Platinum Metals Rev., 1977, 21, (4), 110
Platinum Provides Protection for Steel Structures
An Economic Way to Prevent Corrosion
Increasing concern at the waste caused by corrosion has focused attention on the need to utilise the most advanced methods for preventing, or controlling, this expensive phenomenon. The use of platinum for the cathodic protection of ships and marine structures is by no means new but the high cost of such structures, the aggressive environmental conditions to which they are subjected in service, and the need for maximum reliability over long periods of time all tend to make platinum cathodic protection an economic alternative to other methods. This review, which will appear in two parts, describes how cathodic protection is provided, outlines the many factors that have to be considered when utilising this method and describes some of the most recent applications.
The carbon steels are in many respects ideal structural materials—they are strong, readily fabricated by a variety of techniques and, last but not least, they are comparatively cheap. However, these advantages are offset by their susceptibility to environmental attack with consequent wastage of the metal by rusting, which means that their use is solely dependent on adequate protection at an economic cost.
Platinum, on the other hand, is a noble metal and is highly resistant to corrosion in most environments, but its use as a material of construction is severely restricted by its very high cost. Ideally, a composite consisting of steel with a very thin protective coating of platinum on its surface would combine the desirable properties of the two metals, but this would not be technologically feasible nor could it be achieved economically. Furthermore, should the platinum coating contain discontinuities it would give rise to a galvanic cell in which the noble metal platinum would act as the cathode and stimulate anodic attack on the underlying steel. This is illustrated in Figures 1 (a) and (b) in which it can be seen that the spontaneous direction of electron transfer is from the steel anode to the platinum cathode, with a consequent increase in the potential of the steel (more electropositive) and decrease in the potential of the platinum (more negative); since an increase in potential increases the rate of anodic dissolution corrosion of the steel will be stimulated. Figure 1 (c) shows the converse situation in which the spontaneous transfer of electrons from the steel to the platinum is reversed by interposing a source of direct current e.m.f., and with this system the platinum is made the anode and the steel the cathode of an electrolytic cell. If the potential of the steel is made sufficiently negative (−0.85V with respect to the Cu/CuSO4 saturated reference electrode) by the rapid transfer of electrons from the platinum the rate of the anodic dissolution reaction
will become zero, and the whole of the surface of the steel will become cathodic and the steel is said to be cathodically protected.
The cathodic reactions in natural waters and soils will be oxygen reduction
and hydrogen evolution
Each act of these two reactions will require two electrons, which must be supplied from the platinum by electrochemical oxidation of species in solution, and in natural aqueous environments this will involve the oxidation of water and/orthe oxidation of chloride ions:
(which is actually the reverse of Equation ii)
With an inert anode such as platinum, oxygen evolution (Equation iv) will be the predominant reaction in fresh waters while in saline waters chlorine and oxygen will be evolved simultaneously at relative rates that depend on the concentration of chloride ions and the potential of the electrode (a function of current density); for example in sea water chlorine evolution will be the sole reaction at low potentials, but will be increasingly accompanied by oxygen evolution as the potential is made more positive.
It is apparent that the more inert the anode the smaller the mass required for a given life, and since electrochemical reactions involve only the surface, conservation of the metal may be achieved by using it in the form of a thin foil. Furthermore, if the anodic reaction proceeds at a high rate without excessive polarisation of the anode, it is possible to obtain a high current from a relatively small area of the anode. These considerations apply to platinum in sea water, in which the high concentration of chloride ions and the kinetic ease at which these ions are oxidised to chlorine enables the anode to be used at very high current densities. However, similar considerations do not necessarily apply in fresh waters or in underground environments in which oxygen evolution (Equation iv) is the dominant reaction. Oxygen evolution in comparison with chlorine evolution requires a high overpotential, which means that at high current densities unacceptable polarisation of the anode will occur so that a high e.m.f. will be required for a given current output, and in these circumstances other less inert materials operated at low current densities may prove to be more economical.
Any conducting material may be used as anode, and this includes non-metallic materials such as graphite, magnetite, lead dioxide, ruthenium dioxide, etc., but the most commonly used anode materials are given in the table, which also indicates their rates of consumption and current density of operation. Scrap iron, such as disused railway lines or sunken barges, represents an anode material that in its behaviour may be regarded at one end of the spectrum of anodes for impressed current cathodic protection, with platinum at the other; the former is rapidly and non-uniformly consumed and cannot be used at high current densities owing to excessive polarisation, which means that it must be massive and of large surface area to be effective. Even so it has its uses because of its low cost and availability.
In view of what has been said it is now appropriate to list the desirable properties of an anode for cathodic protection, and to consider whether platinum in the form of a very thin foil would conform to these requirements, which may be summarised as follows:
Low rate of corrosion, irrespective of the nature of the environments or of the reaction products formed during anodic oxidation.
Low anode polarisation, irrespective of the anode reaction(s).
Good electrical conductivity.
High reliability, and a life of 10 to 30 years without maintenance.
Sufficient mechanical strength and stability to withstand damage during installation and maintenance of the structure, and the stresses that it may be subjected to during service.
Low cost in comparison with the overall scheme of protection (rectifier-transformers, cables, monitoring equipment, etc. and more often than not grit blasting and painting of the steel).
Obviously no material, including platinum, will conform to all of these requirements and a thin foil of platinum would have very limited application because of its poor mechanical strength and stability and its low electrical conductivity. However, developments in anode design have to a large extent overcome these disadvantages, and have provided a means of constructing economical anodes in which the anode reaction takes place at a platinum surface, the platinum being supported on a suitable substrate.
Anodic Behaviour of Metals
Passivity, a property that is dependent both on the metal or alloy and the environmental conditions, is a phenomenon that is responsible for the comparative inertness of a number of metals and alloys that are inherently thermodynamically unstable. It is now accepted that this is due to the formation of a protective film of oxide on the surface of the metal, which isolates the metal from the environment, and thus reduces the corrosion rate to a low value (1). The nature of the film, and particularly its ability to conduct electrons and ions, varies widely and it will be seen that this property of passivity can be used with advantage in providing supports for thin layers of platinum.
Metals like iron, nickel, chromium, and the stainless steels, when anodically polarised in for example a reducing acid such as sulphuric acid, are characterised by a sudden transition from the actively corroding state to the passive state, when the potential attains a given value. This is illustrated in Figure 2, which is a diagrammatic representation of an E-log i curve determined potentiostatically in order to reveal the sharp transition from the active to the passive state at the passivation potential Epp. This decrease in the corrosion rate on attaining the passivation potential is due to the formation of a 1 to 3nm thick film of semi-conducting metal oxide or other solid compounds. Since the potential in the passive region curve CD is normally below that for gas evolution—theoretically, 1.23V for oxygen evolution in acid solution and 1.35V for chlorine evolution, both potentials with reference to the Standard Hydrogen Electrode (S.H.E.), transport of charge through the film must be ionic, the film remaining at constant thickness owing to the fact that the rate of film growth and the rate of film dissolution in the solution are equal.
Although passivity is a characteristic of a number of metals, and can be achieved in aggressive solutions such as reducing acids, it is not usually maintained when chlorides are present. In these circumstances localised breakdown of the passive film occurs at the breakdown potential Eb with the consequent formation of pits; even the highly corrosion resistant nickel-chromium alloys Inconel and the Hastelloys pit when anodically polarised in neutral chloride solutions and for this reason they cannot be used as anodes.
Of the noble metals, platinum, iridium and rhodium are almost inert when anodically polarised in chloride solutions; silver forms a non-conducting film of silver chloride which is to some extent protective, while gold and copper corrode rapidly.
It is important to note, however, that although platinum is a typical noble metal it too manifests the phenomenon of passivity, but it differs from the base metals that show active-passive transitions in that passivity may be achieved in chloride solutions without pitting. Determinations of E-log i relationships (2) using clean platinum electrodes immersed in various concentrations of sodium chloride and increasing the current density stepwise showed that a linear curve (Tafel line) is obtained at low current densities (Figure 3). However, at a critical current density, which varied with concentration of chloride and pH, there is a sudden jump in potential with little increase in current density; thus in 2M NaCl this jump occurred at 40mA/cm2 the potential increasing from 1.4 to 1.8V. On increasing the current density, a further linear Tafel line is obtained having a much greater slope than that obtained at the lower current densities before the potential jump. It was postulated that the potential jump corresponded with a change of the bare platinum surface to one filmed with one or two monolayers of a platinum oxide, and various oxides of platinum such as PtO, PtO2 or Pt(OH)2 may be responsible for this effect, with a high electronic conductivity. This filmed surface is not so catalytically active as the bare platinum surface, so that oxidation reactions such as Cl−→Cl2 can only be accomplished at a somewhat higher over-potential, which is of little significance when the platinum is used as anode in an electrolytic cell, but is a serious limitation for fuel cells using platinum as electrodes.
On the other hand, the “valve” metals such as titanium, niobium and tantalum when anodically polarised in a variety of solutions are characterised by the formation of dielectric oxide films (3); these films are amorphous and transparent and give rise to interference colours that provide a means of evaluating film thickness. Except at low potentials conduction in these oxides is almost entirely ionic, and if the metal is anodised at constant current density each new layer of oxide formed requires an additional potential dE to maintain the field in the total oxide and the applied constant ionic current density. Thus the potential increases continuously as the oxide thickens—as can be observed by the change in interference colours of the film—the field strength dE/dD in the new layer of oxide remaining constant. This increase will continue until at a certain potential, which varies with the nature of the metal and the solution, film breakdown occurs with characteristic sparking and oxygen evolution, and with discolouration and thickening of the interference film. At constant potential the growth of the film causes a continuous decrease in field strength with a consequent fall in the ionic current, and eventually this becomes almost zero and the film thickness for all practical purposes attains a constant value that is proportional to the applied potential; for example the oxide film on titanium has a limiting thickness of 2.3nm/V in 0.1 M H2SO4. Thus at any given potential the film will thicken to a limiting value, the ionic current then becoming almost zero.
The Historical Background
The principles of cathodic protection were clearly understood and concisely expressed by Sir Humphry Davy as long agoas 1824. In a paper presented in that year to the Royal Society, of which he was then President, he described a series of experiments he had carried out at the request of the Royal Navy who were concerned at the rapid decay of the copper sheeting of His Majesty’ s ships of war and the uncertainty of the time of its duration”. In the course of this paper he wrote:
Copper is a metal only weakly positive in the electrochemical scale; and, according to my ideas, it could only act upon sea water when in the positive state; and, consequently, if it could be rendered slightly negative, the corroding action of sea water upon it would be null; and whatever might be the differences of the kinds of copper sheeting and their electrical action upon each other, still every effect of chemical action must be prevented, if the whole surfacewere rendered negative. . . . A piece of zinc as large as a pea, or the point of a small iron nail, were found fully adequate to preserve forty or fifty square inches of copper; and this, wherever it was placed, whether at the top, bottom,or in the middle of the sheet of copper, and whether the copper was straight or bent, or made into coils. And where the connection between different pieces of copper was completed by wires, or thin filaments of the fortieth or fiftieth of aninch in diameter, the effect was the same; every side, every surface, every particle of copper remained bright, whilstthe iron or the zinc was slowly corroded.
Davy also investigated the impressed-current system, using a voltaic battery, but he did not consider this to be practical in service conditions. Although the beginnings of cathodic protection date therefore from 1824 this important method of protecting metals—particularly steel—from corrosion was neglected for over a century.
Of the three valve metals tantalum, niobium and titanium, the former is the most resistant to film breakdown and titanium the least resistant, and this applies particularly to breakdown in chloride solutions as is shown in Figure 4.
Thus tantalum in chloride solutions can be anodised to hundreds of volts, niobium to approximately 100V, whereas in the case of titanium, which can be anodised to approximately 80V in sulphuric acid and to over 400V in 95 weight per centformic acid (4), breakdown occurs when the potential of the metal/solution interface attains 8 to 10V. This leads to intense localised attack with the formation of a non-protective precipitate of hydrated titanium dioxide and consequent rapid corrosion of the metal. The breakdown potential is substantially independent of chloride concentration in the range 5g/l NaCl to saturated, and is also independent of temperature up to 50°C but at higher temperatures it decreases with increase in temperature. On the other hand, passivating anions such as sulphate, carbonate and phosphate increase the magnitude of the breakdown potential (5) which in slightly brackish waters may be as high as about 50V. This increasein the breakdown potential is dependent on the ratio of passiving anion/chloride ion rather than upon their actual concentrations. Thus although titanium is far cheaper than niobium or tantalum on a volume basis the cost ratio is approximately Ti : Nb : Ta : 1 : 10 : 20 (6), its pitting propensity at interfacial potentials that may occur during cathodic protection imposes certain limitations on its application in chloride-containing waters.
Thus it is apparent that if a discontinuous coating of platinum is applied to a valve metal such as titanium so that the two are in electrical contact, film growth to a limiting thickness will occur at the titanium exposed at discontinuities when the composite is anodically polarised. This exposed surface will then be protected by a dielectric oxide film,and passage of charge will be confined to the areas of the metal in direct contact with the platinum. Since for the oxidation of Cl−→Cl2 the interfacial potential of the platinum will not exceed about 3.5V (versus S.H.E.) even at very high current densities the film thickness, assuming the surface to be equipotential, willnot exceed 8 to 9 nm. The platinum could be regarded, therefore, as acting as an internal potentiostat that prevents thepotential of the titanium attaining the value at which film breakdown occurs.
Supports for Platinum
Owing to its high cost the use of massive platinum is very limited, and although very small electrodes of platinum sheet have been used for the internal protection of water pipes most practical applications have involved the use of a variety of different types of supports, which can provide a satisfactory means of conserving platinum.
The subject has been extensively reviewed by Walkinden (7), and it is of interest to note that as early as 1922 Baum (8) patented an anode consisting of tantalum partly coated with platinum, either by electroplating or by cladding, for the anodic oxidation of sulphates to persulphates.
Ideally, a substrate for platinum should have the following properties:
It should be resistant to corrosion in chloride-containing environments at elevated anodic potentials, andshould also be resistant to the products of the anodic oxidation of chlorine and water, that is chlorine, hypochloritesand hydrochloric acid.
If the substrate forms an oxide film on anodic oxidation the film should not be broken down by chloride ions at the interfacial potentials that may occur during cathodic protection.
The surface of the substrate must be amenable to coating with thin layers of platinum by technique such aselectroplating, and the coating so produced must be strongly adherent to the substrate and current flow should not be impeded byan intervening film.
The electrical conductivity should be such that excessively high voltages are not required, particularly in large installations where high currents are required.
The mechanical strength should be sufficient to enable relatively thin sections to be used, and good fatigue strength is essential where conditions produce reciprocating stresses.
It should be capable of being fabricated into a variety of forms.
A variety of supports have been used over the years, and these can be classified as follows:
Non-conducting mechanical supports such as plastics and ceramics.
Conducting supports that are not resistant to corrosion, for example copper, silver.
Valve metal supports in which the support forms a dielectric film, these are titanium, niobium and tantalum.
Supports that form electronically conducting films so that both the support and the platinum act as the anode (lead and lead alloys containing micro-electrodes of platinum).
Of these the valve metal supports are the most important, and will be discussed more fully than the others.
Although platinum in the form of thin sheet or fine wire may be supported mechanically on non-conductors such as plastics or ceramics these have obvious disadvantages. However, mention should be made of the “target” anode (9)consisting of an annular ring of a platinum-palladium alloy foil embedded at the outer and inner edges in a reinforced plastic holder. It has been shown that alloys containing up to 50 per cent palladium resists corrosion almost as well as pure platinum, and since palladium is cheaper than platinum the cost is reduced. Anodes of this type have been used forprotecting ships at anode current densities of up to 3200 A/m2, and the small size of the anode has the advantages of reducing drag on the hull, with a consequent economy in fuel as well as in the cost of installation.
Copper (10) and silver supports (11, 12) have the advantage of both good mechanical strength and electrical conductivity, but since they corrode when anodically polarised in chloride solution the platinum must be completely impervious, which precludes the use of thin electroplated coatings. Cladding must be used, therefore, but even so corrosion may occur at the exposed ends, unless they are carefully sealed. A trailing anode consisting of a 6.3mm diameter copper rod clad with 0.0125mm of platinum has been used at about 1900 A/m2 for protecting ships’ hulls (12). Silver is more effective than copper as a substrate, since it reacts with chloride ions to form sparingly soluble silver chloride at the surface of the metal, which gives it some protection.
However, the superiority of valve-metal supports is such that non-metals and reactive metals are now seldom used, and similar considerations apply to tantalum which has been replaced by the equally effective and significantly cheaper valve metal niobium.
Titanium and Niobium Substrates
The use of titanium as a substrate for platinum dates from about 1960 when the principle of the anode was first described in the published literature by Cotton (13), and since that time platinised titanium has become the most important anode for the cathodic protection of marine structure. However, the low breakdown potential of titanium is a disadvantage when the structure is large and/or the resistivity of the water is high since under these circumstances high e.m.f.s will be required from the rectifier-transformer and the interfacial potential of the titanium may then exceed the breakdown potential. This can occur on a cantilever-type anode, which, in order to provide a more uniform current distribution, is plated only at the end remote from the structure; this will result in the interfacial potential increasing with distance from the platinum and should the applied e.m.f. be high and the resistivity of the water low the breakdown potential may be exceeded. It may occur also if the electrodeposited platinum exfoliates to an extent that a significant area of the titanium is exposed. In this connection it should be noted that although electroplating is the obvious technique for applying a thin coating to a metal, the removal of the tenacious and refractory air-formed film on titanium presents difficulties. Furthermore, thin coatings are highly porous and although porosity decreases with thickness the internal stress increases, and if the adhesion is poor the coating may exfoliate.
Since the early days of platinised titanium, during which a number of failures occurred, developments in technology have led to an improvement in the performance of platinised titanium and to a variety of composite anodes in which electrodeposition of platinum has been replaced by cladding so that the platinum is metallurgically bonded to the substrate. These developments were described by Jacobs (6) in a paper presented at a Symposium on Cathodic Protection held in Londonin 1975 which was largely devoted to the use of platinised titanium in cathodic protection.
In the case of platinised titanium, methods of surface preparation to remove the oxide film and roughen the surface and techniques for electrodepositing platinum have both improved and failures are now rare. Although thicknesses of platinum of 2.5 to 5 <m are normally used it is now possible to apply coatings of 12.5 <m with good adhesion, and a variety of sizes and shapes of rod, tube, sheet and mesh are commercially available.
In the case of rod anodes a number of developments have taken place, since in this form it is possible to bond metallurgically a number of metals together, which are exposed only at the ends and can be sealed. The electrical resistivity of titanium is 48 Ω cm at 20°C compared to 15 Ω cm for niobium and 1.7 Ω cm for copper, and the high resistivity of the titanium results in unacceptable e.m.f.s when current outputs required are high and the section of the anode is small. Coppercored platinised titanium rod (6.1mm diameter) was developed in the early 1960s, and is available in long lengths that can be used to produce continuous anodes, which in view of its ductility can be bent to follow the contour of the structure to be protected. Anodes of this material have been widely used in the form of annular rings in Central Electricity Generating Board power-station water-cooling systems to protect the waterboxes from corrosion by sea or saline estuarine waters. Since additional corrosion is produced by the galvanic action of the bronze tube plates on the waterboxes the anode is placed close to the tube plate where maximum protection is required.
Heaton (14) has criticised this type of anode and has pointed out that the location of the anode near to the tube plate has resulted in insufficient spread of current to areas of the waterbox remote from the anode, with consequent corrosion; failures by pitting of the titanium and rapid corrosion of the underlying copper have also been experienced.
More recently a variety of metallurgically bonded rod anodes have become available, which have a number of advantages over platinised titanium rod. Tibond is available as rod 2 to 20mm diameter with a clad coating of platinum (5 to 50 <m thick), and with or without a copper core. Niobond, in which the titanium is replaced by niobium, is available in similar forms, but additionally a steel core can be provided for the larger section rod if high strength is required. Figure 5 shows a section of copper-cored Niobond. These materials are available in long lengths (up to 90m for 2mm diameter and up to 12m for 20mm diameter), either sealed at one or both the ends, or unsealed. Platinum is susceptible to marine fouling, and since the anodes may be exposed to sea water for some time before commissioning, a further refinement is to apply an anti-fouling clad layer of copper over the platinum, which is removed rapidly when the anode is first polarised. Figure 6 illustrates the most complex form of Niobond, which has been machined to reveal the various materials composing the composite anode, that is a core of steel for improved mechanical strength, a layer of copper for improved electrical conductivity, a layer of niobium for corrosion resistance, a layer of platinum, and an outer layer of copper which prevents marine fouling of the anode prior to energising; Figure 7 shows a section of the anode.
The non-porous layer of platinum is an obvious improvement on electroplated platinum, and comparatively thick layers may be used where conditions are severe and where long life is required.
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- 14W. E. Heaton, Ref. 6, p. 47