Corrosion & Degradation Lecture 6 PDF

Corrosion & Degradation Corrosion and Degradation Corrosion Introduction Chemistry of corrosion Classification of corrosion Relevance of corrosion to orthopaedic implants Corrosion Classification of biomaterials for implants reliant on three primary features Biocompatibility of impla...

Corrosion & Degradation Corrosion and Degradation Corrosion Introduction Chemistry of corrosion Classification of corrosion Relevance of corrosion to orthopaedic implants Corrosion Classification of biomaterials for implants reliant on three primary features Biocompatibility of implant Mechanical, chemical and tribological properties of biomaterial Patient profile How healthy patient is Surgical technique Corrosion Defined as a chemical attack on a solid material Occurs primarily in metals Usually a slow process Corrosion Why do materials corrode in the body? Corrosion Why do materials corrode in the body? Hostile electrolyte environment Natural elements H20, O, H, and C etc Additional elements Bone minerals (Ca, P, Mg) Extracellular fluid : Na Cl and K Corrosion Why do materials corrode in the body? Biological molecules upset the equilibrium of corrosion reactions of implant by consuming the products due to anodic and cathodic reactions Rate of corrosion is dependent on: 1. Anode potential 2. Cathode potential 3. Presence of an intermediary resistance material 4. Area of electrode surfaces 5. Temperature Rate of Corrosion Increased rate of release: Faster degradation of bulk metal Greater build-up in tissues More severe tissue reactions Rate of Corrosion Increased rate of release: Determined from the ratio of the net current flow to the surface area of the corroding region (current density) Note: Corroding region may be either an entire implant or a portion of that implant, depending on the type of reactions Standards for testing corrosion resistance of biomaterials ASTM Std. Specifications ASTM G61-86 Corrosion performance of metallic biomaterials ASTM G5-94 ASTEM G71-81 Galvanic corrosion in electrolytes ASTM F746-87 Pitting or crevice corrosion of metallic surgical implant materials ASTM F2129-01 Cyclic potentiodynamic polarisation measurements Corrosion and Degradation Corrosion Introduction Chemistry of corrosion Classification of corrosion Relevance of corrosion to orthopedic implants Chemistry of Corrosion Corrosion occurs when a metal atom: Becomes ionised and goes into the solution Combines with C2 in the solution Forms a compound that flakes off or dissolves Chemistry of Corrosion All corrosion reactions are based on differences in electrical potentials and the establishment of an electrical circuit A potential difference between the anode and the cathode cause current to flow Chemistry of Corrosion V Cathode Anode + - Electrolyte Electrochemical Cell Electrolyte: contains ions Ions complete electrical circuit Anions: Negative ions, migrate towards anode (+ electrode) Cations: positive ions that migrate towards cathode (- electrode) Process of corrosion (anode reactions) 1. Ionisation 2. Oxidation 3. Hydroxylation 4. Reactions 1. Ionisation Direct formation of metallic cations Generally under acidic oxygen poor conditions M M +n +ne− For Iron this would be Fe+2 +2e− 2. Oxidation Gaseous or dissolved Without participation of water 𝑀 + 𝑂2 M𝑂2 +4𝑒 − 3. Hydroxylation Reaction of metal with water Basic or Oxygen rich condition Can lead to formation of passivating film 2𝑀 + 𝑂2 (diss) + 2𝐻2 𝑂 2M(𝑂𝐻)2 +4𝑒 − 4. Reaction: Combination of metal or metallic ions with other cations or anions 𝑀𝑂22− + HCl 𝑀𝑂𝐶𝑙 −+𝑂𝐻− Note: All anodic corrosion process Reduces the amount of bulk metal present Produces metal bearing ions or compounds Cathodic reactions Electroplating 𝑀+ + 𝑒 − M Hydrogen evolution 2𝐻+ + 2𝑒 → 𝐻2 𝐺𝐴𝑆 Water formation 𝑂2 + 4𝐻+ + 4𝑒 − → 2𝐻2 𝑂 Hydroxyl formation 𝑂2 + 2𝐻2 𝑂 + 4𝑒 − → 4𝑂𝐻− Note wont remove any material  Anodic and cathodic reactions are always coupled Electrons released from anodic reactions must react again Do not freely circulate as electrons The metal ion either dissolves in the electrolyte solution or reacts with non-metallic ions Equilibrium in Corrosion No net corrosion will occur if: Potential difference between anode and cathode is zero Concentration of metal ions is such that no more will dissolve Then… Equilibrium in Corrosion Metal ions created by ionisation will not be removed from the bulk material Free electrons will react with metal ions in the reserve reaction Allowing equilibrium to be maintained Electrochemical Series Lists common metals in order from most noble (cathodic) to most anodic Based on comparison of their ionic reaction (half- cell) potential with hydrogen The more noble a metal the less susceptible to corrosion it is in a given solution Electrochemical Series Series also allows determination of relative anodic or cathodic behaviour between materials The more anodic material will act as an anode in a two material cell Nernst potentials Chemistry of Corrosion Corrosion can be halted by: Break in the electrical connection between anode and cathode Depletion of cathodic reactions Build up of anodic products to saturation Chemistry of Corrosion Areas of Oxygen depletion typically become anodic The presence of Oxygen accelerated corrosion O2 acts in a cathodic reaction Anode reaction occurs in O2 deficient region Examples: -In a crack -In an area under surface contamination -In a screw hole thread Chemistry of Corrosion Anodic regions must provide electrons to entire cathodic region If the anode is small compared to the cathode accelerated corrosion can occur Note: localisation corrosion can have a greater effect on mechanical properties of a material than a measure of the amount of metal lost would indicate Chemistry of Corrosion Corrosion process is generally dependent on: Surrounding Ph Local electrical potential Multiple different reactions are possible between metal and water in the presence of oxygen depending on the surrounding conditions General occurrence of reactions can be determined from a pourbaix diagram Pourbaix diagrams Pourbaix diagrams allow to determine the corrosion behaviour of a metal in water solutions i.e. the direction of electro-chemical processes and the equilibrium state of the metal at a certain electrode potential in a water solution at a certain value of PH. Pourbaix Diagrams Pourbaix Diagrams Oxygen Line (top line) Upper limit of stability of water Associated with oxygen rich solution or electrolyte near oxidising materials Above this line oxygen is evolved according to: 2𝐻2 𝑂 + 2𝑒 − → 𝑂2 + 4𝐻+ 4𝑒 − Pourbaix Diagrams Hydrogen line (lower line) Lower limit of stability of water Hydrogen is evolved according to: 2𝐻3 𝑂 + 2𝑒 − → 𝐻2 𝑂 + 2𝐻2 𝑂 Pourbaix Diagrams Regions are based on stability of various species: Immunity Passivation Corrosion Pourbaix Diagrams 1. immunity (M stable) Corrosion process dominated by ionisation Low equilibrium concentration of metal ions (106M Continued degradation to metal region to be avoided Pourbaix Diagrams Note Pourbaix diagrams change with the presence of a chloride solution instead of pure water Pourbaix Diagrams Limitations: 1. Local micro-conditions may be different than averages typically used in interpreting the diagram - Corrosion can be a very localized condition 2. Pourbaix diagrams gibe the equilibrium state of a system - Prolonged non-equilibrium may result in an adverse material/host reaction Pourbaix Diagrams 3. Pretreated metals will behave differently in solution - Example may be relatively stable in corrosion region due to slow dissolution of oxide layer (metastable) 4. Diagrams are available mostly for pure metals in pure water - Not determined for alloys in chloride solutions - However stainless steel and cobalt alloys depend on formation of chromium oxide layers for corrosion resistance Pourbaix Diagrams 5. The presence of active cell products in the solution can modify both the rate of the reaction and the products 6. Physiological variations will change the region of reaction on the diagram and possibly alter the equilibrium process Corrosion and Degradation Corrosion Introduction Chemistry of corrosion Classification of corrosion Relevance of corrosion to orthopedic implants  Classification/Types of corrosion 1. Uniform attack 2. Galvanic 3. Crevice 4. Pitting 5. Inter-granular Uniform attack Occurs when the whole surface of the metal is exposed to cathodic reactions during localized corrosion Results in overall corrosion of the bulk material or oxide layer Will occur for all metals in absence of an equilibrium concentration of metal ions Uniform attack This effect is generally not evident until significant loss of metal occurs -Due to uniform nature of reaction Most implants are relatively highly resistant to uniform attack Galvanic Corrosion Occurs when two metals: -In physical contact with each other -Immersed in a conduction solution Both criteria required to produce necessary chemical circuit Galvanic Corrosion Examples : Plate and screw of different electrical potetials due to differences in processing Multiple component implant using different metals for each component Galvanic Corrosion The less noble metal becomes the anode - Undergoes uniform attack - May not corrode if in passivation region The more noble metal becomes the cathode - Gains released ions - Cannot corrode - cathode protection Galvanic Corrosion Amount of corrosion is mainly dependent on: Size of electronic contact (metal to metal) Size of ionic contact (metal to metal) Specific metal pair Crevice Corrosion Occurs in the presence of a narrow deep crack Examples: Interface between metal pieces Incomplete fatigue crack Characterised by oxygen depletion at the crack Reaction : ionisation Crevice Corrosion Crevice face becomes anodic Crevice mouth of crack becomes cathodic Favoured by non-flow conditions in the Very common in multi component devices is easily seen in contact area between implant parts Pitting Corrosion Special case of crevice corrosion Pitting attacks in the form of spots or pits on the surface Initiated by scratches or handling damage (instead of deep cracks) Pitting Corrosion Basic pitting process: four distinct phases Special case of crevice corrosion Pitting attacks in the form of spots or pits on the surface Initiated by scratches or handling damage (instead of deep cracks) Basic pitting process Stage 0 Represents on un-attached metallic surface which is completely covered with the passive film Stage 1: Formation of multiple pit nucleation's (stable) Prompts development of a minor bare area, without becoming passive film surface on the metal Basic pitting process Stage 2: Conditions for pit growth are met and re- passivation cannot occur anymore Stage 3: Accumulation of damage via stable pit development (prompts re-passivation of metastable pits) Pitting Corrosion Pitting cause stress concentrations - may be come location for crack development Evidenced by changes in the surface appearance - Frosting of surface or matte appearance Severe pitting can result in the accumulation of coloured corrosion products within the pits Pitting Corrosion Implants typically are polished before use to reduce presence of surface scratches Inter-granular Corrosion Also known as inter-crystalline corrosion or inter- dendritic corrosion More common in cast or welded materials and alloys: During solidification grain boundaries can become location of impurities oxides ferrites and carbides Results in a different chemistry at grain boundary compared to surround metal If grain boundary composition is anodic wrt bulk material a galvanic potential is established and corrosion will occur Inter-granular Corrosion Boundaries can rapidly corrode due to the small volume of anodic material present in comparison to the cathodic bulk material Likewise if the grain boundary is cathodic – little effect Once corrosion at grain boundary has begun crevice corrosion can than occur Inter-granular Corrosion The grain boundaries become etched into surface of material Bulk material can crumble under stress if the effect is widespread Inter-granular corrosion increases with number of impurities or inclusions Can be minimised with proper heat treatment to restore an even composition between the grains and the grain boundaries Leaching Corrosion Occurs when components of an alloy are weakly bound and differ in chemical reactivity May be a large difference in loss rate of the various components by uniform attack Leaching Corrosion Induced by: 1.The presence in solution of an agent that attacks a component of the alloy preferentially 2. A multiphasic alloy where the different phases have different susceptibility to corrosion Leaching Corrosion Multiphasic alloys generally not used in implants due to this effect Exception CO-Ni-Cr-Mo alloy The appearance is similar to either pitting or intergranular corrosion – surface changes Rates of Corrosion Rates of corrosion dependent on presence of synergistic factors Example: Corrosion fatigue - Repetitive deformation of metal in a corrosive environment results in acceleration of both the corrosive and fatigue micro damage Note: Ringers solution Rate of Corrosion Fretting Corrosion: Rubbing one part on another - Disturbs the passivation layer - Results in accelerated corrosion Rate of Corrosion Pitting Corrosion: Corrosion ate is accelerated in a local region Localised corrosion can occur if there is inhomogeneity in metal environment - Grain boundaries in metal may be susceptible to the initiation of corrosion Rate of Corrosion Crevice Corrosion: Crevice also vulnerable to corrosion Chemical environment in crevice is different to surrounding medium Rate of corrosion dependent on composition of chemical environment Example: area of contact between screw and bone plate Corrosion and Degradation Corrosion Introduction Chemistry of corrosion Classification of corrosion Relevance of corrosion to orthopedic implants Relevance of corrosion to orthopedic implants Primary reason of malfunction of orthopedic implants is wear Subsequently accelerates corrosion High wear properties: - CoCr alloys - Ceramics Relevance of corrosion to orthopedic implants Wear is a predictable problem an any joint implant Low wear resistance can result in debris - Biologically active Friction can also discharge non-compatible metal ions Mechanical loading accelerates wear process Relevance of corrosion to orthopedic implants Noble metals are immune to corrosion - Gold : widely used in dentistry, highly corrosion resistant, however poor strength, high cost limits its application in orthopedics Materials: CoCr alloys like Ti passive in the human body widely used in orthopedic applications Do not exhibit pitting Relevance of corrosion to orthopedic implants Titanium : Forms a robust passivating layer Remains passive under physiological conditions Superior corrosion resistance Not as stiff or strong as steel Relevance of corrosion to orthopedic implants Stainless steel: Contains enough Cr to convene corrosion resistance (by passivity) Passive layer is not as robust as in the case of Ti or CoCr alloys Only the most corrosion resistant stainless steels are suitable for implants SS implant metals 316, 316L, 317 (contains Mo) Even these types are somewhat vulnerable to pitting and crevice corrosion around screw etc. Surface modifications techniques to reduce corrosion Chemical treatment Plasma ion implantation Laser melting Laser alloy Laser nitration Physical vapour deposition Surface texturing How to minimise corrosion in orthopedics 1. Use appropriate metals 2. Avoid implantation of different types of metal in same region 3. Manufacturing process: provide matched parts from same batch of alloy 4. Design the implant to minimise pits and crevices 5. Clinical setting: avoid transfer of metal from tools to implant tissue - Avoid contact between metal tools and implant 6. Recognise that a metal that resits corrosion in one part of the body environment may corrode in a another part of the body