材料科學導論（雙語版）

第三章（1 / 3）

porosity　[prsＩtＩ]　n.　有孔性，多孔性

propagation　[prpɡeＩn]　n.　傳播；繁殖；增殖

pyrex　[paＩreks]　n.　派熱克斯玻璃（一種耐熱玻璃）

quantum　[kwntm]　n.　量子論

radiation　[reＩdＩeＩn]　n.　輻射，放射物，輻射

reflectivity　[riflektＩvＩtＩ]　n.　[物]反射率；[光]反射性；反射比

refraction　[rＩfrkn]　n.　折射；折光

remanence　[remnns]　n.　[電磁]剩磁；剩餘；剩餘物

reradiation　[rireＩdＩeＩn]　n.　[物]再輻射

samarium　[smerＩm]　n.　[化學]釤

spin　[spＩn]　n.　旋轉vi.　旋轉vt.　使旋轉

translucency　[trnzljusnsＩ]　n.　半透明

transmissivity　[trnzmＩsＩvＩtＩ]　n.　[物]透射率；透光度；過濾係數

trivalent　[traＩveＩlnt]　adj.　三價的

ultraviolet　[ltrvaＩlt]　adj.　紫外的；紫外線的n.　紫外線輻射，紫外光

velocity　[vlstＩ]　n.　[物]速度

yttrium　[ＩtrＩm]　n.[化學]釔

Exercises1.　Translate　the　following　Chinese　phrases　into　English（1）　磁化率（2）　外場（3）　光學現象（4）　絕緣材料（5）　固體材料（6）　價電子（7）　電場（8）　周期表（9）　熱膨脹（10）　熱衝擊（11）　內部溫度分布（12）　電子轉換2.　Translate　the　following　English　phrases　into　Chinese(1)　relative　permeability(2)　magnetic　field(3)　electrical　conduction(4)　energy　band(5)　quantum　theory(6)　forbidden　band(7)　ntype　semiconductors(8)　heat　capacity(9)　thermal　conductivity(10)　thermal　stresses(11)　optical　property3.　Translate　the　following　Chinese　sentences　into　English（1）　近年來，高溫超導體的應用已經超出了實驗室的範圍。（2）　在可見光範圍內，不同波長引起不同的顏色感覺。（3）　超導性意味著零電阻，也就意味著沒有能量的浪費。（4）　矽是半導體工業的基礎材料。（5）　霍爾效應是一種發現、研究和應用都很早的磁電效應。（6）　嚴格區別鐵磁性與順磁性物質有時是很困難的。4.　Translate　the　following　English　sentences　into　Chinese(1)　Magnetic　nanocomposites　with　510　fold　increases　in　magnetocaloric　effects　should　be　used　to　develop　magnetic　refrigerators　that　operate　at　room　temperature.(2)　Donor:　The　impurities　provide　an　energy　level　with　electrons　in　band　gaps.　This　kind　ofsemiconductors　is　conductive　mainly　depending　on　electrons,　which　is　Called　ntype　semiconductor.(3)　At　high　temperature,　the　semiconductor　is　not　different　from　the　conductor　in　the　conductivity.(4)　At　room　temperature,　all　the　metals　are　paramagnetic.(5)　Electron　spin　is　essential　in　understanding　many　atomic　phenomena.5.　Translate　the　following　Chinese　essay　into　English隨著粒子尺寸的減小，粒子中的磁疇數就會減少。一般地，多疇粒子的矯頑力小於單疇粒子的矯頑力。單疇粒子的矯頑力是由所謂的磁晶各向異性與形狀各向異性共同決定的，因此，細長的單疇粒子優先應用於磁記錄材料。具有很高的飽和磁化強度、適中矯頑力的單疇粒子適合於作為磁記錄材料。但是，當粒子變得足夠小，由於熱擾動，磁矩不可能有擇優取向，而展示出超順磁性。6.　Translate　the　following　English　essay　into　ChinesePiezoelectric　ceramic　fibers,　given　their　unique　properties　of　flexibility,　light　weight　andhigher　output　per　pound　of　material,　offer　the　greatest　potential　for　enabling　the　widescale　deployment　of　selfpowered　piezoelectric　ceramic　systems.

掃一掃,查看更多資料Chapter　10Chemical　Properties　of　Materials

材

料

科

學

導

論

（

雙

語

)Chapter　10Chemical　Properties　of　Materials

材

料

科

學

導

論

（

雙

語

)Chapter　10Chemical　Properties

of　Materials10.1IntroductionTo　one　degree　or　another,　most　materials　experience　some　type　of　interaction　with　a　large　number　of　diverse　environments.　Often,　such　interactions　impair　a　materials　usefulness　as　a　result　of　the　deterioration　of　its　mechanical　properties　(e.　g.,　ductility　and　strength),　other　physical　properties,　or　appearance.Deteriorative　mechanisms　are　different　for　the　three　material　types.　In　metals,　there　is　actual　material　loss　either　by　dissolution　(corrosion)　or　by　the　formation　of　nonmetallic　scale　or　film　(oxidation).　Ceramic　materials　are　relatively　resistant　to　deterioration,　which　usually　occurs　at　elevated　temperatures　or　in　rather　extreme　environments;　the　process　is　frequently　also　called　corrosion.　For　polymers,　mechanisms　and　consequences　differ　from　those　for　metals　and　ceramics,　and　the　term　degradation　is　most　frequently　used.　Polymers　may　dissolve　when　exposed　to　a　liquid　solvent,　or　they　may　absorb　the　solvent　and　swell;　also　electromagnetic　radiation　(primarily　ultraviolet)　and　heat　may　cause　alterations　in　their　molecular　structures.10.　2Corrosion　of　MetalsCorrosion　is　defined　as　the　destructive　and　unintentional　attack　of　a　metal;　it　is　electrochemical　and　ordinarily　begins　at　the　surface.　The　problem　of　metallic　corrosion　is　one　of　significant　proportions;　in　economic　terms.　It　has　been　estimated　that　approximately　5　%　of　an　industrialized　nations　income　is　spent　on　corrosion　prevention　and　the　maintenance　or　replacement　of　products　lost　or　contaminated　as　a　result　of　corrosion　reactions.　The　consequences　of　corrosion　are　all　too　common.　Familiar　examples　include　the　rusting　of　automotive　body　panels　and　radiator　and　exhaust　components.10.2.1Cost　of　Corrosion　in　IndustryThe　annual　cost　of　corrosion　in　industry　is　surprising.　Please　see　the　Table　10.1.Table　10.1Corrosion　expense　statistic　for　some　country　in　1984　yearsCountryExpense

(hundred　million)Total　produce

value　\/　%CountryExpense

(hundred　million)Total　produce

value　\/　%America750$4England100￡3.5USSR147　Rbl2France1150　FF1.5gfr300　DM3Japan133～150$1.3It　is　common　knowledge　in　our　life　that　the　rusting　of　steel　is　phenomenon　which　plagues　not　only　the　owner　of　the　domestic　“tin　lizzie①”　but　costs　us　in　Britain　alone　something　in　excess　of　￡500　million　annually　in　the　application　of　protective　measures.10.2.2Classification　of　CorrosionCorrosion　has　been　classified　in　many　different　ways.　Common　classification　of　corrosion　has　been　as　Table　10.2.Table　10.2Classification　of　corrosionCorrosion　processCorrosion　shapeEnvironmental　corrosionGeneral

corrosionLocalized

corrosionDry

corrosionWet

corrosionChemical　corrosionPittingAtmosphericElectrochemical　corrosionCreviceSoilPhysical　corrosionGalvanicSea　waterIntergranularMicrobialStressAcid,　alkali

and　salt　liquidHydrogen　embrittlementFatigueSelectiveWet　corrosion　occurs　when　a　liquid　is　present.　This　usually　involves　aqueous　or　electrolytes　and　accounts　for　the　greatest　amount　of　corrosion　by　far.　A　common　example　is　corrosion　of　steel　by　water.　Dry　corrosion　occurs　in　absence　of　a　liquid　phase　or　above　the　dew　point　of　the　environment.　Vapors　and　gases　are　usually　the　corrosion.　Dry　corrosion　is　most　often　associated　with　high　temperatures.　An　example　is　attack　on　steel　by　furnace　gases.10.2.3Corrosion　Mechanism(1)　Dry　corrosion　(or　oxidation　of　metals)Why　the　materials　have　happen　corrosion?　The　dry　corrosion　mechanism　may　be　explained　that　it　is　an　oxidation　process　in　atmosphere.　For　example,　whilst　many　metals　tend　to　oxidize　to　some　extent　at　all　temperatures,　most　engineering　metals　do　not　scale　appreciably　except　at　high　temperatures.　When　iron　is　heated　strongly　in　an　atmosphere　containing　oxygen　it　becomes　coated　with　a　film　of　black　scale.　A　chemical　reaction　has　taken　place　between　atoms　of　iron　molecules　of　atmospheric　oxygen:2Fe+O2　2FeO(101)Although　the　reaction　can　be　expressed　by　the　above　simple　equation　in　fact　atoms　of　iron　have　been　oxidized　whilst　atoms　of　oxygen　have　been　reduced.　These　processes　are　associated　with　a　transfer　of　electrons　from　one　atom　to　the　other.Oxidation　FeFe+++2　electrons　(e-)

Reduction　O+2　electrons　(e-)O-　-(102)It　should　be　noted　that,　in　the　chemical　sense,　the　terms　oxidation　and　reduction　have　a　wider　meaning　than　the　combination　or　separation　of　a　substance　with　oxygen.　Thus,　a　substance　is　oxidized　if　its　atoms　lose　electrons　whilst　it　is　reduced　if　its　atoms　(or　groups　of　atoms)　gain　electrons.　For　this　reason　we　say　that　iron　is　oxidized　if　it　combines　with　sulphur,　chlorine　or　any　other　substance　which　will　accept　electrons　from　the　atoms　of　iron.In　some　cases　the　formation　of　the　oxide　film　protects　the　metal　from　further　oxidation　yet　in　many　instances　oxidation　and　scaling　continue.　For　this　to　occur　either　molecules　of　oxygen　must　pass　through　a　very　porous　film　of　oxide,　or　ions　of　either　oxygen　of　the　metal　must　migrate　within　a　continuous　film.Figure　10.1　indicates　the　state　of　affairs　which　exists　within　the　film　of　iron　oxide　scale.　The　positively　charged　Fe++　ions　are　attracted　outwards　towards　the　cathodic　regions,　i.　e.　those　rich　in　negative　charge,　whilst　the　negatively　charged　O-　-　ions　are　attracted　inwards　towards　the　region　which　is　richer　in　positive　charge,　i.　e.　that　near　the　metal　surface.　Since　metallic　ions　are　generally　smaller　than　oxygen　ions,　the　diffusion　of　the　metal　ions　outwards　is　quicker　than　the　diffusion　of　oxygen　ions　inwards.　The　rate　of　oxidation　of　the　metal　will　depend　partly　on　the　mobility　of　the　ions　through　the　oxide　film　but　also　upon　the　rate　of　flow　of　electrons　outwards.　These　mobilities　are　in　turn　affected　by　the　nature　and　structure　of　the　film,　particularly　in　terms　of　“vacancies”,　that　is,　positions　from　which　ions　are　missing.　The　number　of　vacancies　is　affected　by　the　presence　of　solute　atoms　such　as　chromium　and　ionic　mobilities　are　reduced　as　a　result.Figure　10.1The　mechanism　of　the　scaling　of　iron　at　high　temperatures

As　mentioned　above　oxidation　rates　also　depend　upon　the　porosity　of　the　oxide　film.　Some　metals　form　ions　which　are　much　smaller　than　the　original　atoms.　Consequently　as　ions　are　formed　there　is　a　volume　reduction　of　the　scale　which,　as　a　result,　becomes　porous　and　permits　easier　access　of　oxygen　to　the　metal　surface.　Contraction　of　the　oxide　film　may　also　cause　it　to　flake　off　thus　exposing　a　fresh　metal　surface.　In　some　cases　expansion　of　the　film　may　occur　causing　it　to　buckle　and　become　detached.An　oxide　film　which　adheres　tightly　to　the　surface　of　the　metal　generally　offers　good　protection.　Good　adhesion　is　the　result　of　coherence　between　the　film　and　the　metal　beneath.　In　Figure　10.2　(a)　there　is　good　“matching”　between　the　ions　in　the　metal　surface　and　the　metallic　ions　in　the　oxide　film　such　that　the　structure　is　virtually　continuous,　whilst　in　Figure　10.2　(b)　matching　is　absent　so　that　the　two　surfaces　will　be　noncoherent　and　there　will　be　little　adhesion.　The　high　degree　of　coherence　between　aluminum　and　its　oxide　leads　to　the　effective　protection　of　this　metal　especially　when　the　film　is　artificially　thickened　by　anodizing.Figure　10.2Adhesion　between　a　metal　surface　and　the　covering　oxide　film.　In　the　row　of　metal　ions　at　the　interface　“matches　up”　with　those　in　the　oxide　film　(a)　,　but　in　ions　are　“out　of　step”　at　the　interface　(b)

Some　alloys　are　prone　to　attack　by　atmospheres　containing　sulphurous　gases.　Since　nickel　readily　forms　a　sulphide　at　high　temperatures　it　is　particularly　liable　to　be　oxidized　by　gases　containing　sulphur.　Heatresisting　steels　used　in　such　conditions　must　be　of　the　highchromium　type,　preferably　containing　little　or　no　nickel.(2)　Wet　corrosion　(or　electrochemical　oxidation)What　is　wet　corrosion?　Whats　wet　corrosion　mechanism?Iron　dose　not　rust　in　a　completely　dry　atmosphere　nor　will　it　rust　in　completely　pure,　oxygenfree　water②,　but　in　a　moist　atmosphere　the　wellknown　reddishbrown　deposit　of　ferric　hydroxide　soon　begins　to　develop.　The　overall　chemical　reaction　representing　resting　can　be　expressed　by　a　simple　chemical　equation:4Fe+6H2O+3O24Fe(OH)3(103)This　general　result,　however,　is　achieved　in　a　number　of　stages　and　the　fundamental　principle　involved　is　that　atoms　of　iron　in　contact　with　oxygen　and　water　are　oxidized,　that　is　they　lose　electrons　and　enter　solution　as　ferrous　ions　(Fe++):FeFe+++2　electrons　(2e-)(104)These　ferrous　ions　are　ultimately　oxidized　further　to　ferric　ions　(Fe+++)　by　the　removal　of　another　electron:Fe++Fe++++e-(105)As　iron　goes　into　solution　in　the　form　of　ions　the　corresponding　electrons　are　released.　These　electrons　immediately　combine　with　other　ions　so　that　overall　equilibrium　is　maintained.The　ease　with　which　a　metal　can　be　oxidized　in　this　way　depends　upon　the　ease　with　which　valency　electrons　can　be　removed　from　its　atoms.　Thus　metals　like　calcium,　aluminium　and　zinc　hold　their　valency　electrons　comparatively　loosely　and　can　therefore　be　oxidized　more　easily　than　iron;　but　the　noble　metals　gold,　silver　and　platinum　retain　their　valency　electrons　more　strongly　and　are　therefore　much　more　difficult　to　oxidize　than　iron.Copper　holds　on　to　its　valency　electrons　more　strongly　than　does　iron　and,　under　suitable　conditions,　copper　ions　will　“steal”　valency　electrons　from　atoms　of　iron.　Thus　when　a　penknife　blade　is　immersed　in　copper　sulphate　solution　the　blade　becomes　coated　with　metallic　copper.　Copper　ions　at　the　surface　of　the　blade　have　removed　electrons　from　the　atoms　of　iron　there　so　that　the　resultant　ferrous　ions　have　gone　into　solution　thus　replacing　the　copper　ions　which　have　been　deposited　as　copper　atoms.FeFe+++2e-(106)Cu+++2e-Cu(107)Copper　sulphate　solution　is　sometimes　used　to　coat　the　surface　of　steel　with　a　thin　layer　of　copper　as　an　aid　to　“marking　out③”,　in　the　tool　room.10.2.4Electrochemical　ConsiderationsFor　metallic　materials,　the　corrosion　process　is　normally　electrochemical,　that　is,　a　chemical　reaction　in　which　there　is　transfer　of　electrons　from　one　chemical　species　to　another.　Metal　atoms　characteristically　lose　or　give　up　electrons　in　what　is　called　an　oxidation　reaction.　Examples　in　which　metals　oxidize　areFeFe2++2e-(108a)AlAl3++3e-(108b)The　site,　at　which　the　oxidation　takes　place,　is　called　the　anode.　The　electrons　generated　from　each　metal　atom　that　is　oxidized　must　be　transferred　to　and　become　a　part　of　another　chemical　species　in　what　is　termed　a　reduction　reaction.　For　example,　some　metals　undergo　corrosion　in　acid　solutions,　which　have　a　high　concentration　of　hydrogen　(H+)　ions;　the　H+　ions　are　reduced　as　hydrogen　gas.　Other　reduction　reactions　are　possible,　depending　on　the　nature　of　the　solution　to　which　the　metal　is　exposed.　For　an　acid　solution　having　dissolved　oxygen,　reduction　according　toO2+4H++4e-2H2O(109)

will　probably　occur.　Or,　for　a　neutral　or　basic　aqueous　solution　in　which　oxygen　is　also　dissolved,O2+2H2O+4e-4OH-(1010)The　location　at　which　reduction　occurs　is　called　the　cathode.　Furthermore,　it　is　possible　for　two　or　more　of　the　reduction　reactions　above　to　occur　simultaneously.As　a　consequence　of　oxidation,　the　metal　ions　may　either　go　into　the　corroding　solution　as　ions,　or　they　may　form　an　insoluble　compound　with　nonmetallic　elements.Not　all　metallic　materials　oxidize　to　form　ions　with　the　same　degree　of　ease.　Metallic　materials　may　be　rated　as　to　their　tendency　to　experience　oxidation　when　coupled　to　other　metals　in　solutions　of　their　respective　ions.　Table　10.2　represents　the　corrosion　tendencies　for　the　several　metals;　those　at　the　top　(i.　e.,　gold　and　platinum)　are　noble,　or　chemically　inert.　Moving　down　the　table,　the　metals　become　increasingly　more　active,　that　is,　more　susceptible　to　oxidation.　Sodium　and　potassium　have　the　highest　reactivities.Most　metals　and　alloys　are　subject　to　oxidation　or　corrosion　to　one　degree　or　another　in　a　wide　variety　of　environments;　that　is,　they　are　more　stable　in　an　ionic　state　than　as　metals.　In　thermodynamic　terms,　there　is　a　net　decrease　in　free　energy　in　going　from　metallic　to　oxidized　states.　Consequently,　essentially　all　metals　occur　in　nature　as　compoundsfor　example,　oxides,　hydroxides,　carbonates,　silicates,　sulfides　and　sulfates.　Two　notable　exceptions　are　the　noble　metals　gold　and　platinum.　For　them,　oxidation　in　most　environments　is　not　favorable,　and,　therefore,　they　may　exist　in　nature　in　the　metallic　state.Even　though　Table　10.2　was　generated　under　highly　idealized　conditions　and　has　limited　utility,　it　nevertheless　indicates　the　relative　reactivities　of　the　metals.　A　more　realistic　and　practical　ranking,　however,　is　provided　by　the　galvanic　series,　Table　10.3.　This　represents　the　relative　reactivities　of　a　number　of　metals　and　commercial　alloys　in　seawater.　The　alloys　near　the　top　are　cathodic　and　unreactive,　whereas　those　at　the　bottom　are　most　anodic;　no　voltages　are　provided.Table　10.2The　corrosion　tendencies　for　common　metalsElectrode　reaction

Increasingly　inter　(cathodic)

Increasingly　active　(anodic)

Au3++3e-Au

O2+4H++4e-2H2O

Pt2++2e-Pt

Ag++e-Ag

Fe3++e-Fe2+

O2+2H2O+4e-4OH-

Cu2++2e-Cu

2H++2e-　H2

Pb2++2e-Pb

Sn2++2e-Sn

Ni2++2e-Ni

Co2++2e-Co

Cd2++2e-Cd(to　be　continued)

Electrode　reaction

Increasingly　inter　(cathodic)

Increasingly　active　(anodic)

Fe2++2e-Fe

Cr3++3e-Cr

Zn2++2e-Zn

Al3++3e-Al

Mg2++2e-Mg

Na++e-Na

K++e-KTable　10.3The　galvanic　series

Increasingly　inter　(cathodic)

Increasingly　active　(anodic)

Platinum

Gold

Graphite

Titanium

Silver

316　Stainless　steel　(passive)

304　Stainless　steel　(passive)

lnconel　(80Ni13Cr7Fe)　(　passive)

Nickel　(　passive)

Monel　(70Ni30Cu)

Coppernickel　alloys

Bronzes　(CuSn　alloys)

Copper

Brasses　(CuZn　alloys)

Inconel　(active)

Nickel　(active)

Tin

Lead

316　Stainless　steel　(active)

304　Stainless　steel　(active)

Cast　iron

Iron　and　steel

Aluminum　alloys

Cadmium

Commercially　pure　aluminum

Zinc

Magnesium　and　magnesium　alloys10.2.5Corrosion　RatesThe　corrosion　rate,　or　the　rate　of　material　removal　as　a　consequence　of　the　chemical　action,　is　an　important　corrosion　parameter.　This　may　be　expressed　as　the　corrosion　penetration　rate　(CPR),　or　the　thickness　loss　of　material　per　unit　of　time.　The　formula　for　this　calculation　isCPR=KWρAt(1011)

where　W　is　the　weight　loss　after　exposure　time　t;　ρ　and　A　represent　the　density　and　exposed　specimen　area,　respectively,　and　K　is　a　constant,　its　magnitude　depending　on　the　system　of　units　used.　The　CPR　is　conveniently　expressed　in　terms　of　millimeters　per　year　(mm\/a).10.2.6PassivitySome　normally　active　metals　and　alloys,　under　particular　environmental　conditions,　lose　their　chemical　reactivity　and　become　extremely　inert.　This　phenomenon,　termed　passivity,　is　displayed　by　chromium,　iron,　nickel,　titanium　and　many　of　their　alloys.　It　is　felt　that　this　passive　behavior　results　from　the　formation　of　a　highly　adherent　and　very　thin　oxide　film　on　the　metal　surface,　which　serves　as　a　protective　barrier　to　further　corrosion.　Stainless　a　result　of　steels　are　highly　resistant　to　corrosion　in　a　rather　wide　variety　of　atmospheres　as　a　passivation.　They　contain　at　least　11　%　chromium　that,　as　a　solidsolution　alloying　element　in　iron,　minimizes　the　formation　of　rust;　instead,　a　protective　surface　film　forms　in　oxidizing　atmospheres.　(Stainless　steels　are　susceptible　to　corrosion　in　some　environments,　and　therefore　are　not　always　“stainless”.)　Aluminum　is　highly　corrosion　resistant　in　many　environments　because　it　also　passivates.　If　damaged,　the　protective　film　normally　reforms　very　rapidly.　However,　a　change　in　the　character　of　the　environment　(e.　g.,　alteration　in　the　concentration　of　the　active　corrosive　species)　may　cause　a　passivated　material　to　revert　to　an　active　state.　Subsequent　damage　to　a　preexisting　passive　film　could　result　in　a　substantial　increase　in　corrosion　rate,　by　as　much　as　100000　times.10.2.7Environmental　EffectsThe　variables　in　the　corrosion　environment,　which　include　fluid　velocity,　temperature　and　composition,　can　have　a　decided　influence　on　the　corrosion　properties　of　the　materials　that　are　in　contact　with　it.　In　most　instances,　increasing　fluid　velocity　enhances　the　rate　of　corrosion　due　to　erosive　effects.　The　rates　of　most　chemical　reactions　rise　with　increasing　temperature;　this　also　holds　for　the　great　majority　of　corrosion　situations.　Increasing　the　concentration　of　the　corrosive　species　(e.　g.,　H+　ions　in　acids)　in　many　situations　produces　a　more　rapid　rate　of　corrosion.　However,　for　materials　capable　of　passivation,　raising　the　corrosive　content　may　result　in　an　activetopassive　transition,　with　a　considerable　reduction　in　corrosion.Cold　working　or　plastically　deforming　ductile　metals　is　used　to　increase　their　strength;　however,　a　coldworked　metal　is　more　susceptible　to　corrosion　than　the　same　material　in　an　annealed　state.　For　example,　deformation　processes　are　used　to　shape　the　head　and　point　of　a　nail;　consequently,　these　positions　are　anodic　with　respect　to　the　shank　region.10.2.8Forms　of　CorrosionIt　is　convenient　to　classify　corrosion　according　to　the　manner　in　which　it　is　manifest.　Metallic　corrosion　is　sometimes　classified　into　eight　forms:　uniform,　galvanic,　crevice,　pitting,　intergranular,　selective　leaching,　erosioncorrosion　and　stress　corrosion.　In　addition,　hydrogen　embrittlement　is,　in　a　strict　sense,　a　type　of　failure　rather　than　a　form　of　corrosion;　however,　it　is　often　produced　by　hydrogen　that　is　generated　from　corrosion　reactions.(1)　Uniform　attackUniform　attack　is　a　form　of　electrochemical　corrosion　that　occurs　with　equivalent　intensity　over　the　entire　exposed　surface　and　often　leaves　behind　a　scale　or　deposit.　In　a　microscopic　sense,　the　oxidation　and　reduction　reactions　occur　randomly　over　the　surface.　Some　familiar　examples　include　general　rusting　of　steel　and　iron　and　the　tarnishing　of　silverware.　This　is　probably　the　most　common　form　of　corrosion.　It　is　also　the　least　objectionable　because　it　can　be　predicted　and　designed　for　with　relative　ease.(2)　Galvanic　corrosionGalvanic　corrosion　occurs　when　two　metals　or　alloys　having　different　compositions　are　electrically　coupled　while　exposed　to　an　electrolyte.　The　less　noble　or　more　reactive　metal　in　the　particular　environment　will　experience　corrosion;　the　more　inert　metal,　the　cathode,　will　be　protected　from　corrosion.　For　example,　steel　screws　corrode　when　in　contact　with　brass　in　a　marine　environment;　or　if　copper　and　steel　tubing　are　joined　in　a　domestic　water　heater,　the　steel　will　corrode　in　the　vicinity　of　the　junction.　Depending　on　the　nature　of　the　solution,　one　or　more　of　reduction　reactions　will　occur　at　the　surface　of　the　cathode　material.The　rate　of　galvanic　attack　depends　on　the　relative　anodetocathode　surface　areas　that　are　exposed　to　the　electrolyte,　and　the　rate　is　related　directly　to　the　cathodeanode　area　ratio;　that　is,　for　a　given　cathode　area,　a　smaller　anode　will　corrode　more　rapidly　than　a　larger　one.A　number　of　measures　may　be　taken　to　significantly　reduce　the　effects　of　galvanic　corrosion.　These　include　the　following:1)　If　coupling　of　dissimilar　metals　is　necessary,　choose　two　that　are　close　together　in　the　galvanic　series.2)　Avoid　an　unfavorable　anodetocathode　surface　area　ratio;　use　an　anode　area　as　large　as　possible.3)　Electrically　insulate　dissimilar　metals　from　each　other.4)　Electrically　connect　a　third,　anodic　metal　to　the　other　two;　this　is　a　form　of　cathodic　protection.(3)　Crevice　corrosionElectrochemical　corrosion　may　also　occur　as　a　consequence　of　concentration　differences　of　ions　or　dissolved　gases　in　the　electrolyte　solution,　and　between　two　regions　of　the　same　metal　piece.　For　such　a　concentration　cell,　corrosion　occurs　in　the　locale　that　has　the　lower　concentration.　A　good　example　of　this　type　of　corrosion　occurs　in　crevices　and　recesses　or　under　deposits　of　dirt　or　corrosion　products　where　the　solution　becomes　stagnant　and　there　is　localized　depletion　of　dissolved　oxygen.　Corrosion　preferentially　occurring　at　these　positions　is　called　crevice　corrosion　(Figure　10.3).　The　crevice　must　be　wide　enough　for　the　solution　to　penetrate,　yet　narrow　enough　for　stagnancy;　usually　the　width　is　several　thousandths　of　an　inch.Figure　10.3On　this　plate,　which　was　immersed　in　seawater,　crevice　corrosion　has　occurred　at　the　regions　that　were　covered　by　washers