by R. A. Cottis’ Corrosion is not a problem -

but the cost of corrosion is

Corrosion of metals presents aperpetual problem. Althoughtechnology is available to prevent itin many cases, costs are involved.The author reviews various aspectsof corrosion and methods for itsprevention - stressing that suchcosts should be weighed againstthose of extensive metal damage.Referenced to specific illustrativeexamples, the invaluable roles ofeducation, research and thecorrosion specailist are outlined.particular emphasis being placed onhigh temperature oxidation,anodisrng of aluminium, inter-granular damage and corrosion inmarine environments.

As a subject, the study of corrosionis ruttier like weather forecasting. Virtually everyone is aware of the moreobvious manifestations,and all too oftenthe ‘experts’ appear to have difficultyin getting it right. After all, everyoneknows that cars do rust, but no oneseemsto be able to prevent this. 1-lowthen can the title of this article bejustified?

The author’s justification lies in thenature of the problems resulting fromcorrosion. To take the example of therusty car, the experts can offer solutions,one being the use of stainlesssteel forthe constructionof bodywork,which hasbeen shown to give at least 20 yearsservice. However, it is doubtful whethermany customers would pay the considerably higher price, and it is equallyquestionablewhether the world’s supplyof chromium could support such achange. More generally there are fewcorrosion problems which cannot betechnically overcome if in doubt tryplatinum! I, what is much more difficultis to contain corrosion in the most costeffective manner.

Before we examine some of the costimplications of particular corrosion problems, and ways in which these problemsare being tackled, it is worth puttingthe overall national costs of corrosioninto perspective. Various studies haveexamined the costs of a corrosion inthe UK, the USA and elsewhere. Ingeneral, for developed countries theseshow costs resulting from corrosion andits control of around 3% of the grossnational product. This figure may perhaps take on greater significance whenexpressed in terms of a correspondingproduct. Thus the cost of corrosionin South Africa is comparable to itsannual production of diamonds, whileNorth Sea oil more or less pays Britain’scorrosion bill. It is clear that corrosionoffers plenty of scope for saving money.It is estimated that 152S% of the costof corrosion could be saved simply by

the use of known methods of corrosioncontrol. These cost savings can beachieved in many ways. An obviousexampleis the use of a more corrosionresistant material to give a longer liftand hencea lower annual cost. Equally,however, it will often make economicsense to use a very cheap materialaccepting the more frequent replacement which becomes necessary.

A familiar example of this balancebetween cost and corrosion resistancecan be seen in the selection of materialsfor a car exhaust system. Traditionally,these parts have been made in mildsteel, which gives a life in the regionof 15 months, resulting in frequent, butrelatively inexpensive replacements. Inan effort to prolong the life of vehicleexhausts two alternatives have beendeveloped. Of these stainless steel isconsiderably more expensive than mildsteel, hu offers very long service inexcess of five years. A lower costoption is to use aluminium coated steel.which gives more modest improvementsin corrosion performance, about twoyears life, but at a cost which t notvery much greater than that of mildsteel.

The costs and expecteu ott. ‘ thestthree alternatives are shown . ‘

Table 1 Costs and lives a* typicalexhaust system8 tor a medium-sized

saloon car

Lifeimonths,

In simple terms, this suggests thaistainless steel is the most cost-effectivechoice for an expected life of 10 years.costing a total of £140. compared to£175 for aluminised steel and £240 fomild steel figure 1.

‘The cost of 8fl 8lU,nlfl,sed system 6 estimetedfew echeust replacement centres otter thesel

CostIii

Mild steelAluminised steelStainless steel

303570

152460

La corrosion n’est pas Unproblème - mais son coot en estUnLa corrosion des métaux oat unprobléme perpetual. Bion quo Istachnologie permettant do I’évhterexisto dons do nombreux cas, ohiocoUte chor. L’auteur passe en revuedivers aspects do Ia corrosion at desméthodes preventives et it insiste surhe fait qu’iI taut mettre en balanced’une part los frais et d’autro part losdegats importants causes aux métaux.ihlustrés par des examples particuliers.los réles inostimables do l’éducation.do Ia recherche at du specialiste do- acorrosion SOnt exposés. L’auteui insistetout particuliérement sur ‘oxydation ahaute temperature. ‘anodisation dol’aluminium. los dégéts interyranulairosat to corrosion en milieu mann.

Korrosion setbst ist kein Problem- aber die Kosten sind emsDie Korrosion von Motailen stout emsténdiges Problem dar. Obwohl hauteTechniken verfflgbar sind. die sie invlelon Féllan verhindern können. sobringen diese Kosten mit slch. DorAuto.’ befaBt sich mit verschiedenenAspekton von Korrosion und denMethoden zu lhrer Verhindorung; a,betont, dat oventuello Koston gegenstarke Metallschãden abgewogenwarden müssen. Anhand von einzelnenanschautichen Beispielen werden diebedeutenden Funklionen vonInformation. Forschung undKorroslonsspeziaiisten gezeigs.Besonders hervorgehoben werdenhiorbel Oxidierung tinter hohenTemperaturen, AluminiumAnodislerung, interkrlstalllne Korrosionund Schãden in Meeresumgebungan.

1 Even stainless

course of thcii everyday work.ItIVUi%I.’, Ut’ fl3WhII I I ‘.1111 U 1.III IIIII

In addition to the actual cost of theexhausts, a significant bill for interestwill also be accumulated, Because thestainless steel exhaust requires the outlayof large sums of money early in the lifeof the car, it accumulates a proportionately larger amount of interest. Theamount owing to the bank after tenyears is shown in Table 2 for a rangeof interest rates.

Table 2 Total

It can be seen that aluminised steelsystems become more competitive withstainless steel as interest rates increase,whereas ordinary mild steel Systems arealways significantly more expensive. Onewonders why it is that mild steel systemsdominate the replacement market!

Even this analysis is incomplete,since it does not take account of factorssuch as inflation and the time for whichthe owner expects to keep the car.Imagine the problems of the corrosionengineer when recommending materialsfor the construction of a large chemicalplant!

In the more complex industrial situation there may often he greater advantage in opting for more expensivecorrosion resistant materials and systems,since the costs of lost production incurred during replacement of less resistant components may far outweighactual component costs. Thus a criticalvalve in a chemical process plant maycost £1000 in an alloy expected to lastfor one year or £10000 in an alloyexpected to last five years. Clearly, thesecond option is not a reasonable choiceon the basis of components cost alone,but if valve replacement requires aplant shut-down resulting in loss ofproduction worth £2000, the balanceswings in favour of the component withthe longer life.

This example is clearly oversimplified,and a wise selection would also takeaccount of reliability and maintenancestrategy. Thus a component with aknown history of reliable operationwould be preferred to a less reliablecomponent, or one with no servicehistory. Equally it might be reasonableto use a lower cost, shorter life component if its replacement can bescheduled to coincide with regular maintenance periods, so that replacementdoes not incur any additional productionlosses.

Control of corrosion costsHaving seen that corrosion is primarilyan economic problem, what can he doneto minimise the cost of corrosion? Oneof the more unfortunate features of

of small ‘losses, such as the failure ofmillions of car exhaust systems at a costof a few tens of pounds each, or thegradual deterioration of thousands oftons of structural steework exposed tothe elements. Three major aspects ofmethods of corrosion control are education, the work of corrosion specialistsand applied corrosion research.

Role of educationBecause of the widely dispersed natureof corrosion problems, one of the mostpowerful weapons in the battle to reducecorrosion losses is education. This canoperate at several levels, as examplifiedby the work of the UMIST Corrosionand Protection Centre. The primary aimof the Centre is the training of corrosionspecialists by means of the M.Sc. coursein Corrosion Science and Engineering,with more specialised training beinggiven in the form of research programmes for the degrees of M.Sc. byresearch and Ph.D. These coursesattract students from a wide range ofacademic and industrial backgrounds,and may form the basis for a careeras a corrosion scientist or engineer.Alternatively the one-year M.Sc. courseprovides a sound understanding of corrosion and its control for professionalmetallurgists, chemical engineers andsimilar disciplines.

While the training of professionalcorrosion scientists and engineers is animportant activity as is evidenced bythe continuing demand from employers,despite the current recession, it has arelatively small immediate impact on thelarge number of ‘minor’ corrosion losseswhich do not merit the attention of afull-time corrosion specialist. This presents a very difficult problem of conveying a relatively limited ‘corrosionawareness’ to a large number of people.Members of staff of the Corrosion andProtection Centre attempt to play theirpart in the solution of this problem bythe presentation of lectures to nonspecialist audiences, by the preparationof articles and publications for the nonspecialist and by the presentation of aseries of short courses introducing thepractical side of corrosion control toengineers and technicians. As far asindustrial corrosion problems are concerned it is economically attractive toan organisation for a wide range ofpersonnel to acquire a general tinder-standing of the principles of corrosion.These can range from the more obviousexamples of engineers involved in thedesign and opeiat ion of plant and equip-

Role of the corrosion specialistThere are relatively few specialist corrosion scientists and engineers in theUK probably numbering a fewthousand. Many of these are employedin industrial plant with significant corrosion problems. In this situation it isimportant that the corrosion specialistis seen as an integral part of the design,operation and maintenance team sothat corrosion problems can be minimised by design, rather than beingtackled as a failure investigation afterthe plant has been constructed.

A second role for the corrosionspecialist is in the provision of a researchand/or advisory service for suppliers ofmaterials for use in the control of corrosion. These specialists play an important part in the development of newmaterials, and, as technical service/salesengineers, in the encouragement of corrosion awareness among non-specialists.

Finally, a number of independentconsultants offer a corrosion adviceservice to smaller organisations whocannot justify the employment of full-time staff to handle corrosion questions.As an example the Corrosion and Protection Centre Industrial ServiceCAPCIS provides a corrosion failureinvestigation and advice service toindustry. All too often however, theconsultant is investigating a failurewhen it would have been far moreeconomical for him to have given adviceat the design and installation stage!

4

Role of researchThe more widespread awareness ofknown principles of corrosion control,and the efforts of specialist corrosionscientists and engineers, can realise significant savings from the application ofexisting knowledge. The role of appliedcorrosion research is to extend thisknowledge to provide more economicalmaterials and corrosion control systems,and to increase the reliability of corrosion control systems by providing abetter understanding of the mechanismsinvolved.

In common with most universitydepartments, research at the UMISTCorrosion and Protection Centre is ablend of pure and applied work, butthe industrial importance of corrosion,and the Centre’s many contacts withindustry probably results in a greaterproportion of studies with direct relevance to practical corrosion problems.Some examples of this class of researchwhich are currently in progress at theCorrosion and Protection Centre arepresented here.6

‘Interested ede’s cn Oø obtein a eopy, onrequest, of the Centre’s rr’search in progressbrochure. which gives brief detail’ of all the90 o’ so prnje-ts crrrrerrtTy 0 proq,es,

77

costs of providIng exh.aust systems for ten years wIth moneyborrowed at various rates of Interest

5% 10% 15% 20%MUd steel 324 440 597 809Alumlnisecj stoolStainless steel

237203

321294

437424

595608

METALLURGIA FEt3RUARY 1982

UE

‘C0

I-i

C

Corrosion monitoringAs was indicated previously, corrosionproblems are often far more severe ifthey occur unexpectedly. A known,steady rate of corrosion can be toleratedin a chemical process plant by meansof a planned replacement policy. Byordering replacements well in advanceof shut down periods the time andhence production lost can be kept toa minimum. In contrast an unexpectedfailure can cause prolonged shut downand large production losses while replacements are being obtained. For thisreason the monitoring of corrosion processes plays an important part in ensuring the reliable operation of such plant.

Existing techniques of corrosionmonitoring cover a wide range of applications. In particular, the linear polarisation resistance technique offers anaccurate direct reading of corrosion rateon a near-continuous basis. However,the accuracy of this technique is restricted in certain circumstances,especially in environments where theelectrical resistance of the corrosive fluidis very high. A large research group atUMIST is working on electrochemicalaspects of corrosion processes, and techniques developed by this group offermethods of monitoring corrosion inthese, more difficult, environments.

The best-established of these techniques is the measurement of the a.c.impedance of the interface between ametal sample and its environment overa range of frequencies. If the real andimaginary components of the impedanceare plotted with frequency as a parameter figure 2, the resulting complexplant or Nyquist diagram permits theanalysis of the thickness of some surfacefilms by means of the capacitance, thecorrosion rate proportional to I /R0..and the resistance of the solution R801..

More advanced techniques are alsobeing investigated, and offer somepromise for the detection and measurement of localised corrosion, such asflitting, crevice corrpsion and c.I vitation.

2 A.C. impedancediagram for a typicalcorroding metal.

High temperature/oxidationcorrosionThe performance of alloys at hightemperatures in industrial environmentsis often limited by their resistance to

oxidation or corrosion by componentsof the environment. For many thermalprocessing and energy conversion systemssuch as boilers and gas turbines, theefficiency of the process is a function

of the operating temperature. Hence inthis case the economies of the corrosionproblem are controlled more by the costsof the operating effIciency than bymaterial costs. Consequently there Is acontinuing search for alloys capableof operating at higher temperatures andin more hostile environments.

It is well known that the theoreticalmaximum efficiency of conversion ofheat to other forms of energy is controlled by the difference in temperaturesof the heat source and sink. The gasturbine can play a useful part in thisconversion process, since it can extractenergy directly from a hot, pressurisedgas stream without the intermediateprocess of heat transfer in a boiler.Consequently there is considerable interest in materials to withstand coal

$

3 Severe hotCorrosion of a gasturbine stator blade.

combustion or gasification products athigh temperatures, and the availabilityof suitable materials could materiallyinfluence coal gasification and powergeneration economics.

These environments are extremelycorrosive figure 3, consequently research work at the Centre is concentrating on the development of coatingmethods for existing alloys, rather thantrying to achieve both good mechanicalproperties and corrosion resistance inone alloy.

Anodised aluminiumThe corrosion resistance of aluminiumis well known and plays a large part inits widespread use for architecturalmetalwork, particularly window framesWhat may not he so widely recognisedis that untreated aluminium will sufferfrom fairly rapid pitting corrosion aresult of atmospheric pollutants or saltspray from the sea. The good atmospheric corrosion performance of aluminium is obtained by anodising, artificiallythickening and reinforcing the naturaloxide film which protects aluminium

Real Impedance ohms

4 Section through a porous anodic filmon aluminium.

against corrosion.Despite its generally good perform

ance, anodised aluminium can sufferfrom corrosion in the atmosphere, particularly if pollutants are allowed tocollect on the surface, and the cleaningof anodised aluminium components canbe a significant part of the maintenancecosts for modern aluminium clad buildings. The corrosion may take the formof localised pitting attack, and muchwork has been done at the Centre inorder to understand this process, andthereby to develop methods of corn-batting it. As an example, figure 4shows a section through the normalanodic oxide film on aluminium. Theporous structure which can be seen issubsequently immersed in boiling waterto hydrate the oxide, causing it toexpand and block the pores. This process is known as sealing, and increasesthe corrosion resistance of the material.

In contrast, figure 5 shows a filmformed on an aluminium alloy containing iron, where a particle of inter-metallic is present at the surface. Thishas caused a major change in the natureand thickness of the film, which providesa local point of weakness which is liableto initiate the growth of a corrosion pit.

lntergranular corrosion ofsacrifical anodesA very cost-effective method of protecting steel against corrosion in moderately aggressive environments such asseawater is to use cathodic protection.In essence this consists of making thesteel electrically more negative, therebyslowing down or stopping the normalcorrosion process of iron dissolving togive positively charged ferrous ions. Thiscan be done either by means of anexternal power supply, or by connectingthe steel to a more reactive metal suchas zinc or aluminium. The latter isknown as the sacrificial anode methodof cathodic protection, and is particularly convenient to use, since no externalpower source is required, and, if properly designed, is essentially self-regulating. For these reasons a majority ofthe steel oil production platforms areprotected by sacrificial anodes whichare attached to the structure when itis fabricated. In general these performwell, and corrosion of the submergedstructure of the platform is not a majorproblem. However when zinc sacrificialanodes get hot, as is the case near thehot riser, where the hot oil emergesfor the well, the problems become rathermore severe. Because of the high temperature, corrosion reactions occur morerapidly, and it becomes more importantthat the cathodic prulectio system

5 Anodic rum over an aluminium-ironintermetal/ic.

provides sufficient current to protect thestructure. Unfortunately it has beenfound in practice that the zinc alloysacrificial anodes normally used for theprotection of this area fail prematurelybecause of an intergranular corrosionsee figure 6. Work is in progress atthe Corrosion and Protection Centre todetermine the cause of the prematurefailure of the anodes, and thereby todevelop solutions to the problem and

permit this econom ily attractivemethod of corrosion protection to beapplied in this particular situation.

Marine corrosionThe development of the North Seaoilfields has provided a spur for thedevelopment of marine technology, inaddition to the study of materials andprotection systems for conventionalstructures, work at the Corrosion and

Protection Centre also reflects the demand for higher performance materialswhich will permit lighter and less expensive structures to be used for oil production platforms, and in the more distantfuture, for wave energy devices. Thus,as a part of a considered SRC programme investigating the performanceof steel wire ropes in the marine environment, workers at the Centre are studyingthe feasibility of cathodic protection asa means of providing a long servicelife for large wire ropes used to tetherwave energy devices and floating oilproduction platforms. This is particularly important for the economics ofpower generation from wave energy,since the cost of rope tethers, and particularly the cost of their replacement,is a major proportion of the cost ofthe overall system.

The marine environment is alsocomplicated by the inevitable growthsof seaweed, algae, barnacles and so on,which can4 modify the conditions at thesteel surface in a variety of ways. Inorder to clarify the possible influenceof such fouling growths, a number ofjoint programmes are commencing atthe Corrosion and Protection Centre,the joint UMIST/University of Manchester Pollution Research Unit andCAPCIS. These are studying the influence of marine fouling growths on possible hydrogen embrittlement problems,on the efficiency of the cathodic protection, and on the performance ofsacrificial anodes figure 7. The resultsof this work will assist in the designof new structures, allowing the cost ofthe corrosion protection system to beminimised, and will give operators anindication of the extent to which it isnecessary to remove fouling growths forcorrosion control purposes.

The UMIST Corrosion and Protection Centre is of course not alone in itsefforts to develop improvements in themethods available to control the costsof corrosion, and many other universitydepartments and government and commercial laboratories are also engaged insimilar work. However it is only by thecontinuing and increasing awarenessamong non-specialist engineers, managers and technicians of the potentialcosts of corrosion, that this researchwork can he translated into the maximum possible financial benefit.

*

AcknowledgementThe author wishes to thank his colleagues at the Centre for their assistancein the preparation of this article.

6 lnzergranular corrosionsacrificial anode x 100.

or a zinc

7 Growths of marine fouling organismson steel.