Electrochemical Corrosion Protection
by: John Morgan, 1988
In 1681, accelerated corrosion having been observed between iron and lead sheathing, the Navy Board decided locally to remove the lead sheathing from ships' hulls to prevent the rapid corrosion of the rudder irons and bolt heads, Charles II and Samuel Pepys being the instigating experts. (reference)
Wollaston (circa 1815) regarded corrosion by acids to be an electrochemical process, and a few years later, in 1819, a French writer suggested that rusting was also an electrochemical phenomenon. In 1824, Davy showed that when two dissimilar metals were electrically connected and immersed in water, the corrosion of one was accelerated while the other received a degree of protection. From this work he suggested that the copper bottoms of ships could be protected by attaching iron or zinc plates to them, the earliest example of practical cathodic protection.
In 1830 de la Rive published a paper showing that impure zinc was corroded rapidly by the great number of bimetallic junctions that it contained, the corrosion cells being formed between the zinc and the impurities. This work was followed by the investigations of Faraday into the correlation of electrical and chemical phenomena. Much of Faraday's work could be described as corrosion experiments, and from these he was able to derive his laws of electrochemical action which give the relationship between the current flowing and the associated rate of corrosion.
The metals were arranged in decreasing order of activity by de la Rive who also showed that this order was dependent upon the electrolyte. The generally accepted theory assumed that an electrochemical reaction demanded the presence of two metals or a metal and a metal oxide. Sturgeon (circa 1830) considered that a single metal could have a surface that was `unequally electrical and consequently electropolar' and Faraday set about to prove this by his experiments involving a single metal. In these he was able to produce potential differences by variations in the electrolyte concentration and temperature.
In 1837, the British Association for the Advancement of Science commissioned Robert Mallet to investigate the effects of sea water at various temperatures and of foul river water whether fresh or salt on cast and wrought iron. During his tests he exposed a great number of specimens to these types of waters all over the British Isles and he observed the differential concentration cell effect on the corrosion of extended iron structures where sea and river water became stratified. At about this time considerable interest was being aroused by Davy's work on the protection of iron by zinc anodes and the development of hot dip galvanizing that followed. Mallet showed that zinc so used became covered with a thick layer of zinc oxide and calciferous crystals `which retards or prevents its further corrosion and thus permits the iron to corrode.' The variation in the corrosion rate of alloyed zinc reported by de la Rive led Mallet to experiment with zinc alloy anodes. He found that metals cathodic to zinc decreased its efficiency while those which were anodic, notably sodium, tended to increase this and that an addition of mercury was an advantage. A workable anode could be made from zinc when alloyed with mercury and sodium and this produced superior galvanizing.
Towards the end of the nineteen century, electrochemical corrosion received little attention and the view that two metals were required to produce this type of corrosion became accepted, the corrosion resistance of metals in aqueous solutions being associated with their purity. The corrosion of two metals in contact was investigated in detail by Heyn and Bauer about 1910. The nobler metal (the cathode) was known to corrode at a slower rate while the more base metal (the anode) corroded more rapidly. The investigators were able to establish that this corrosion increased with the relative separation of the metals in the electrochemical potential table. Other factors played an important part including the relative areas of the metals and the rate of arrival of oxygen at the cathode.
In a paper published in 1924, Evans described several mechanisms which led to the establishment of corrosion currents on a single metal surface and called this `The Newer Electrochemical View on the Corrosion of Metals.' Some of these principles had been noted earlier, including observations of the differential temperature cell by Walcker (1825), the differential stress cell by Davy (1826), the differential concentration cell by Becquerel (1827) and the differential aeration cell by Marianini (1830). Evans and his colleagues, Hoar, Thornhill and Agar, continued their work at Cambridge and produced direct quantitative evidence of these electrochemical corrosion mechanisms. In 1938 Hoar published a discussion on the basic electrochemical theory of cathodic protection and, independently, a similar theory was suggested by Brown and Mears. Anodic protection of metals that passivate was proposed by Edeleanu in Cambridge in 1955.
Having (in 1823) commissioned Sir Humphrey Davy to investigate the corrosion of the copper sheathing of the hulls of wooden naval ships, the Admiralty were the first users of cathodic protection. Davy experimented with anodes of tin, iron and zinc to protect the copper. The last two metals were used and in a later paper (1824) he favored the use of cast iron because it lasted longer and remained electrically more active than zinc. Zinc remained in use, however, and no doubt gave considerable protection to the copper sheathing. When wooden hulls were superseded by iron and steel, zinc anodes or protectors were still fitted. Though there was every reason to believe that zinc would successfully protect steel, its continued use seems to have rested more on tradition. The zincs were placed close to the stern gear and `yellow' metal parts, such as circulating pipe inlets, as these areas proved to be the most susceptible to corrosion. The practice became universal in shipping circles and protectors were even placed in boilers, though it is doubtful whether any complete protection resulted. Zincs were reported as being in sound order, that is uncorroded, and this was often regarded as good practice.
Edison tried to achieve cathodic protection of a ship at sea from trailing impressed current anodes but the materials and techniques available to him in the eighteen nineties proved to be inadequate. Most early users of impressed current in sea water were concerned with attempts to effect antifouling or to prevent the scaling that would occur in boilers which were replenished with sea water. The polarity of this current was often considered unimportant and anticipation of the present cathodic protection trends can hardly be claimed.
By the late twenties leaks were few and could have been tolerated had not the leak frequency curve begun to rise alarmingly. In the early thirties all the major pipeline owners were applying anti-corrosive measures to the external protection of their pipes, including various coatings and cathodic protection. The earliest schemes were applied to the worst sections where the pipes had been laid in corrosive soils, and great success was achieved. The cathodic protection was derived from zinc anodes or from impressed current supplied either by d c wind generators or by transformers and copper oxide rectifiers from a c power supplies.
In 1936 the Mid-Continent Cathodic Protection Association was formed to discuss and exchange information on cathodic protection. This association later became the foundation of the National Association of Corrosion Engineers.
The other area where oil pipelines were used extensively was the Middle East; the first cathodic installation protected a group of sea water loading pipelines at Bahrain in 1939.
There are a great number of patents on methods of preventing the corrosion of buried metals, particularly pipes and cable sheaths. Seventy or so years ago a major cause of corrosion of buried metal pipes was the electrolysis effect, or interference, caused by stray currents from the electric traction systems. The first patents describe the connection of the pipes to the negative pole of the station generator; this method was universally adopted and is still used. The introduction of a further d c generator between the negative return of the electric traction and the structure was claimed to give superior results. In 1911 a German, Herman Geppert, obtained letters patent on `a method of protecting articles from earth currents' and substantially described cathodic protection. Since then patents have applied to more specific devices such as reverse current switches, anodes, boosters, etc.
From these early beginnings cathodic protection has developed rapidly and its use has become widespread. New materials such as sacrificial alloys of magnesium and aluminum and superior impressed current anodes together with developments in electrical and electronic engineering have allowed great advances in the techniques. Cathodic protection is now established as an essential engineering service with a sound and comprehensive scientific background.
On the object of stray current corrosion see also: DC traction, Cathodic protection, Coating, Contour plots, Definition, Detection, Examples, External currents, Historical perspective, Impressed current, Interference, Mechanisms, Modeling. Pipeline, Potential distribution, Prevention, Stray fields and leakage, Transit systems