CHAPTER 16: 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

THE COST OF CORROSION • Corrosion: • Cost: --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

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

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

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

CORROSION IN A GRAPEFRUIT 6

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

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 Iron/Steel Aluminum Alloys Cadmium Zinc Magnesium 8

FORMS OF CORROSION • Stress corrosion • Uniform Attack 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

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

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 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

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)

Causes: abrasive fluids impinging on surfaces 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 Solutions: 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

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. Adapted from Fig. 17.14, Callister 6e. 11

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!)

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