platinum provides protection for steel structures...platinum provides protection for steel...

Platinum Provides Protection for Steel Structures AN ECONOMIC WAY TO PREVENT CORROSION

By Lionel L. Shreir Department of Metallurgy and Materials, City of London Polytechnic, London

Increasing concern at the waste caused by corrosion hasfocused attention on the need to utilise the most advanced methods for preventing, or controlling, this ex- pensive phenomenon. The use of platinum for the cathodic protection oj' 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 malce platinum cathodic protection an economic alternative to other methods. This review, which will appear in tzoo parts, describes hoau cathodic protection is provided, outlines the many .factors hat haae to be considered when actilising 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

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. Further- more, 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 I (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 electro- positive) 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 stimu- lated. Figure I (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/CuSO, saturated reference electrode) by the rapid transfer of electrons from the platinum the rate of the anodic dissolution reaction

metal and is highly resistant to corrosion in will become zero, and the whole of the surface most environments, but its use as a material of of the steel will become cathodic and the steel construction is severely restricted by its very is said to be cathodically protected.

T h e ilorth Sea subjects oil andgas rigs to extremely severe environmental conditions so it i s not surprising that considerable care must be taken to protect such structures from corrosion, with the consequent possi- bility of mechanical failure. W i t h a typical production platform costing of the order of a 0 millions, and expected to have a life of 25 years, a properly applied cathodic protection system i s able to offer considerable safeguards to such large and crucially important investments. Th is Shell photograph shows the production platform Indefatigable K, which i s operating in the southern section of the North Sea o n behalf of Shell U.K. and Esso Petroleum, upon which trials of a new cathodicprotection system developed by Solus Scholl have been carried out. Th is Solanode system, which i s more ful ly described in Part I I of this article, uses a buoyant platinised-niobium anode moored approximately 3 metres above the sea bed at a position 100 metres from the platform, thus giving many advantages over more conventional anode systems

The cathodic reactions in natural waters and soils will be oxygen reduction

+O, +H,O +ze-+zOH- (ii) and hydrogen evolution

2H,O f zep-+H, +2OH- (iii) 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/or the oxidation of chloride ions :

3H,0+2H,0t$~0, +2e- (iv) (which is actually the reverse of Equation ii) and 2C1-+C1, +2e- (v> With an inert anode such as platinum, oxygen evolution (Equation iv) will be the pre- dominant 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

Platinum Metals Rev., 1977, 21, ( 4 )

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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


No reliable figure available, but 0.005 kgjamp yr at 2000

thin foil would conform to these requirements, which may be surnmarised as follows :

I Low rate of corrosion, irrespective of the nature of the environments or of the reaction products formed during anodic oxidation.

2 Low anode polarisation, irrespective of the anode reaction(s).

3 Good electrical conductivity. 4 High reliability, and a life of' 10 to 30

years without maintenance. 5 Sufficient mechanical strength and stab-

ility to withstand damage during installation and maintenance of the structure, and the stresses that it may be subjected to during service.

6 Low cost in comparison with the overall scheme of protection (rectifier-trans- formers, 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 in- herently thermodynamically unstable. I t 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 (I). 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 E,,. This decrease in the corrosion rate on attaining the passivation potential is due to the formation of a I to jnrn 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 Elec- trode (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 relation- ships (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 4omA/cm2 the potential increasing from 1.4 to r.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, PtO, or Pt(OH), 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 C1-+Cl, can only be


accomplished at a somewhat higher over- On the other hand, the "valve" metals such potential, which is of little significance when as titanium, niobium and tantalum when the platinum is used as anode in an electrolytic anodically polarised in a variety of solutions cell, but is a serious limitation for fuel cells are characterised by the formation of dielectric using platinum as electrodes. oxide films (3); these films are amorphous and

The Historical Background The principles of cathodic protection the whole surface were rendered negative. . . .

were clearly understood and concisely A piece o f zinc as large as a pea, or the point expressed by Sir Humphry Davy as long of a small iron nail, toere found fully adequate ago as 1824. In a paper presented in that $0 Preserve forty Or @Y square inches of

copper; and this, wherever i t was placed, year to the of which he was at top, bottom, or in the middle then President, he described a series of the sheet of copper, and whether the copper

he had carried Out at the was straight or bent, or made into coils. A n d request of the Royal Navy who were con- where the connection between different pieces cerned at "the rapid decay of the copper of copper was completed by wires, or thin sheeting of His Majesty's ships of war and filaments of the fortieth or jifiieth of a n inch the uncertainty ofthe time of its duration". in diameter, the effect was the same; every

In the course of this paper he wrote: side, every surface, every particle of copper remained bright, whilst the iron or the zinc

Copper is o metal only zueaicly positive in was slowly corroded. the electrochemical scale; and, according to Davy also investigated the impressed- m y idem, it could only act upon sea water current system, using a voltaic battery, but when i n the ~ o s i t i v e state; and, consequently, he did not consider this to be practical in

i t could be rendered slightly negative, the corroding action of sea water upon it would

service conditions. Although the begin- be null; and whatever might be the differences nings of cathodic protection date therefore of the kinds of copper sheeting and their from 1824 this method of pro- electrical action upon each other, still every tecting metals-~articularl~ steel-from effect ?f chemical action must be prevented, if corrosion was neglected for over a century.

.dr.

H.M.S. Samarang. the original ship to be cathodirally protected by Sir Humphry Davy

Platinum Metals Rev., 1977, 21, ( 4 ) 115

transparent and give rise to interference colours that provide a means of evaluating film thickness. Except at low potentials con- duction 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 oxidc 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 z.3nm/V in 0.1 M H,SO,. Thus at any given potential

the film will thicken to a limiting value, the ionic current then becoming almost zero.

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 IOOV, whereas in the case of titanium, which can be anodised to approximately 8oV in sulphuric acid and to over 4ooV in 95 weight per cent formic acid (4), breakdown occurs when the potential of the metal/solution interface attains 8 to IOV. 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 temper- atures 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 increase in 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 T i : Nb : Ta :: I : 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 C1-+Cl, 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, will not exceed 8 to g nm. The platinum could be regarded, therefore, as acting as an internal potentiostat that prevents the potential 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 Baurn (8) patented an anode consisting of tantalum partly coated with platinum, either by electroplating or by cladding, for the anodic oxidation of sul- phates to persulphates.

Ideally, a substrate for platinum should have the following properties :

I It should be resistant to corrosion in chloride-containing environments at ele- vated anodic potentials, and should also be resistant to the products of the anodic oxidation of chlorine and water, that is chlorine, hypochlorites and hydrochloric acid.

2 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.

3 The surface of the substrate must be amenable to coating with thin layers of platinum by technique such as electroplating, and the coating so produced must be strongly adherent to the substrate and current flow should not be impeded by an intervening film.


4 The electrical conductivity should be such that excessively high voltages are not required, particularly in large instal- lations where high currents are required.

5 The mechanical strength should be sufficient to enable relatively thin sections to be used, and good fatigue strength is essential where conditions produce re- ciprocating stresses.

6 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 :

I Non-conducting mechanical supports such as plastics and ceramics.

2 Conducting supports that are not resistant to corrosion, for example copper, silver.

3 Valve metal supports in which the support forms a dielcctric film, these are titanium, niobium and tantalum.

4 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.

Non-conducting Supports

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. I t 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 for protecting 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.

Active-metal Supports

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 Alma 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

Fig. 5 Section of copper- cored Niobond in which the copper, niobium and platinum (not visible at this magni- $cation) are metallurgically bonded to one another. The inner core of copper is used to improve electrical conductivity, and is completely iso- lated from the platinum by the corrosion resistant layer of niobium

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 diffi-

cuities. 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

Fig. 6 Composilc Niobond anode in which layers have been removed by machining to reveal the various metals. These are from right to left: steel core, copper, niobium, platinum, copperfor antqouling


Fig. 7 Section of Figure 6 , a composite Niobond anode, showing the inner core of steel surrounded by a layer of copper and then niobium Acknowledgements are made to Marston Excelsior for providing Figures 5, 6 and 7

is metallurgically bonded to the substrate. These developments were described by Jacobs (6) in a paper presented at a Sym- posium on Cathodic Protection held in London in 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 pm are normally used it is now possible to apply coatings of 12.5 pm 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 0 cm at 20°C compared to 15 62 cm for niobium and 1.7 0 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. Copper- cored platinised titanium rod (6.1mm diameter) was developed in the early 1g6os, 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 con- tour 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-cool- ing 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 pm thick), and with or without a copper core. Niobond, in which the titanium is replaced by nicbium, 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

corrosion resistance, a layer of platinum, and an outer layer of copper which prevents marine fouling of the anode prior to energis- ing; Figure q 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.

References I T. P. Hoar, "Modern Aspects of Electro-

chemistry", ed. J. O'M. Bockris, Butter- worths, London, 1959, Vol. 2, p. 262, "Corrosion", ed. L. L. Shreir, Newnes- Butterworths, London, 1976, Vol. I, p. 1-114

long lengths (up to for 2mm diameter 2 E. L. Littauer and L. L. Shreir, Electrochim. Acta, 1966, 11, (51, 527

and up to 12m for 2omm diameter), either L. young, oxide ~ i l ~ ~ - , ~ ~ ~ d ~ ~ i ~ sealed at one or both the ends, or unsealed. - Press, I ~ I

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 com- plex 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

A. R. Piggott, H. Leckie and L. L. Shreir, Corrosion Sci., 1965, 5, (3), 165 I. Dugdale and J. B. Cotton, Corrosion Sci., 1964, 43 (4)> 397 W. R. Jacobs, "Proceedings of Symposium on Cathodic Protection", (Marston Excelsior Ltd.), London, May 1975 G. W. Walkiden, Corrosion Technol, 1962, 9, (11, 14 and 1962, 9, (21, 38 G. Baum, U.S. Patent 1,477, 099; 1922 E. P. Anderson, French Patent 1,253,058; 1966 E. E. Nelson, Corrosion, 1957, 13, I22t H. S. Preiser and F. E. Cook, Ibid., 1t5t H. S. Preiser, U.S. Patent 2,863,819; 1955 J. B. Cotton, Chem. and Znd., 1958, 68 W. E. Heaton, Ref. 6, p. 47

The second part of Dr. Shreir's review will be published i n the January 1978 issue of Platinum Metals Review.

Catalysts for Automobile Emission Control While the hydrocarbons and carbon mon-

oxide emitted from automobile exhausts can now be catalytically oxidised very satis- factorily a number of different approaches are still being pursued for the control of nitrogen oxides. In a recent paper J. C . Schlatter and K. C . Taylor of General Motors Research Laboratories report work carried out on the addition of platinum and palladium to rhodium catalysts to improve their oxidising capacity when used for the simultaneous control of all three pollutants (J. Catalysis, 1977, 49, (I); 42-50). I t was found that an improvement could be achieved

in the laboratory and initial associated disadvantages were later avoided.

The results obtained using laboratory feedstream are not always substantiated when engine exhaust tests are carried out and these have still to be done. It is however reported that for enhanced performance the platinum or palladium, should not be deposited on the same support beads as the rhodium. The two sets of beads can then be separately posi- tioned, with the rhodium catalyst in the front of the catalytic bed. In this way each material is in the environment most suited to its catalytic purpose.

Platinum Metals Rev., 1977, 2 1 , ( 4 )

platinum provides protection for steel structures...platinum provides protection for steel...

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