ISSUES TO ADDRESS... Why does corrosion occur ? 1 What metals are most likely to corrode? How do temperature and environment affect corrosion rate? How do we suppress corrosion? CHAPTER 16: CORROSION AND DEGRADATION

2 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** THE COST OF CORROSION

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

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

5 EMF series Metal with smaller V corrodes. Ex: Cd-Ni cell metal o Au Cu Pb Sn Ni Co Cd Fe Cr Zn Al Mg Na K V metal V o  V = 0.153V o STANDARD EMF SERIES

6 CORROSION IN A GRAPEFRUIT

7 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 V Ni - V Cd by --increasing X --decreasing Y EFFECT OF SOLUTION CONCENTRATION

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 GALVANIC SERIES

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

Stress & Saltwater... --causes cracks! Heat treatment: slows crack speed in salt water! 4m4m --material: 7150-T651 Al "alloy" (Zn,Cu,Mg,Zr) 10 DETERIORATIVE

Uniform Corrosion: Rust! Prevention: Paint Plate Sacrificial anode

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

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

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)

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

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

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

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

11 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 O 2 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. Adapted from Fig , Callister 6e. CONTROLLING CORROSION

Corrosion prevention Sacrificial Anode Applied Voltage

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!)

12 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. SUMMARY