Recommended Books. Course Objectives. Course Objectives. Prerequisites. Forms of Assessment 2/24/2018. MSE 506 Environmental Degradation of Materials

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1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana Course Objectives MSE 506 Environmental Degradation of Materials Ing. Anthony Andrews (PhD) Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering Website: To introduce students to 1. the nature of interactions between engineering materials and environments 2. degradation in properties of engineering materials due to exposure to environment 3. Reduction/prevention of degradation of materials Course Objectives Specific objectives include the following: Explain corrosion phenomena and use that to identify different forms of corrosion Discuss the corrosion of metals, polymers and ceramics Describe common corrosion control methods Discuss other degradation methods such as wear and radiation damage. Prerequisites Knowledge of electrochemistry/physical chemistry Forms of Assessment Recommended Books No specific text book Class Assignment 20 Term paper 20 Final Exam 60 Total 100 M. G. Fontana: Corrosion Engineering, 3rd. Ed., McGraw Hill, 1998 D. A. Jones: Principles and Prevention of Corrosion, Macmillan Publ. Co., 1996 J. M. West: Electrodeposition and Corrosion Control, J. Wiley W. Revie (ed.): Corrosion Handbook, Electrochemical Society Series, John Wiley and Sons,

Course Outline 1. Degradation Economics 2. Forms of Corrosion 3. Electrochemical Concepts of Degradation 4.

$(s) + 2 HA (aq) MA 2 (aq) + H 2 (g) Erosion-corrosion Wear influenced corrosion (chemical-mechanical) Environmentally assisted cracking Brittle fracture below yield strength$ (material-stressenvironment) What is Corrosion?

(material-stressenvironment) What is Corrosion?

2 Course Outline 1. Degradation Economics 2. Forms of Corrosion 3. Electrochemical Concepts of Degradation 4. Corrosion Control Methods Types of Environmental Degradation Environmental degradation can be classified into three main categories: 1. Corrosion and oxidation 2. Wear 3. Radiation damage 8 Sublimation M (s) M (g) Reactivity Types Dissolution MA (s) M + (aq) + A - (aq) Oxidation (Dry corrosion) xm (s) + ½yO 2 (g) M x O y (s) Aqueous (Wet) corrosion M (s) + 2 HA (aq) MA 2 (aq) + H 2 (g) Erosion-corrosion Wear influenced corrosion (chemical-mechanical) Environmentally assisted cracking Brittle fracture below yield strength (material-stressenvironment) What is Corrosion? Chemical or electrochemical reaction of a material (usually a metal) with its environment or Extractive metallurgy in reverse Leads to deterioration of the material and its properties and thus its performance Ceramics and engineering polymers also undergo deterioration and hence corrode Examples of degradation of materials include rusting of steel, deterioration of paint, etc The Corrosion Cycle of Steel 2

3 Degrading Environments All environments are corrosive to some degree Oxidation Vacuum Atmosphere (rural, marine, industrial) Chemical Gases (H, O, C, S, Cl, N, etc.) Aqueous corrosion Atmosphere (rural, marine, industrial) Soil (pipelines, storage tanks) Fresh Water (potable Sea Water Chemical (acids, bases, solvents) Biological (biofilms, human body) Atmospheric Corrosion Corrosion of materials exposed to air and its pollutants, rather than immersed in liquid Classification: Dry very low humidity Dump created at certain critical humidity level (adsorption of water molecule) Wet associated with dew, ocean spray, rainwater and other forms of water splashing 14 Types of atmospheric corrosion 1. Rural Least corrosive Does not contain chemical pollutants Contain organic and inorganic particulates Principal corrodents are moisture, oxygen and carbon dioxide 2. Urban Similar to rural type with little industrial activity Contaminants include SO x and NO x variety from motor vehicles and domestic fuel emissions Types of atmospheric corrosion 3. Industrial Associated with heavy industrial processing facilities Contain concentration of SO 2, chlorides, phosphates, nitrates 4. Marine Windswept chloride particles gets deposited on surfaces Highly corrosive and strongly depends on wind direction, wind speed, and distance from coast Equivalent environment is created by use of deicing salt on the road Theory of atmospheric corrosion Condition: Presence of electrolyte Proceeds by balancing anodic and cathodic reactions Anodic reaction dissolution of the metal For Fe: 2Fe = 2Fe e Cathodic reaction oxygen reduction reaction O 2 + 2H 2 O + 4e = 4OH - Practical variables in atmospheric corrosion 1. Time of wetness (TOW) Directly determines the duration of the electrochemical corrosion processes. Depends on relative humidity 2. Sulphur dioxide Produce from combustion of sulphur containing fossil fuels It has high solubility in water and forms H 2 SO 4 3. Chlorides Enhance surface electrolyte formation by hygroscopic salts such as NaCl and MgCl 2 Increases atmospheric corrosion rates

Practical variables in atmospheric corrosion 4.

Temperature Effect of temperature on atmospheric corrosion rate is complex Corrosion rate can increase or decrease Classification of

Electrochemical Appearance of corrosion Uniform versus Localised 19 Degradation of some common materials Wood The environmental factors

susceptible to attack when the moisture content exceeds 20% Degradation of some common materials Physical and mechanical effects of

content increases Durability is affected Coatings can be compromised Degradation of some common materials Plastics UV light will weaken

4 Practical variables in atmospheric corrosion 4. Other atmospheric contaminants Hydrogen sulphide, hydrogen chloride, and chloride present in the atmosphere intensifies corrosion rate 5. Temperature Effect of temperature on atmospheric corrosion rate is complex Corrosion rate can increase or decrease Classification of Corrosion Three major classification system: Nature of corrodant Wet corrosion versus Dry corrosion Mechanism of corrosion Chemical versus Electrochemical Appearance of corrosion Uniform versus Localised 19 Degradation of some common materials Wood The environmental factors that affect degradation in wood include: Biological organisms fungi and insects Risk of wetting or permanent contact with water Wood is susceptible to attack when the moisture content exceeds 20% Degradation of some common materials Physical and mechanical effects of degradation in wood include: Change in cross-sectional dimensions, swelling and shrinkage Strength and stiffness decrease as moisture content increases Durability is affected Coatings can be compromised Degradation of some common materials Plastics UV light will weaken certain plastics and produce a chalky faded appearance on the exposed surface Degradation of some common materials Metals Most metals corrode because they react with oxygen in the atmosphere, particularly under moist conditions this is called oxidation 4

Degradation of some common materials Metals Ferrous metals are susceptible to oxidation and require ongoing maintenance or they will suffer inevitable structural failure Corrosion resistant metals

downtime Loss of product due to leakage Loss of efficiency due to corrosion Overdesign Loss of human lives due to explosion, fire, etc 27 Effects of Corrosion Economics - Direct and Indirect: Direct:

leakage Loss of efficiency due to corrosion Overdesign 28 Effects of Corrosion Social: Safety: Bridges, aircraft, submarines Health Depletion of natural resources.

5 Degradation of some common materials Metals Ferrous metals are susceptible to oxidation and require ongoing maintenance or they will suffer inevitable structural failure Corrosion resistant metals Metals Some non-ferrous metals are particularly resistant to corrosion Choice of metal, environmental location and design features must all be considered carefully Copper Cladding Zinc Cladding Co$t of Corrosion Direct cost: 4.2% GNP Replacement of damaged/corroded equipment Use of corrosion resistant alloys Use of coatings and inhibitors Electrochemical protection measures Indirect cost Loss of production during plant downtime Loss of product due to leakage Loss of efficiency due to corrosion Overdesign Loss of human lives due to explosion, fire, etc 27 Effects of Corrosion Economics - Direct and Indirect: Direct: Replacement of damaged equipment Redundant equipments Maintenance and repair Preventive maintenance (corrosion control) Indirect: Loss of production during plant downtime Loss of product due to leakage Loss of efficiency due to corrosion Overdesign 28 Effects of Corrosion Social: Safety: Bridges, aircraft, submarines Health Depletion of natural resources. Corrosion is metallurgy in reverse Aesthetics ( Eye Sore ) Empire destruction: Postulated that storage of wine in lead-lined vessels led to lead poisoning of the Romans Effect of corrosion on mechanical and physical properties of metals Reduction of metal thickness leading to loss of strength or complete structural failure Localised corrosion leading to crack like structure. Produces a disproportionate weakening in comparison to the amount of metal lost Fatalities and injuries from structural failure, e.g. bridges, buildings, or aircraft Damage to valves or pumps due to solid corrosion products 29 5

Environmental Considerations Contamination of fluids/foodstuffs in pipes and containers Corrosion in

production/design and ongoing maintenance costs.

environment 32 Corrosion in Action Corrosion in Action 33 2/24/2018 34 Forms of Corrosion Uniform

uniform over the entire exposed surface Metal becomes thinner and eventually fails Corrosion environment

6 Environmental Considerations Contamination of fluids/foodstuffs in pipes and containers Corrosion in Action Leakage of potentially harmful pollutants and toxins into the environment Increased production/design and ongoing maintenance costs. This results in greater use of scarce resources and the release of harmful CO 2 gasses into the environment 32 Corrosion in Action Corrosion in Action 33 2/24/ Forms of Corrosion Uniform Corrosion Responsible for the greatest destruction of structural materials (30% of failures) Relatively uniform over the entire exposed surface Metal becomes thinner and eventually fails Corrosion environment has same access to all parts of the surface Corrosion rate can be predicted and therefore not a major concern Acceptable for design Jones, Principles and prevention of corrosion,

Aqueous Uniform Corrosion Aqueous Uniform Corrosion White liquor storage tank Material: Carbon steel 100g/L NaOH + 30g/L Na 2 S 93 o C 25 years Continuous digester Material: Carbon steel field test

!!! Atmospheric Uniform Corrosion Black liquor evaporator Material: Carbon steel Coastal rain (NaCl + H 2 O) 35 years Prevention/Control Slow down or stop the movement of electrons by: Coating

surface Use corrosion resistant metal Use metal that forms protective oxides 40 Galvanic (Bimetallic) Corrosion Takes place when two metals are in physical contact with each other and are immersed in

7 Aqueous Uniform Corrosion Aqueous Uniform Corrosion White liquor storage tank Material: Carbon steel 100g/L NaOH + 30g/L Na 2 S 93 o C 25 years Continuous digester Material: Carbon steel field test coupon Acid cleaning (non-process) Uninhibited 3 wt.% HCl o C 4 hours!!!! Atmospheric Uniform Corrosion Black liquor evaporator Material: Carbon steel Coastal rain (NaCl + H 2 O) 35 years Prevention/Control Slow down or stop the movement of electrons by: Coating surface with non-conducting medium such as paint, lacquer, or oil Reducing the conductivity of solution in contact with the metal Using cathodic protection Slow down or stop oxygen from reaching the surface Use corrosion resistant metal Use metal that forms protective oxides 40 Galvanic (Bimetallic) Corrosion Takes place when two metals are in physical contact with each other and are immersed in a conducting fluid. Corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. Examples: Plate and screw of different electrical potentials due to differences in processing Multiple component implant using different metals for each component Copper and steel tubing are joined in a domestic water heater, the steel will corrode in the vicinity of the junction Galvanic (Bimetallic) Corrosion The following fundamental requirements have to be met for galvanic corrosion: Dissimilar metals (or other conductors, such a graphite). Electrical contact between the dissimilar conducting materials (can be direct contact or a secondary connection such as a common grounding path). Electrolyte (the corrosive medium) in contact with the dissimilar conducting materials

Galvanic Corrosion Heat exchanger Materials: Carbon steel baffle Stainless steel tube Cooling water in shell (45-47 o C, 85ppm Cl - ) Product in tubes 10

Corrosion Environmental effect: metal with lesser resistance to the given environment becomes the anodic member of the couple Distance effect: greatest near

of anodic material increases. Large anodes coupled with small cathodes may not give problems even with unfavourable pairs of materials.

Galvanic corrosion between stainless steel screw and Aluminium. Fig. Galvanic corrosion between Steel and Brass.

8 Galvanic Corrosion Heat exchanger Materials: Carbon steel baffle Stainless steel tube Cooling water in shell (45-47 o C, 85ppm Cl - ) Product in tubes 10 years Galvanic Corrosion Door frame Materials: Aluminium frame Copper hinge Industrial atmosphere 10 years Galvanic Series in Seawater (courtesy Inter. Nickel Ltd.) This gives no indication of the corrosion rate Useful for predicting galvanic relationships Series depends on environment 45 Factors Affecting Galvanic Corrosion Environmental effect: metal with lesser resistance to the given environment becomes the anodic member of the couple Distance effect: greatest near the junction, with attack decreasing with increasing distance from that point Area effect: large cathodic area coupled to small anodic area rate of attack of anodic material increases. Large anodes coupled with small cathodes may not give problems even with unfavourable pairs of materials. 46 Galvanic Corrosion Area effect Cu rivets on a steel bar and steel rivets on Cu bar submerged in 3% NaCl solution Start of exp. After 6 months Fig. Galvanic corrosion between stainless steel screw and Aluminium. Fig. Galvanic corrosion between Steel and Brass. After 10 months Steel plates with Cu rivets Tolerable corrosion of steel plates Small cathode/large anode Cu plates with steel rivets Steel rivets severely attacked Large cathode/small anode 47 Fig. Anodic- cathodic behavior of steel with zinc and tin outside layers exposed to the Atmosphere. (a) zinc is anodic to steel and corrodes (b) steel is anodic to tin and corrodes 48 8

Prevention/Control Beneficial Applications Ensure one or more of the features do not exist Break the electrical contact

g. galvanized (zinc coated) steel Zinc corrodes preferentially and protects the steel Zinc acts as a sacrificial anode

soda (NaHCO 3 ) The current generated by the contact between Ag and Al causes silver sulphide to be reduced back to

usually associated with a stagnant solution on the micro-environmental level.

material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.

inhibitor in the crevice Depletion of oxygen in the crevice A shift to acid conditions in the crevice Build-up of

9 Prevention/Control Beneficial Applications Ensure one or more of the features do not exist Break the electrical contact Select metals close together in the galvanic series Prevent ion movements by coating junction 49 Cathodic protection: E.g. galvanized (zinc coated) steel Zinc corrodes preferentially and protects the steel Zinc acts as a sacrificial anode Cleaning silverware: Stains due to silver sulphide Clean by placing the silver in an Al pan containing water and baking soda (NaHCO 3 ) The current generated by the contact between Ag and Al causes silver sulphide to be reduced back to silver Rinse silver and wash with soapy water 50 Crevice Corrosion Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps. Well-known examples of such geometries including flanges, gaskets, disbonded linings/coatings, fasteners, lap joints and surface deposits. Crevice Corrosion Crevice corrosion is initiated by changes in local chemistry within the crevice: Depletion of inhibitor in the crevice Depletion of oxygen in the crevice A shift to acid conditions in the crevice Build-up of aggressive ion species (e.g. chloride) in the crevice Crevice Corrosion Cooling water condenser sieve Material: 316L stainless steel Flowing 40 o C 2 years Zebra mussels- an example of marine environment Underside of panel where severe corrosion was found Close-up picture showing the severity of corrosion 54 9

Prevention/Control Avoiding sharp corners and design out stagnant areas Use of sealants Use welds instead of bolts or rivets Select resistant materials Pitting Corrosion Pitting corrosion is a

Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films. Localized pitting attack is found where these passive films have broken down.

causing it to lose a significant amount of weight.

10 Prevention/Control Avoiding sharp corners and design out stagnant areas Use of sealants Use welds instead of bolts or rivets Select resistant materials Pitting Corrosion Pitting corrosion is a localized form of corrosive attack that produces holes or small pits in a metal. the bulk of the surface remains unattacked. Pitting is often found in situations where resistance against general corrosion is conferred by passive surface films. Localized pitting attack is found where these passive films have broken down. Pitting can be one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent, relatively difficult to detect, occurs very rapidly, and penetrates a metal without causing it to lose a significant amount of weight Pitting Corrosion PM whitewater pipe Material: 316L stainless steel Biofilm Cl - - and S 2 O 3 rich 50 o C Not available Control Environment Reduce chloride content Lower temperature Remove oxidants Use inhibitors Lower potential Control ph Control flow velocity of fluids Control of Pitting Selection of Metals Increase alloying content (e.g. Mo, Cr, N) Use protective coatings. Maintain the material s own protective film. Filiform Corrosion Filiform Corrosion Occurs under painted or plated surfaces when moisture permeates the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Starts at small, sometimes microscopic defects in the coating. Filiform corrosion causing bleed-through on a welded tank. "worm-like" filiform corrosion tunnels forming under a coating at the Atmospheric Test Site. 10

Control of Filiform Corrosion Careful surface preparation prior to coating. Use coatings that are resistant to this type of corrosion. E.g. zinc-rich coatings to coat carbon steel.

11 Control of Filiform Corrosion Careful surface preparation prior to coating. Use coatings that are resistant to this type of corrosion. E.g. zinc-rich coatings to coat carbon steel. Careful inspection of coatings to ensure that holidays or holes in the coatings are minimized. Microbiologically Influenced Corrosion (MIC) Deterioration of an alloy by conventional corrosion processes which are stimulated/modified as a result of activity of living organisms. Micro-organisms can produce slimes and deposits which give rise to crevice corrosion. Create corrosive conditions through their metabolic products, or by destroying materials added to the system to provide corrosion inhibition. Microbial Corrosion Bacteria in untreated river water cause deposits in a lowflow cooling water line. Control of MIC Chemical/biocide dosing of water systems to destroy the bacteria Protective organic coating or wrapping Use resistant materials (highly alloyed stainless steel, titanium, plastics) Control the oxygen content Cathodic protection Thermogalvanic Corrosion Temperature changes can alter the corrosion rate of a material. 10 o C rise doubles the corrosion rate. If one part of component is hotter than another, the difference in the corrosion rate is emphasized by the thermal gradient. Concrete Corrosion Concrete is a widely-used structural material that is frequently reinforced with carbon steel reinforcing rods. The steel is necessary to maintain the strength of the structure, but it is subject to corrosion. The cracking associated with corrosion in concrete is a major concern in areas with marine environments and in areas which use deicing salts. Local attack occurs in the zone between the maximum and minimum temperatures. 11

12 Concrete Corrosion Two theories on how corrosion in concrete occurs: Salts and other chemicals enter the concrete and cause corrosion. Corrosion of the metal leads to expansive forces that cause cracking of the concrete structure. Concrete Corrosion Cracking and staining of seawall Cracks in the concrete allow moisture and salts to reach the metal surface and cause corrosion. Control of Concrete Corrosion Embedding the steel deep enough so that chemicals from the surface don't reach the steel (adequate depth of cover). Keep the water/cement ratio below 0.4. For already corroded concrete, remedial action can include repairing the cracked and spalled concrete, coating the surface to prevent further entry of corrosive chemicals into the structure, and cathodic protection. Factors Affecting Corrosion Rate Effect of temperature: Corrosion rate increase with increase in temperature Reaction rate doubles with 20 to 70 o C temperature rise Effect of velocity: High velocity tends to remove the protective oxide layer Effect of oxygen and oxidizers: Oxygen/oxidizers in water increases the corrosion rate This is due to the rapid reaction between oxygen and the polarizing layer of atomic hydrogen absorbed on the oxide layer 70 Factors Affecting Corrosion Rate Effect of corrosive concentration: Materials which exhibit passive effects have slight influence on corrosive concentration others is the reverse Effect of galvanic coupling: Zn coupled with Pt in HCl. Pt is noble and tends to increase the surface area on which hydrogen evolution can occur 71 12

Prevention Strategies Design and Coatings

Lecture 14 Prevention Strategies Design and Coatings Keywords: Corrosion Prevention, Designs, Protective Coatings. There are a number of methods to control corrosion. The choice of any one control technique