CORROSION AND DEGRADATION

1. CORROSION AND DEGRADATION ISSUES TO ADDRESS... • Why does corrosion occur? • What metals are most likely to corrode? • How do temperature and environment affect corrosion rate? • How do we suppress corrosion? 1

2. THE COST OF CORROSION • Corrosion: --the destructive electrochemical attack of a material. --Al Capone's ship, Sapona, off the coast of Bimini. • Cost: --4 to 5% of the Gross National Product (GNP)* --this amounts to just over $400 billion/yr** 2

3. CORROSION OF ZINC IN ACID • Two reactions are necessary: -- oxidation reaction: -- reduction reaction: • Other reduction reactions: -- in an acid solution -- in a neutral or base solution 3

4. STANDARD HYDROGEN (EMF) TEST • Two outcomes: --Metal sample mass --Metal sample mass --Metal is the anode (-) --Metal is the cathode (+) (relative to Pt) (relative to Pt) Standard Electrode Potential 4

5. STANDARD EMF SERIES • EMF series • Metal with smaller V corrodes. • Ex: Cd-Ni cell o V metal o metal metal +1.420 V +0.340 - 0.126 - 0.136 - 0.250 - 0.277 - 0.403 - 0.440 - 0.744 - 0.763 - 1.662 - 2.262 - 2.714 - 2.924 Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K o DV = 0.153V 5

6. CORROSION IN A GRAPEFRUIT 6

7. EFFECT OF SOLUTION CONCENTRATION • Ex: Cd-Ni cell with standard 1M solutions • Ex: Cd-Ni cell with non-standard solutions n = #e- per unit oxid/red reaction (=2 here) F = Faraday's constant =96,500 C/mol. • Reduce VNi - VCd by --increasing X --decreasing Y 7

8. GALVANIC SERIES • Ranks the reactivity of metals/alloys in seawater Platinum Gold Graphite Titanium Silver 316 Stainless Steel Nickel (passive) Copper Nickel (active) Tin Lead 316 Stainless Steel Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium 8

9. FORMS OF CORROSION • Stress corrosion Stress & corrosion work together at crack tips. • Uniform Attack Oxidation & reduction occur uniformly over surface. • Erosion-corrosion Break down of passivating layer by erosion (pipe elbows). • Selective Leaching Preferred corrosion of one element/constituent (e.g., Zn from brass (Cu-Zn)). • Pitting Downward propagation of small pits & holes. • Intergranular Corrosion along grain boundaries, often where special phases exist. • Galvanic Dissimilar metals are physically joined. The more anodic one corrodes.(see Table 17.2) Zn & Mg very anodic. • Crevice Between two pieces of the same metal. 9

10. DETERIORATIVE • Stress & Saltwater... --causes cracks! • Heat treatment: slows crack speed in salt water! --material: 7150-T651 Al "alloy" (Zn,Cu,Mg,Zr) 4mm 10

11. Uniform Corrosion: Rust! Prevention: • Paint • Plate • Sacrificial anode

12. Galvanic Corrosion Causes: Dissimilar metalsElectrolyteCurrent Path Described by Galvanic Series Solutions: Choose metals close in galvanic series Have large anode/cathode ratios Insulate dissimilar metals Use “Cathodic protection”

13. Pitting and Creviced Corrosion Causes: concentration gradients in electrolyte cause some areas high in ion concentrations that accelerate oxidation Prevention: Weld – don’t rivet Use non-absorbing gaskets Polish surfaces Add drains – avoid stagnant water Adjust composition; e.g., add Mo to SS

14. Intergranular Corrosion Occurs in specific alloys – precipitation of corrosive specimens along grain boundaries and in particular environments e.g. : Chromium carbide forming in SS, leaving adjacent areas depleted in Cr Solutions: High temp heat treat to redissolve carbides Lower carbon content (in SS) to minimize carbide formation Alloy with a material that has stronger carbide formation (e.g., Ti or Nb)

15. Erosion Corrosion Causes: abrasive fluids impinging on surfaces Commonly found in piping, propellers, turbine blades, valves and pumps • Solutions: • Change design to minimize or eliminate fluid turbulence and impingement effects. • Use other materials that resist erosion • Remove particulates from fluids

16. Selective Leaching • Occurs in alloys in which one element is preferentially removed – e.g., in Brass, Zinc is electrically active and is removed, leaving behind porous Copper • Occurs in other metals, such as Al, Fe, Co, Cr Solutions: • Use protective coating to protect surfaces • Use alternative materials

17. Stress Corrosion Aka: stress corrosion cracking Cracks grow along grain boundaries as a result of residual or applied stress or trapped gas or solid corrosion products e.g., brasses are sensitive to ammonia Stress levels may be very low Solutions: Reduce stress levels Heat treatment Atmosphere control

18. Hydrogen Embrittlement • Metals loose strength when Hydrogen is absorbed through surface, especially along grain boundaries and dislocations • Often occurs as a result of decorative plating • High strength steels particularly susceptible • Can be removed by “baking” the alloy

19. CONTROLLING CORROSION • Self-protecting metals! --Metal ions combine with O to form a thin, adhering oxide layer that slows corrosion. • Reduce T (slows kinetics of oxidation and reduction) • Add inhibitors --Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). --Slow oxidation reaction by attaching species to the surface (e.g., paint it!). • Cathodic (or sacrificial) protection --Attach a more anodic material to the one to be protected. 11

20. Corrosion prevention Sacrificial Anode Applied Voltage

21. Surface coatings & Passivation Some materials, such as Aluminum or Stainless Steel, form oxide barrier coatings that prevent oxidation at active surface – this is called “passivation” Surface can be coated with protective layers: painted, anodized, plated (Caution!!! Cracks in plating or paint can lead to crevice corrosion!)

22. Schematic of the interface of a passivating alloy surface in contact with a biological environment Modular junction taper connection of a total hip arthroplasty showing corrosion of the taper connections. Macrograph of deposits of CrPO4 corrosion particle products on the rim of a modular Co-Cr femoral head.

25. Metal Degradative concerns • High release of ionic metallic debris • Toxicity: Metal-on-metal bearings are not recommended for patients with poorly functioning kidneys because metal ions excreted through the kidneys can build up in the blood. • osteolysis and implant loosening in total hip patients with metal-on-metal bearings may be associated with hypersensitivity to metallic debris • Surface replacement with metal on metal is a new technology that has gained a great deal of recent interest. Hip surface replacement preserves more bone in the patient than conventional hip replacement. This has the potential of being a first-line treatment of end-stage arthritis in younger, active patients.

26. Metal on Polyethylene Bearings • The adverse effects of oxidation during radiation sterilization • Polyethylene components, like most medical devices, are sterilized by exposure to gamma radiation. The radiation, while penetrating through the component, has sufficient energy to break the chains that form the molecular backbone of the polymer. If the radiation exposure is performed while the component is exposed to air, the broken ends can react with oxygen, causing harmful changes, including a decrease in molecular weight, a dramatic loss of ductility, and a decrease in strength. The combined effect may make the polyethylene markedly more susceptible to wear.

27. Approaches to minimize degradation of PE • Placing polyethylene joint replacement components into sealed packages that contain either a vacuum or an inert gas, such as nitrogen or argon, instead of air. • Replacing radiation altogether, instead exposing polyethylene components to ethylene oxide or gas plasma, neither of which imparts sufficient energy to cause oxidation. • Increasing dose of radiation to promote crosslinking of polymer chains • early results show a dramatic decrease in wear of between 30 and 96 percent in total hip replacements over that seen with conventional polyethylene. • Disadvantage: increased crosslinking makes material more brittle

28. SUMMARY • Corrosion occurs due to: --the natural tendency of metals to give up electrons. --electrons are given up by an oxidation reaction. --these electrons then are part of a reduction reaction. • Metals with a more negative Standard Electrode Potential are more likely to corrode relative to other metals. • The Galvanic Series ranks the reactivity of metals in seawater. • Increasing T speeds up oxidation/reduction reactions. • Corrosion may be controlled by: -- using metals which form a protective oxide layer -- reducing T -- adding inhibitors -- painting --using cathodic protection. 12