CORROSION Engineering Material Class Monday, 25 September 2006

REFERENCES Chandler, H., Metallurgy for the Non Metallurgist, ASM, Ohio, 1998. (1 copy in Student Design Facility) Fontana, M. G., Corrosion Engineering, 3rd ed., McGraw-Hill, Singapore, 1987. (3 copies in Petra library) ASM Handbook volume 13: Corrosion, ASM, Ohio, 1987. (reference book in Petra library) Roberge, P. R., Handbook of Corrosion Engineering, McGraw-Hill, New York, 2000. (1 copy in Student Design Facility) Jones, D. A., Principles and Prevention of Corrosion, 2nd ed., Prentice Hall, New Jersey, 1996.

THE COST OF CORROSION Economic Human Life and Safety Corrosion represents a constant charge to a nation’s GNP in the order of 3-5%. Human Life and Safety Country Year Cost# % GNP India Germany United Kingdom Australia Japan United States Indonesia 1961 1969 1970 1973 1974 1975 1985 1990 2002 1986 1.54*109 19*109 1.36*109 470*109 2.55*109 82*109 150*109 180*109 300*109 109 - 3.5 1.5 4.9 1

COST OF CORROSION - Economic Corrosion of bridges – they age and require replacement A report by the New York department of transport: by 2010, 95% of all New York bridges would be deficient if maintenance remained at the same level as it was in 1981. Corrosion in chemical company Corrosion in paper industry Corrosion in oil industry Corrosion of automobiles - fuel systems, radiators, exhaust systems, and bodies

Indirect Costs Plant Downtime Loss of Product Loss of efficiency Some of the more important sources of indirect costs : Plant Downtime Loss of Product Loss of efficiency Contamination Overdesign

DEFINITION OF CORROSION The destruction or deterioration of a material because of reaction with its environment. (Fontana) The destructive result of chemical reaction between a metal or metal alloy and its environment. (Jones) CORROSION SCIENCE is the study of the chemical and metallurgical processes that occur during corrosion. CORROSION ENGINEERING is the design and application of methods to prevent corrosion. Ideally SCIENCE + ENGINEERING = to invent new and better methods of prevention and apply existing methods more intelligently and effectively.

ENVIRONMENT Practically all environments are corrosive to some degree. Air and moisture Fresh,distilled, salt and mined water Rural, urban and industrial atmospheres Steam and other gases such as chlorine, ammonia,hydrogen sulfide, sulfur dioxide, and fuel gases. In general, the ‘inorganic materials are more corrosive than the ‘organic.’

Corrosion as Extractive Metallurgy in Reverse

Factor affecting choice of an engineering material Corrosion resistance Cost Appearance Fabricability Mechanical behaviour Availability F

Factors affecting corrosion resistance of a metal Thermodynamics and electrochemistry : understanding and controlling corrosion. Metallurgical factors Physical chemistry: for studying mechanism of corrosion reactions, surface condition of metals, and other basic properties.

Electrochemical Aspects Figure 1 Schematic diagram of metal M dissolution, liberating into solution a metal ion M2+ and into the metal electrons, e-, which are consumed by reduction of H+ to H2.

FORMS OF CORROSION Classification based on the appearance of the corroded metal. Can be identified by naked eye or magnification. Examination before cleaning is particularly desirable. See various forms of corrosion pictured shematically in the acetate film

Forms of Corrosion Characteristics – Mechanisms – Preventive Measures Some of the 8 forms of corrosion are unique, but all of them are more or less interrelated. Uniform corrosion Galvanic, or two-metal corrosion Crevice corrosion Pitting Intergranular corrosion Selective leaching or parting Erosion corrosion Stress corrosion

UNIFORM CORROSION The most common form of corrosion. A uniform, regular removal of metal from the entire exposed surface. Metal ? - Environment ? - The metal becomes thinner and eventually fails. Represents the greatest destruction of metal on a tonnage basis. The life of equipment can be accurately estimated on the basis of comparatively simple tests.

Examples of Uniform Corrosion Steel or zinc in dilute H2SO4 Atmospheric corrosion : a sheet iron roof Carbon steel storage tank for sour (H2S-containing) crude oil after only two years of service.

Uniform attack can be prevented or reduced by Proper materials including coatings Inhibitors Cathodic Protection

Galvanic Corrosion Two dissimilar metals are coupled in the presence of corrosive solution. There is a potential difference – the less resistant metal becomes anodic and the more resistant metal cathodic. The driving force for current and corrosion is the potential developed between the two metals. Dry-cell battery is an example of this type of corrosion.

Prevention of Galvanic Corrosion Select combinations of metals as close together as possible in the galvanic series. Avoid the unfavourable area effect of a small anode and large cathode. Insulate dissimilar metals wherever practicable. Apply coatings with caution. Add inhibitors. Avoid threaded joints for materials far apart in the series. Design for the use of readily replaceable anodic parts or make them thicker for longer life. Install a third metal that is anodic to both metals in the galvanic contact.

CREVICE CORROSION Localised forms of attack that result in relatively rapid penetration at small discrete areas. Location : within crevices and other shielded areas on metal surfaces exposed to corrosives (small volumes of stagnant solution) caused by : Holes Gasket surfaces Lap joints Surface deposits Crevices under bolt and rivet heads Wet packing materials Stainless steel bolt (bottom) inappropriately used in seawater after a five year exposure. (photo copyright 2000 by George Dinwiddie, used by permission of www.alberg30.org)

Cases of CREVICE CORROSION

Environmental Factors – Crevice Corrosion Deposits that may produce crevice corrosion: sand, dirt, corrosion products (permeable), and other solids. To function as a corrosion site, a crevice must be wide enough to permit liquid entry but sufficiently narrow to maintain a stagnant zone.

PITTING A form of extremely localised attack that results in holes in the metal. Holes – small or large in diameter, in most cases are relatively small. Pitting is unpredictable, especially in conditions forming deep pits. Difficult to detect pits because of their small size and because of the pits often covered with corrosion products. The rate is variable, depending on uncertain migration of corrodents into and out of the pit. Difficult to predict quantitatively and by laboratory tests : Varying depths and number of pits. Sometimes the pits require a long time – several months or a year to show up in actual service.

Prevention Similar methods as suggested for combating crevice corrosion. Addition of 2% Mo to 18-8S (type 304) to produce 18-8SMo (type 316) results in a very large increase in resistance to pitting. To use materials that are known not to pit in the environment under consideration: hastelloy and titanium. Adding inhibitors.

Effects of Metallurgical Structure on Corrosion Intergranular corrosion of austenitic stainless steels: When heat treatments deplete the g.b. of Cr by metallurgical reaction with C (in the temp. range 425-815oC). In that temp. range, chromium carbides (Cr23C6) are insoluble and precipitates at g.b. What is the influence of Ni and Mo ?

Control for Austenitic Stainless Steels Employing high-temperature solution heat treatment (quench-annealing or solution-quenching). Adding elements (stabilisers) that are strong carbide formers. Lowering the carbon content to below 0.03%.

SELECTIVE LEACHING The removal of one element from a solid alloy by corrosion processes. That element is more susceptible to corrosion than the rest, more active electrochemically and are anodically dissolved in galvanic contact with the more noble elements. Uniform dezincification – layer type (favour the high brasses/high Zn content) Localised dezincification – plug type (favour the low brasses/low Zn content)

Prevention Reducing the aggressiveness of the environment (i.e. oxygen removal) Cathodic protection Use less susceptible material: 15% Zn (red brass) Addition 1% Sn to a 70-30 brass (admiralty metal) Addition of As, Sb, P as ‘inhibitor’ (70%Cu, 29% Zn, 1% Sn, 0.04% As) Cupronickel (70-90%Cu, 30-10%Ni) for severely corrosive environments.

EROSION CORROSION Thinning or removal of surface films by erosion from the flowing stream results in accelerated corrosion, called errosion corrosion or impingement attack. The attack is accelerated at elbows, turbines, pumps, tube constrictions, and other structural features that alter flow direction or velocity and increase turbulence (agitator, blowers, propellers, turbine blades, etc.) Corrosive mediums: gases, aqueous solutions, organic systems, and liquid metals. Appears as grooves, gullies, waves, rounded holes, and valleys.

CASES OF IMPINGEMENT

Stress Corrosion Cracking (SCC) Conditions to produce SCC Critical environment A susceptible alloy Some component of tensile stress