.² 

Davy described “the nature of chemical changes taking place in the constituents of sea water by the agency of copper” and especially the importance of oxygen in the process.

Davy repeats his hypothesis from 1807¹²  (read 20 November 1806) “that chemical and electrical changes may be identical”; a feature clarified and enumerated by his assistant Michael Faraday⁴  several decades later with what is now popularly called Faraday’s Law of electrochemical equivalence.⁴  Davy then describes how, by this hypothesis, “that chemical attractions may be exalted, modified or destroyed, by changes in the electrical states of bodies”. In other words, change the electrical state from its natural positive or negative state (i.e., change the potential, another word not yet in electrochemical use) and you will cause the chemistry to change. Davy then notes that it was the “application of this principle that, in 1807, I separated the bases of the alkalies from the oxygene with which they are combined, and preserved them for examination; and decomposed other bodies formerly supposed to be simple”.

All of these past works by Davy led him “to the discovery which is the subject of this Paper.”

It is a testament to Davy’s scientific method that he supported his hypothesis with past research results, created an understanding of the processes involved, and then went on to test it in the laboratory and with full-scale trials, collecting and analyzing the data for comparison with the predictions of the hypothesis.

Once he had established the basis of his hypothesis, he went on to suppose that if copper “could be rendered slightly negative, the corroding action of sea water upon it would be null; (i.e. polarise the copper negative) and whatever might be the differences of the kinds of copper sheeting and their electrical action upon each other (i.e. galvanic effects), still every effect of chemical action must be prevented, if the whole surface were rendered negative”. He then astoundingly says “But how was this to be effected? I first thought of using a Voltaic battery; but this could be hardly applicable in practice”. So, Davy first thought of using an impressed current CP system! He could hardly know that future DC power supplies would make impressed current an easy and common means of cathodically polarizing a structure. Davy then says “I next thought of the contact of zinc, tin, or iron”; i.e., galvanic anode CP.

Davy, assisted by “Mr Faraday” then conducted a series of experiments using these three metals to mitigate corrosion on copper. He then reports that although tin was initially effective, “it was found that the defensive action of the tin was injured, a coating of sub-muriate (chloride compound of tin) having formed, which preserved the tin from the action of the liquid”; the tin chloride deposits reduced the effectiveness of tin as an anode. “With zinc or iron, whether malleable or cast, no such diminution of effect was produced”. Davy now knew for certain that he had effective anodes for the galvanic CP of copper.

Davy and Faraday then proceeded to conduct numerous experiments using zinc and iron in various shapes and sizes attached to copper, including small pieces “as large as a pea”, wires, nails, sheets connected directly by wires, filaments, soldering, etc., and always with areas of zinc or iron much smaller than the copper being protected.

Near the end of the paper, Davy notes “.… that small pieces of zinc, or which is much cheaper, of malleable or cast iron, placed in contact with the copper sheeting of ships, which is all in electrical connection, will entirely prevent its corrosion”. It is an important note by Davy that the iron anodes were much cheaper and we will return to this issue when discussing the next two papers presented to the Royal Society.

Finally, Davy says that in future communications he might describe other applications that the principle that can be used “to the preservation of iron, steel, tin, brass, and various useful metals”. He was aware that the principle is widely applicable to any metal or alloy, a feature we enjoy today.¹³ 

Davy reports the results of sheets of copper connected to zinc, malleable, and cast iron for many weeks in Portsmouth Harbour. He notes that cast iron, which is the cheapest and most easily procured of the materials tested “is likewise most fitted for the protection of copper” and lasts longer than malleable iron or zinc. Davy later, however, after further research, recommends a preference to use zinc anodes rather than iron.³ 

Davy anticipated and observed “the deposition of alkaline substances” on the copper being “carbonated lime and carbonate and hydrate of magnesia”. Nicholson and Carlisle had discovered the decomposition of water using a voltaic pile¹¹  (described by Davy as a “capital fact”⁷ ) and included a description of “the separation of alkali on the negative plates of the apparatus”, hence Davy’s anticipation. These now familiar calcareous deposits of calcium carbonate and magnesium hydroxide are crucial to the efficiency of CP systems in seawater. Davy was aware of the increase in alkalinity at the cathode and acidification at the anode. (Note: Even though the concept of pH and the quantification of acidity and alkalinity was not formulated until 1909,¹⁴  Davy talks extensively about the alkalinity and acidity produced during galvanic coupling of different metals).

Davy also understood and documented that when the calcareous deposit completely covered the copper sheets, it could result in “weeds” and “insects” collecting on them. Davy then considers the amounts of calcareous deposits generated by various quantities of anode material. He found that using zinc and iron anode to copper area ratios from 1/35 to 1/80, the copper became coated with calcareous deposits but “weeds” eventually adhered to the surface as well. He then reports that when the ratio was reduced to 1/150 “…the surface, though it has undergone a slight degree of solution, has remained perfectly clean; a circumstance of great importance as it points out the limits of protection; and makes the application of a very small quantity of the oxidable metal, more advantageous in fact than that of a larger one”. So, Davy here cautions about the excessive application of anodes for the specific protection of copper in seawater when fouling is unwanted and illustrates that fouling could also be mitigated if careful selection of the anode quantities was made for the specific circumstances in which they were used.

Davy’s full-scale trials were generally very successful; he prevented the copper corrosion, and he made the comment that the fouling was usually not an issue if the vessel was on the move. He notes that mooring stationary in harbor allows calcareous deposits to form more readily (surface pH will rise more than when in motion) and weeds etc can adhere. He also mentions the quality of the copper may be important and that the proportion of the anode:cathode area ratio affects deposits.

He observed that the marine growths are often initiated on the iron oxides deposited near the anodes (“protectors”); he recommends here a preference to use zinc anodes rather than iron, “Zinc, in consequence of its forming little or no insoluble compound in brine or seawater, will be preferable to iron…”.

Davy defends his work from p. 341 onward in his 1825 paper where he says “A false and entirely unfounded statement respecting this vessel (the 28-gun “Sammarang”) was published in most of the newspapers, that the bottom was covered in weeds and barnacles. I was present at Portsmouth soon after she was brought into dock: there was not the smallest weed or shell-fish upon the whole of the bottom from a few feet round the stern protectors to the lead on her bow.” He goes on to describe other instances of fouling and nonfouling when protected and at least attempts to understand the various circumstances.

It appears that cast iron anodes were used in most of the field trials on ocean-going copper-sheathed ships. For instance, Davy states in the Bakerian Lecture of 1826 (p. 420),⁷  when discussing the field trials and operations “…in the only experiment in which zinc has been employed for this purpose in actual service, the ship returned… perfectly clean”. Davy wanted to conduct further experiments on ships in service because the mitigation of corrosion was proven and fouling only occasionally for reasons he thought could have been elucidated by further work.

It seems reasonable to expect the vessel captains, wanting the most from the protection system, to add iron anodes at possibly excessive rates because they were relatively cheap and easily procured and then complain that fouling was unacceptable. This is corroborated by Davy’s observation³  when discussing several ships returned from the West Indies: “The proportion of protecting metal in all of them (our emphasis) has been beyond what I have recommended, 1/90 to 1/70; yet two of them have been found perfectly clean, and with the copper untouched after voyages to Demarara; and another nearly in the same state, after two voyages to the same place. Two others have had their bottoms more or less covered with barnacles; but the preservation of the copper has been in all cases judged complete”. Davy was therefore not reluctant to report fouling on some ships but balanced this with positive reports. Clearly, it was possible to obtain both corrosion protection and no fouling; it would just require continued methodical research (and with ship owners/captains installing the recommended quantities of anodes rather than excessive amounts).

On the issue of fouling, F. James’ otherwise excellent article on Davy⁵  claims that “Davy does not seem to have appreciated the side effect, and he was certainly unable to overcome it”. This is incorrect on two points; Davy did appreciate the issue if one refers to the scientific articles as we have above where Davy addresses this specific issue and he was in the throes of trying to better establish the conditions in which corrosion mitigation and acceptable amounts of fouling could be achieved. Unfortunately, ongoing pressure finally led the Admiralty to issue orders to the Navy Board on July 19, 1825 to discontinue the project,⁵  thus ending further research just 2 y after first being initiated.

The historian S. Ruston, in an essay discussing Davy as the philosopher,⁶  seems to draw heavily on James’ article where Ruston writes “Unfortunately, what had worked in the laboratory did not work at sea …”. This is incorrect because there was no disputing that the corrosion was fully mitigated; it worked perfectly well and was the original aim of the Admiralty’s directions, with only fouling being a troublesome and sometimes unacceptable side effect. On this Ruston goes on to write… “the electro-plating (sic.) had a chemical side effect, which stopped the poisonous copper salts from going into the sea and resulted in ships’ bottoms being fouled thus slowing them considerably”. Although Ruston mistakes electrochemical protection used by Davy as “electro-plating”, the essay ignores the actual scientific words of Davy within his papers to the Royal Society where, as discussed above, he understood the effect of calcareous deposits, the effect on fouling, and the need to strike a practical balance between corrosion protection and fouling.

A lot was riding on Davy’s work and competition from other inventors tied with the newspapers.⁵  Plenty of “fake news” and “alternative truths”—not much has changed! If we study these works with our scientific, objective eyes we can establish a good understanding of the success or otherwise of Davy’s work. Davy makes so many great and insightful statements about his observations, many of which are equally valid today. Also don’t forget that he had the greatest assistant one could imagine in Michael Faraday in these works. The veracity of Davy’s publications is not in doubt, especially with Faraday on board.

The world’s navies to this day use CP on virtually every ship, submarine, and marine vessel on the oceans and waterways across the globe to mitigate corrosion, both external to the hull and within internal water-filled spaces.^15-17  Ships hulls are of course now predominantly coated steel but Davy would surely have been pleased to know that zinc anodes are still used extensively as shown in Figure 7, using the same basic principles.¹⁷  Impressed current systems are used for larger current demand applications on bigger vessels but even these are often supplemented with zinc anodes around high current demand areas and shielded locations such as sea chests, ballast tanks, propellers, or shafts. Aluminum alloy anodes also provide excellent, cost-effective performance in seawater but zinc is especially versatile when vessels experience waters of varying salinity such as estuaries and harbors with fresh water inflow.

Another example of special interest to the Australian authors lies within the waters of the Sea of Marmara. “On 25 April 1915—the day the Anzacs landed at Gallipoli—Lieutenant Commander Dacre Stoker set out as captain of the Australian submarine AE2 on a mission to breach the treacherous Dardanelles Strait to disrupt Turkish supply lines to the isolated Gallipoli peninsula. Facing dangerous currents, mines, and withering enemy fire, Stoker and his men succeeded where British and French submarines had come to grief”. So begins the teasing summary on the back jacket of the book “Stoker’s Submarine” by Fred and Elizabeth Brenchley¹⁸  in which the extraordinary story is told about the courageous efforts of Stoker and his men. They created havoc for 5 d before being critically damaged by the Turkish torpedo boat Sultanhisar and were forced to scuttle with no loss of life.

The AE2, shown in Figure 8, lay at rest on the floor of the Sea of Marmara until 1998 when Selçuk Kolay, director of the Rahmi M. Koç Museum in Istanbul, finally located and dived on the submarine after a 3 y search. Kolay was awarded an Order of Australia for his efforts. The AE2 is being preserved where it lay and a key part of her preservation is CP, as shown in the schematic in Figure 9 from the AE2 Commemorative Foundation.¹⁹  CP serves a highly significant role in preserving Australia’s largest relic from the Gallipoli campaign.

• Although this paper intended to focus on Davy’s work, it is important to note that in the decades and now centuries following Davy’s work extraordinary advances were made in the understanding of the science that underpins CP,^13,20-21  amongst other fields of science and engineering. This included Michael Faraday’s discovery and experimental proof about the electrochemical equivalence between electric current and corrosion already mentioned; Josiah Gibbs’ development of the thermodynamics that lets us determine whether an electrochemical reaction can occur; Julius Tafel’s investigations and descriptions during the 1890s and early 1900s about how changes in the metal potential can regulate the anodic and cathodic reaction rates and Walther Nernst who showed how the potential of metal could be calculated if the concentrations of reactants and products were known and in doing so, the stability of chemical species could be predicted if the potential and pH were known. Marcel Pourbaix summarised all of these features into his first beautiful Pourbaix diagrams.^22-23  Mears and Brown²⁴  also provided a clear kinetic description of CP in 1938 that is still valid today. Kuhn^25-26  first suggested the earliest criterion for CP of polarising to −0.85 V_CSE or more negative, which was shown to be suitable for steel, not only in seawater²⁷  but also in soils.²⁸  The increase in pH at the metal surface²⁹  and the subsequent development of passive oxide films during cathodic polarization and their role in mitigating corrosion for steel, when considering both the kinetics and thermodynamics, is now well established for iron and steel alloys^30-32  and our deeper understanding continues to evolve, as good science always does.

• It was recognized very early on³³  that the primary protective action of the calcareous deposits is to: (i) act as a barrier to oxygen or other depolarizers, (ii) increase the internal resistance of the local corrosion cells, and (iii) increase the pH of the water film in contact with the metal surface above that of normal seawater. The benefits to marine CP systems are now well known, particularly in reducing the current density requirement for corrosion mitigation,^33-37  a feature reflected in various industry standards for marine structures.^38-40 

• The formation of calcareous deposits anticipated and observed by Davy is also still of special interest today and the circumstances of their formation continue to be investigated.^17,41  Kuhn²⁶  also noted the formation of calcareous deposits that varied from “…practically nothing in neutral areas to an inch in thickness in heavily drained areas”, and hence in the degree of the protective or beneficial efficiency.

• The calcareous deposits formed by CP can also have detrimental effects such as accelerated bearing wear in water-lubricated propeller shafts and seizing of hull valves due to clogging,¹⁷  or in nonseawater applications such as limiting heat transfer of pumps causing overheating. Means of avoiding these adverse effects continue to be investigated,¹⁷  along with understanding the effects of CP upon biofouling.⁴² 

• Davy achieved remarkable success even though after 200 y, issues with the protection of metals and alloys using CP are still the subject of ongoing research and refinement.

• The future remains bright, with vigorous research continuing worldwide into CP, advancing the science and range of applications to an ever-widening array of structures. Science never sleeps and no doubt each new generation of scientists and engineers will, bit by bit, keep adding to our knowledge and advance Davy’s legacy.