Corrosion and Wear.pdf

PROCESS ENGINEERING Corrosion and Wear Lecturer: Mr. Craig NSAKABWEBWE Outline ❑Introduction ❑Material selection ❑Engineering materials ❑Metals and alloys MINING INDUSTRY ❑Types of corrosion on metals BASICS OF TRIBOCORROSION ❑Corrosion control ❑Tribology...

PROCESS ENGINEERING Corrosion and Wear Lecturer: Mr. Craig NSAKABWEBWE Outline ❑Introduction ❑Material selection ❑Engineering materials ❑Metals and alloys MINING INDUSTRY ❑Types of corrosion on metals BASICS OF TRIBOCORROSION ❑Corrosion control ❑Tribology ❑Wear mechanisms ❑Friction process and mechanisms 2 Cont.… ❑Lubrication regimes ❑Tribocorrosion MINING INDUSTRY BASICS OF TRIBOCORROSION 3 Materials selection in mining processes Corroded pipes Corroded impeller Corroded infrastructure 4 Cont.… ❑ Engineers have a key role to play to mitigate corrosion and wear-related MINING INDUSTRY costs in the mining industry. BASICS OF TRIBOCORROSION ❑ Selection of materials is one of the solutions to consider to cope with corrosion and wear-related damages. 5 Engineering materials ❑Engineering materials are referred to as solids materials useful for humankind. ❑A great proportion of these materials are in combined forms as alloys, compounds or mixtures. ❑The application of pure elements is usually limited. 6 Metals and alloys ❑Metals are pure engineering materials bonded by metallic bonds. They are also referred to as metallic elements. ❑Metals have high density, high electrical and thermal conductivity, bright lustre, good ductility , malleability and opacity. ❑Application of pure metals is limited because of their inherent properties. 7 Cont.… ❑Adding one or more element(s) to pure metal can help in improving their properties and consequently their applicability. ❑ The addition of one or more elements to a pure metal is known as alloying and the resulting product is called an alloy. 8 Corrosion 1.Corrosion 2.Corrosion Control 3.Corrosion Definition Inhibitors 9 Corrosion 10 Cont.… ❑Corrosion is defined as a reaction of a material with its environment leading to subsequent deterioration of the inherent properties of the material. ❑ The reaction can be chemical, electrochemical, physical or their combination. ❑The vulnerable material can either be metals, ceramics or polymers. ❑The environments can either be aqueous and non-aqueous liquids and gases. 11 Cont.… To study the corrosion process on a material, three factors from the definition of corrosion are involved: ❑ The material (metal), ❑ The environment and ❑The material/environment interface. 12 Electrochemical occurrence of corrosion ❑Oxidation in an electrochemical cell always takes place at the anode while reduction occurs at the cathode. ❑In the case of steel corroding in hydrochloric acid, the product of the chemical reaction is generally a ferrous chloride with hydrogen release as shows Equations below. ❑Fe + 2HCl → FeCl2 + H2 13 Cont.… ❑Fe + 2H + + 2 Cl− → (Fe 2+ + 2Cl− ) ❑FeCl2 + ( 2H + + 2e/H2 ) ❑ Corrosion of steel generates a similar reaction, but the only difference resides in the reacting species and the corrosion product. ❑The product of the corrosion reaction is generally ferric hydroxide. ❑When the part is taken out the corroding medium, the corrosion product shifts from ferric hydroxide to familiar red rust. 14 Requirement for electrochemical corrosion of metal The following are the key requirements for the electrochemical corrosion of metals: 1. The metal oxidation takes place at the anode of an electrolytic cell; 2. The reduction of some ions occurs at the cathode; 3. A potential is applied between the anode and cathode; 4. An electrolyte is present to facilitate the movement of charges from the anode to the cathode; 15 Cont.… 5. An electrical bridge must be completed (somehow the anode must be coupled to the cathode if any of these conditions are not met, electrochemical corrosion will not occur. 16 Thermodynamic aspects of corrosion ❑Many factors can assist to evaluate the susceptibility for a material to corrode, however material properties and the nature of the environment are probably the most important. ❑The tendency to undergo chemical reactions, as well as the interaction of a metal with its environments, is evaluated by the Gibbs free energy (∆G). ❑The more negative the value of ∆G, the higher the susceptibility for material to corrode. 17 Cont.… 1 ❑Mg + H2 O l + O2 (g) → Mg(OH)2 s ∆G0 = −142,600 cal 2 Equation 2-5 1 ❑Cu + H2 O l + O2 (g) → Cu(OH)2 (s) ∆G0 = −28,600 cal 2 Equation 2-6 3 3 ❑Au + H O l + O2 (g) → Au(OH)3 (s) ∆G0 = +15,700cal 2 2 4 Equation 2-7 18 Cont.… ❑the susceptibility for metal to corrode is not directly related to the corrosion rate. ❑ A more negative Gibbs energy (∆G0 ) may not absolutely be indicative of a high dissolution rate of metal ❑ When ∆G0 is positive, it can be concluded with confidence that the corrosion reaction will never occur. ❑The difference in the Gibbs energy (∆G) between the metal and the product of its corrosion is only indicative of the tendency of the metal to corrode. 19 Kinetics effect of metals corrosion ❑Thermodynamics give insight into the susceptibility of a system to corrode and an equilibrium condition is necessary, and this requires no net flow of current. ❑Corrosion reactions, however, cause current to flow, so to examine them fully, we must also consider the kinetics of corrosion reactions. ❑ The equilibrium between the anodic and cathodic reactions at an electrode is not static and the charged entities are generated and discharged concurrently at the conductive surface. 20 Cont.… ❑Since they are electrically charged species, the ions flow generates two equivalent and different electric currents. ❑ The electric current flowing to and from the metal surface, designated 𝐢→ and 𝐢← respectively, are termed as partial currents. ❑The extent at equilibrium is termed as the exchange current density 𝐢𝐨 and it is represented by Equation 2-8. −∆𝐆∗ ❑ 𝐢→ = 𝐢← = 𝐢𝐨 = 𝐤 𝐞𝐱𝐩 Equation 2-8 𝐑𝐓 21 Cont.… Where: ❑∆G∗ is the activation energy; ❑T is the absolute temperature; ❑R is a constant related to ideal gas; ❑k is a constant depending on the process and on the ion activity. 22 Cont.… ❑Using the denoted 𝐢𝐚 and 𝐢𝐜 current densities of the respective forward (anodic) and backward (cathodic) reaction, the mild steel dissolution reaction can be rewritten as shows in Equation 2-9. →ia ❑Feic ← Fe2+ + 2e− Equation 2-9 ❑Hence, when the steady state is reached, the current density at the anode becomes equal to the current density at the cathode 𝐢𝐚 = 𝐢𝐜. 23 Cont.… ❑There is a current flowing, but it is equal and opposite in both directions and cannot be measured. ❑ As soon as corrosion starts on the metal, the equilibrium situation no longer exists, and the anodic current density becomes bigger than the cathodic current density (𝐢𝐚 > −𝐢𝐜 ). ❑The potential of metal is, therefore, determined by the number of negative charges developed on it when placed in an electrolyte 24 Cont.… ❑ The corrosion rate of a metal is generally associated with the activation energy ❑ Therefore, metal atoms must first overcome this energetic barrier before they may corrode, and more energy must be supplied for this to occur + ❑ ∆G+ represents the free energy of activation 25 Forms of corrosion Based on literature it has been established that corrosion existed in eight 2 forms. The eight types of corrosion are: 1 ❑Uniform corrosion (general corrosion) ❑Galvanic corrosion 8 3 4 ❑Pitting corrosion ❑Crevice corrosion; ❑Selective leaching corrosion (parting corrosion); 26 Cont.… ❑Inter-granular corrosion; 6 ❑ Stress corrosion; ❑Erosion-corrosion. 7 27 Economic cost of corrosion ❑The overall cost of rust was estimated (2016) to be US$2.5 trillion, which is nearly 3.4 out of a hundred compared to the overall gross domestic product (GDP) in developing countries. ❑This significant amount of money can be easily recovered through the implementation of appropriate corrosion prevention protocols which can help saving between 15-35 per cent of the global damage cost or $375-875 billion (USD), 28 Cont.… ❑However, in South Africa, the physical cost of rust was evaluated at around R130 billion in 2013. ❑ According to Mintek and the Corrosion Institute of South Africa (CorrISA), half of each ton of steel that is manufactured is manufactured simply to substitute rusted steel 29 Corrosion Impacts It is extremely expensive It is extremely wasteful financially of natural resources It causes considerable inconvenience to human beings. 30 Corrosion Control 31 Coating and Painting ❑Coatings: Protective coatings are possibly the most broadly utilized products for corrosion prevention. The key purpose of a coating is to protect structural reactive components from the corrosive environment. ❑Painting: The purpose of corrosion protection painting is to protect the metal substrate against atmospheric corrosivity 32 Impressed current cathodic protection ❑An impressive current cathodic protection controls the corrosion of a metal surface by making it the cathode of an electrochemical cell ❑The method of protection connects the metal to be protected to a more easily corroded ‘‘sacrificial metal’’ to act as the anode 33 Cathodic and anodic protection ❑ Cathodic protection: Cathodic protection (CP) is a corrosion controlling technique which consists of making the surface of the metal the cathode of an electrochemical process ▪ By applying a cathodic current on the protected metal surface, the anodic dissolution is minimalized. ❑Anodic protection: The technique involves the formation of a protective layer on metals by applying an external anodic current. 34 Cont.… There are two methods of generating the cathodic current: i) by using connected sacrificial anodes to create a galvanic cell. ii) by applying an external power current source. ❑ Sacrificial anode(galvanization): A sacrificial anode is part of a cathodic protection system and is attached to a steel structure to protect it against corrosion. It is made of a more active, less noble metal (usually zinc or aluminium) than that of the structure itself 35 Corrosion inhibitor ❑The anodic inhibitors block the anodic reactions upon the surface of the investigated material. ❑These cathodic inhibitors block the cathodic reactions by mitigating the flow of reducing entities to the working electrode ❑These mixed-type inhibitors concomitantly retard both the anodic and cathodic corrosion reactions. 36 Corrosion Inhibitors Inorganic Inhibitors Organic Inhibitors -Good corrosion Polymeric Inhibitors inhibitors -Good Corrosion -Cost-sensible Inhibitors -Good corrosion -Toxic -Eco-friendly inhibitors Environmentally- -Cost-prohibitive -Eco-viable hazardous. -Industrially- -Efficient restricted. -Cost-effective. 37 Polymeric Corrosion Inhibitors Economical and practical means of controlling metallic Through their functional corrosion in different groups, they form corrosive media. complexes with metal ions. They possess They are low cost and multiple adsorption stable to metallic sites. materials in acid media. The complexes on the metal surface occupy a large surface, increasing the inhibition coverage. 38 Corrosion evaluation ❑ Gravimetric analysis (Weight loss) 87.6∗W CR = tAd Where CR is the corrosion speed (mm/year), W is the change in mass (g), d is the density of the mild steel, A is the surface of the mild steel substrates (mm2) and t corresponds to the immersion time (h). 39 Cont.… W2 IE = 1 − ∗ 100 W1 Where IE (%) is the inhibition efficiency, W1 (g) change in the mass value of the steel exposed to an uninhibited 1MHCl solution and W2 (g) is the change in the mass value of mild steel exposed to an inhibited 1MHCl solution. CR0×CR θ = Where (𝜃) is the surface coverage; CRO and CR are the CR0 dissolution rates of mild steel exposed to an uninhibited and inhibited solution respectively. 40 Cont.… ❑Electrochemical analysis ❑The electrochemical corrosion rate can be determined using the potentiodynamic polarization(PDP) or Tafel method. ❑The value of either the anodic or the cathodic current density at the intersection is Icorr. The current density represents the corrosion rate of a material exposed to an aggressive environment 41 Cont.… ❑The PDP technique of analysis involves changing the potential of the working electrode and monitoring the resulting current as a function of potential or time. ❑The potential of the electrode is varied at a selected frequency by the application of a current through the electrolyte. ❑ It is the most used polarization testing technique for measuring corrosion resistance and is also used for a broad variety of functions. 42 Tribology ❑ The science and technology of interacting surfaces in relative motion. ❑It focuses on friction, wear and lubrication of contacting surfaces in relative motion. 43 Wear Mechanism ❑Wear is a progressive damage, which takes place on the surfaces of materials because of their motion. ❑ It is an undesired removal of metal particles from the surface of a hardware component that is continually sliding or rolling/rotating by a different surface. ❑In materials science, wear is also referred to as the erosion of material from a solid surface by the action of another solid. 44 Cont.… ❑There are four important wear mechanisms: adhesive wear, abrasive wear, corrosive wear and fatigue wear. However, other types of wear also exist, these are cavitation wear, fretting wear and diffusive wear. 45 Adhesive wear ❑Adhesive wear is referred to as scoring, galling or seizing. ❑ It happens when two solid surfaces slide over one another under pressure. 46 Abrasive wear ❑Abrasive wear: The type of wear mechanism that results in the disintegration of the material on the surface due to the influence of the hard particle in contact with the surface. ❑It takes place when a solid object is loaded against particles of a material that have equal or greater hardness. 47 Cont.… The mechanisms of abrasive wear can be: ❑Micro cutting; ❑Fracture; ❑Ploughing or ❑Grain pull-out: 48 Corrosion wear ❑Corrosive Wear also known as oxidation or chemical wear, ❑This type of wear is caused by chemical and electrochemical reactions between the surface and the environment. ❑The fine corrosive products on the surface constitute the wear particles in corrosive wear. 49 Fatigue wear ❑The wear occurring due to surface fatigue is termed fatigue wear ❑Wear under these conditions is determined by the mechanics of crack initiation, crack growth and fracture 50 Friction process and mechanism ❑Friction is the resistance encountered when one body moves over another when in contact ❑Generally, friction is either dry or lubricated. ❑ Dry friction is a consequence of surface imperfections 51 Theory on metallic friction Friction includes three major factors: ❑ The area of real contact (A) between the surfaces, ❑ The nature of the adhesion or bonding at the areas of true contact, and ❑How the contacts so formed are sheared during sliding. ❑The friction is proportional to the load and independent of the size of the bodies 52 Friction of hard solids ❑The friction of hard solids in the air is usually negligible partly because of lack of ductility and partly because of the presence of surface layers ❑The adhesion at the interface will always be weaker than the cohesion within the solid itself at higher temperatures. 53 Lubrication regimes ❑Lubrication is provided when contacting rough surfaces are isolated by a lubricant film ❑Lubricants minimize friction, reduce wear and tear on motional parts ❑It also ensures lower operating temperature, keeping the machinery in optimal working condition and improving its useful life. 54 Lubrication regimes There are four lubrication regimes which are: ❑ Boundary ❑Mixed ❑Elastohydrodynamic ❑Hydrodynamic 55 Cont.… ❑Electrohydrodynamic lubrication: Is the type of lubrication that occurs where very high contact pressure causes elastic deformations of the contacting surfaces of at least the same order of magnitude as the film thickness separating them. ❑Hydrodynamic lubrication: The hydrodynamic lubrication regime is formed when there is no contact between the surfaces at the sliding interface and there is a gap of separation at the interface due to thick lubricant film formation 56 Cont.… ❑Boundary lubrication occurs when the separation between two surfaces is only partially achieved and there are areas of direct contact between surface asperities (microscopic roughness) ❑In these areas, the lubricant molecules form a thin adsorbed film, or a chemical reaction occurs providing some protection against wear and friction 57 THANK YOU FOR YOUR ATTENTION !! Any Questions? 59