5 Prilagodba materijala

7/30/2019 5 Prilagodba materijala

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6. Prilagodba materijala6. PRILAGODBA MATERIJALA ....................................................................1

6.1 Osnove prilagodbe materijalaProvedbom pogodnih postupaka mogu se svojstva materijala u velikoj mjeri prilagoditi

potrebama te time omoguiti njegovo koritenje za izradu aktualnog proizvoda. Na primjer,razvoj metalnih zrakoplova omoguen je Wilmovim otkriem (1909. godine) kako sealuminiju dodatkom magnezija i bakra, te toplinskom obradom, moe 5 puta poveativrstoa (Rm).

6.1.1 Izbor materijala

Proizvod se moe izraditi od razliitih materijala, a sve jaa konkurencija natritu nameesve stroe zahtjeve koje nije mogue ispuniti bez sustavnog pristupa izboru materijala.

U izboru manje ili vie su znaajna brojna razliita svojstva materijala (S-05.1).


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06 Prilagodba materijala 2

Slika S-05.1 Svojstva materijala

Dio svojstva materijala opisuje se jednoznanim ili kombiniranim "karakteristikama" kojesu definirane u tehnikim normama, a njihove se vrijednosti nalaze u literaturi u obliku:

semikvantitativnih opisa i slika, dijagrama, tablica i matematikih izraza, te jednoznanih podataka.

esto se karakteristike materijala moraju i odrediti/provjeriti provedbom normiranihlaboratorijskih mjerenja/pokusa te rijetko utvrditi provedbom eksperimentima na modelima iliu pogonskim uvjetima.

U izboru materijala najee su znaajne karakteristike:

Karakteristika materijalanaziv i oznakakemijski sastav

graagustoavlana vrstoatlana vrstoamodul elastinostiPoissonov koeficijentgranica razvlaenjaistezljivostlomna ilavost

tvrdoa po Vickersudinamika izdrljivost

maksimalna radna temperaturaminimalna radna temperaturaspecifina toplinatoplinska vodljivosttoplinska rastezljivostnapon probojaelektrina otpornostreciklinostcijena

otpornost na: troenjeoksidaciju pri 500

paljenjeultra-ljubiaste zrakeslatku vodumorsku vodu

jake kiselinejake luineslabe kiselineslabe luineorganska otapala

U pripremi proizvodnje, strunjak ili tim za izbor materijala danas koristi raunalnupodrku koja obuhvaa potrebne baze podataka i pogodne programe za izbor materijala.

Osobito je znaajan zahtjev minimalnog ukupnog troka ivotnog ciklusa proizvoda:

tu = t1m + t2 + t3 + t4 + t5 , kn/proizvodu

gdje je: t1 trokovi nabavke samog materijala (svi pratei trokovi, transport, carina,akciza, skladitenje), kn/kg,

m masa materijala potrebna za izradu jednog proizvoda, kg/proizvodu,t2 trokovi izrade jednog proizvoda od aktualnog materijala, kn/proizvodu,


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razni oblici otapanja/korozije/starenja materijala

d) elektrinih djelovanja krateri/nakupine uslijed kratkih spojeva

promjene elektrinih i/ili magnetnih svojstavaVei dio nepogodnih promjena svojstava materijala moe se rijeiti provedbom pogodnih

postupaka prilagodbe materijala (uzeti u obzir poveanje tu).

U izradi proizvoda povrinama se posveuje posebna panja zbog: fizikih svojstava lijepog izgleda sigurnosti pri rukovanju izraenog utjecaja na trenje i troenje

Svojstva se povrina takoer mogu znaajno poboljati postupcima prilagodbe materijala.

6.2 Toplinska obradaToplinskom se obradom svojstva materijala u manjoj ili veoj mjeri mogu prilagoditi

potrebama. Toplinski se obrauju najee elici, ali nerijetko i drugi metali te keramike(stakla).

Vrste su toplinske obrade:

1. termika komadi se griju, dre na odreenoj temperaturi i potom hlade primjeri su: arenje pribliavanje stanju ravnotee gaenje udaljavanje od stanja ravnotee

poputanje ponovno neznatno pribliavanje stanju ravnotee:2. termokemijska termika se obrada obavlja u kemijski aktivnoj sredini primjeri su:

pougljenienje poveavanje sadraja ugljika (C) u povrinskom sloju komada nitriranje poveavanje sadraja duika (N) u povrinskom sloju komada karbonitriranje poveavanje sadraja C i N u povrinskom sloju komada boriranje poveavanje sadraja bora (B) u povrinskom sloju komada

3. termomehanika termika se obrada kombinira s mehanikim deformiranje.

Toplinska se obrada obavlja: prije oblikovanja komada plastinom deformacijom i odvajanjem strugotine,

da bi metal omekao,

poslije oblikovanja komada, da bi se uklonila zaostala naprezanja, zavrno, za postizanje konane vrstoe i tvrdoe proizvoda.

6.2.1 Termika obradaOvisno o materijalu i geometriji komada te eljenom uinku odreuje se dinamika

promjena temperature.


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5 Strojarska tehnologija I

U gornjem dijagramu su prikazanepromjene temperature komada tijekomvremena.

Intervali su vremena: tu ugrijavanja tp progrijavanja td dranja to ohlaivanja

U donjem dijagramu su prikazanepromjene razlike temperature povrine ijezgre komada tijekom vremena.

Na krajevima intervala vremena, s temperaturom pei (pei) izjednaavaju se:

interval ugrijavanja (tu) temperatura povrine komada (povrine = pei) interval progrijavanja (tp) temperatura jezgre (jezgre = pei).Tijekom intervala vremena dranja (td) postie se eljena struktura materijala. Na trajanjeintervala vremena ohlaivanja (to) pored svojstava materijala te veliine i geometrije komadautjeu i svojstva usvojenog sredstva za hlaenje.

Na intervale temperatura ravnotea faza (Fe-C dijagram) znaajno utjee sadraj ugljika ueliku. Na primjer, austenit je ravnotena faza elika s 1 % C u intervalu temperature od oko800 do oko 1300 C (jezgreaustenita, 1%C). Meutim, pri grijanju/hlaenju ravnotene faze seformiraju samo ako su brzine grijanja/hlaenja manje od brzina faznih transformacija.

BCC reetka FCC reetka BCC reetka

Fe3C cementit

Cw masenipostotakugljika

Few masenipostotakeljeza

Fe3Cw masenipostotak

cementita

T talinaF feritA austenitP perlitL ledeburitC cementitC'' sekundarni

cementit

S porastom sadraja ugljika znaajno se mijenja graa metalnog materijala na bazi eljeza ugljik se u sve veoj mjeri izdvaja iz kristalne reetke eljeza. elicima se nazivaju eljeza swC = 0,1 2 % dok se lijevanim eljezima nazivaju eljeza s wC = 2,5 3,7 %.


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Martenzitna transformacija pri neravnotenoj transformaciji austenita u uvjetima brzoghlaenja, u kristalnoj reetki eljeza bivaju zarobljeni atomi ugljika i formiraju se kristalmartenzita, volumno centrirane tetragonalne strukture BCT. Martenzit ima vrlo visokutvrdou, to je posljedica deformacije reetke (BCC BCT) te time formirane barijere zaklizanje.

elik s cw = 0,8 %, t > 723 C elik ugrijan na temperaturu otvrdnjavanjaGraa austenit FCC austenit Graa austenit FCC austenit

elik ohlaen ispod temperature od 723 C Otvrdnuti elikGraa perlit BCC perlit Graa martenzit BCC martenzit

Pri brzinama hlaenja veim od brzina faznih transformacija formiraju se neravnotenestrukture materijala (razliitih svojstava od ravnotenih), to se opisuje dijagramimavrijeme-temperatura-transformacija TTT dijagrami (eng. Time-Temperature-Transformation).


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Tvrdoe od 670 HBodnosno 330 HB dobivajuse hlaenjem komada izaustenitnog podruja utoploj vodi (bre) odnosnoulju (sporije). Pri jo

sporijem hlaenju dobivase jo manja tvrdoa od230 HB.

Uslijed razlika temperatura povrina i jezgre komada u intervalima vremena ugrijavanja,progrijavanja (povrinepei):

= povrine jezgre 0 C

i ohlaivanja (povrinepei):

= povrine jezgre 0 C

javljaju se toplinska naprezanja:

top = E, N/mm2

gdje je: E= f1() modul elastinosti materijala, N/mm2 = f2() toplinska rastezljivost materijal, mm/(mmC)

Vrste termike obrade


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arenje

Kaljenje

Austenitizacija elik se grije na dovoljno visoku temperature dok se ne formira austenit,

bar djelomino

Gaenje obavlja se u slanoj vodi, vodi, ulju ili zraku

Temperiranje smanjuje se krhkost martenzita (temperirani martenzit) precipitacijom sitnih estica karbida BCT BCC

Jominyjeva proba:

Poputanje

Metal se grije dri neko vrijeme na dovoljno visokoj temperaturi i potom sporo hladi.Svrhe su poputanja:

smanjenje tvrdoe i krhkosti promjena mikrostrukture omekavanje metala kako bi se lake deformirao materijal ili odvajala strugotina rekristalizacija na hladno obraenog materijala otputanje zaostalih naprezanja

Vrste su poputanja:a) potpuno poputanje grijanje materijala na bazi eljeza do podruja austenita

te sporo hlaenje (u pei) radi formiranja krupnozrnog perlitab) normalizacija kao i potpuno poputanje, uz neto bre hlaenje (na zraku)

radi formiranja sitnozrnog austenita (vea vrstoa i tvrdoa)c) procesno poputanje poputanje radi omoguavanja dodatne deformacijed) poputanje kao i procesno poputanje, bez dodatnog deformiranjae) oporavno poputanje zadravanje glavnine tvrdoa uz poveanje ilavostif) poputanje za uklanjanje naprezanja eliminacija zaostalih naprezanja


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Poboljavanje

Precipitacijsko ovrivanje

toplinska obrada kojom se poveava vrstoa

obrada otopine legura se grije na temperaturu Ts iznad linije solvus u oblast faze ina toj temperaturi dri dovoljnog dugo za otapanje faze

gaenje hlaenje do sobne temperature kako bi se formirala presiena kruta otopina

precipitacijska obrada izaziva se precipitacija finih estica faze pri temperaturi Tp

starenje prirodno starenje starenje pri sobnoj temperaturi umjetno starenje starenje pri povienoj temperaturi

prestarivanje (slino poputanju)


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6.2.2 Termokemijska obrada

Povrinsko otvrdnjavanje

Pougljenienje sa: krutinom s ugljinim materijalom u kutiji, debljina 0,6 3,8 mm, plinom ugljikovodino gorivo u komori, debljina 0,13 0,75 mm, tekuinom kupka sa taljenom soli, debljine 0,13 0,75 mm.

Kromiranje iziskuje veu temperaturu i dulju obradu: postiu se ne samo vee tvrdoe i otpornosti na troenje nego i vee otpornosti

na koroziju. Nitriranje elici sa 0,85 1,5 Al te 5 ili vie postotaka Cr, koji formiraju sitne

estice spojeva nitrida, debljina 0,025 0,05 mm: plinsko nitriranje grijanje u amonijanoj atmosferi pri 510 C, tekue nitriranje potapanje u talinu cijanidnih soli pri 510 C.

Karbonitriranje grijanje u pei s ugljikom i amonijakom, debljine 0,07 0,5 mm. Boriranje alatni elici, legure na bazi nikla i kobalta te lijevano eljezo:

velike tvrdoe i mali koeficijenti trenja.


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Difuzija

Pri termokemijskoj obradi atomi supstancije sadrane u okolini pri visokim temperaturama

difundiraju kroz granini sloj u povrinu osnovnog materijala komada. Povrina komada inakon termokemijske obrade ima visok sadraj osnovnog materijala dok sadraj difundiranesupstancije brzo opada od vanjske povrine k unutranjosti komada.

6.2.3 Termomehanika obrada

6.3 ienje povrinaTijekom proizvodnje skoro se svi komadi moraju jedan ili vie puta oistiti. ienjem se:

uklanjaju oneienja koja kemijski reagiraju s povrinama komada (korozija,bubrenje) priprema povrina komada za sljedee proizvodne operacije (prevlaka, lijepljenje) poboljanje higijenskih uvjeta za zaposlenike i korisnike poboljanje izgleda i svojstava proizvoda

Za uklanjanje oneienja s povrina komada koriste se kod: kemijskih postupaka kemikalije mehanikih postupaka mehanike operacije

Izbor postupka ienja ovisi o:

vrste prisutne neistoe potrebnog stupnja istoe materijala od koga je komad izraen svrsi ienja zahtijeva zatite na radu i ekolokih propisa oblika i dimenzija komada prihvatljivim trokovima

Neistoe na povrinama komada su posljedica obavljenih proizvodnih operacija i/ilidjelovanja okoline. U proizvodnim pogonima su najea oneienja:

ulja i masti (emulzija za obradu metala) vrste estice (strugotina, abrazivne estice, praina)


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niti i sredstva za poliranje film oksida, hra, naslaga

Stupanj istoe informacije o koliini neistoa koja se zadri na komadu nakon ienja.Moe se procijeniti na temelju koliine neistoe koja se zadri na krpi nakon brisanjakomada.

Kemikalije za ienje ne smiju izazvati oteenje materijala komada, na primjer: aluminij se otapa u veini kiselina i luina, elik je otporan na djelovanje luina a reagira praktino sa svim kiselinama

Za ienje se biraju postupci i potrebne kemikalije s kojima se izbjegavaju zagaenja iugroavanja zdravlja.

6.3.1 Postupci kemijskog ienjaPostupci se kemijskog ienja provode sa:

luinom emulzijom otapalom kiselinom ultrazvukom (mehaniko titranje visokih frekvencija) kombiniranim postupkom, npr. kemikalija ultrazvuk

ienje s luinama:

Uses an alkali to remove oils, grease, wax, and various types of particles (metal chips,silica, light scale) from a metallic surface

Most widely used industrial cleaning method

Alkaline solutions include sodium and potassium hydroxide (NaOH, KOH), sodiumcarbonate (Na2CO3), borax (Na2B4O7)

Cleaning methods: immersion or spraying, usually at temperatures of 50-95Cfollowed by water rinse to remove residue

ienje s emulzijom:

Uses organic solvents (oils) dispersed in an aqueous solution

The use of suitable emulsifiers (soaps) results in a two-phase cleaning fluid(oil-in-water), which functions by dissolving or emulsifying the soils on the partsurface

Can be used on either metal or nonmetallic parts

Must be followed by alkaline cleaning to eliminate all residues of the organic solventprior to plating

ienje s otapalom:

Organic soils such as oil and grease are removed from a metallic surface by means ofchemicals that dissolve the soils


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Common application techniques: hand-wiping, immersion, spraying, and vapordegreasing

Vapor degreasing (a solvent cleaning method) uses hot vapors of chlorinated orfluorinated solvents Because the chemicals are hazardous, vapor degreasing has been largely

discontinued as an industrial cleaning process, at least in the U.S.

ienje s kiselinom:

Removes oils and light oxides from metal surfaces using acid solutions combined withwater-miscible solvents, wetting and emulsifying agents

Common application techniques: soaking, spraying, or manual brushing or wipingcarried out at ambient or elevated temperatures

Cleaning acids include hydrochloric (HCl), nitric (HNO3), phosphoric (H3PO4), andsulfuric (H2SO4)

Jetkanje s kiselinom:

A more severe acid treatment to remove thicker oxides, rusts, and scales

The distinction between acid cleaning and acid pickling is a matter of degree

Generally results in some etching of the metallic surface which serves to improveorganic paint adhesion

ienje s ultrazvukom:

Mechanical agitation of cleaning fluid by high-frequency vibrations (between 20 and 45kHz) to cause cavitation - formation of low pressure vapor bubbles that scrub the surface

Combines chemical cleaning and mechanical agitation of the cleaning fluid

Cleaning fluid is generally an aqueous solution containing alkaline detergents

Highly effective for removing surface contaminants

6.3.2 Postupci mehanikog ienjaPhysical removal of soils, scales, or films from the work surface by means of abrasives or

similar mechanical action Often serves other functions also, such as deburring, improving surface finish, and

surface hardening

Processes: Blast finishing Shot peening Mass finishing processes

ienje s mlazom

High velocity impact of particulate media to clean and finish a surface

The media is propelled at the target surface by pressurized air or centrifugal force


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Most well-known method is sand blasting, which uses grits of sand as the blastingmedia

Other blasting media: Hard abrasives such as aluminum oxide (Al2O3) and silicon carbide (SiC) Soft media such as nylon beads

Samarenje

High velocity stream of small cast steel pellets (calledshot) is directed at a metallic surfaceto cold work and induce compressive stresses into surface layers

Used primarily to improve fatigue strength of metal parts

Purpose is therefore different from blast finishing, although surface cleaning isaccomplished as a byproduct of the operation

arno ienje

Finishing parts in bulk by a mixing action in a container, usually in the presence of anabrasive media

The mixing causes the parts to rub against the media and each other to achieve thedesired finishing action

Parts are usually small and therefore uneconomical to finish individually

Processes include: Tumbling

Vibratory finishingienje u bubnju

Use of a horizontally oriented barrel of hexagonal or octagonal cross-section in which partsare mixed by rotating the barrel at speeds = 10 to 50 rev/min

Finishing by "landslide" action - media and parts rise in the barrel as it rotates, thentop layer tumbles down due to gravity

Drawbacks: slow, high noise levels, and large floor-space required

Figure 28.1 - Diagram of tumbling (barrel finishing) operation showing "landslide" actionof parts and abrasive media to finish the parts

Vibracijsko ienje


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An alternative to tumbling

Vibrating vessel subjects all parts to agitation with the abrasive media, as opposed toonly the top layer as in barrel finishing

Consequently, processing times for vibratory finishing are significantlyreduced

The open tubs in this method permit inspection of the parts during processing, andnoise is reduced

Sredstva za arno ienje

Most are abrasive; however, some media perform nonabrasive finishing operationssuch as burnishing and surface hardening

Natural media (corundum, granite, limestone) - generally softer and

nonuniform in size Synthetic media (Al2O3 and SiC) - greater consistency in size, shape, and and

hardness

Steel - used for burnishing, surface-hardening, and light deburring operations

Figure 28.2 - Typical preformed media shapes used in mass finishing operations:

(a) abrasive media for finishing, and (b) steel media for burnishing

6.4 Korozija i zatita od korozijeJedan od oblika djelovanja okoline na metalne materijale je korozija tetno

elektrokemijsko otapanje metala u elektrolitima, uz formiranja sloja produkata korozije ili beznjega. Pored gubitaka metala javljaju se i indirektne tetne posljedice korozije, koje u

pravilu znaajno nadmauju tete uzrokovane gubicima metalnog materijala. Na primjer, krozprobojno korozijsko oteenje vodovodne cijevi promjera 1 mm, pod tlakom od 3 bar,dnevno e biti izgubljeno oko 1 m3 pitke vode.

6.4.1 KorozijaKorozija:

elektrokemijska reakcija metala s agensima korozije sadranim u okolini, odvija se na dodirnoj povrini metal/okolina, izaziva mjerljive promjene svojstava metala i okoline.

Razvoj se korozije moe opisati na primjeru korozije ispod kapljice vode na metalnojpovrini. Uzrok je korozije neravnomjerna aeracija (razliite koncentracije otopljenog kisika).


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U biti, na tijek korozije uslijedneravnomjerne aeracije znaajnoutjee formiranje produkata

korozije. Brzina korozije je vea upodruju manje koncentracije kisikazbog odsustva sloja produkatakorozije koji imaju uinakkorozijski zatitnog sloja. Naime, uodsutnosti ovog uinka korozija bi

bila najbra u podruju najveekoncentracije kisika.

Korozijske veliine pokazatelji svojstava metala i okoline koja bitno utjeu namogunost (termodinamika) i brzinu (kinetika) razvoja korozije.

Razlikuju se:(a) vrste korozije, prema uzrocima:

(b) vidovi korozije, prema posljedicama:

Korozijsko oteenje gubitak funkcije metalnog dijela/sustava izazvan korozijom.

Osnovni je uzrok korozije elektrokemijska neravnotea.

Metal se otapanju u elektrolitu opire formiranjem elektrinog dvojnog sloja. to je jaatenja otapanja metala u elektrolitu i slabije privlaenje iona i elektrona u metalu to e zauspostavljanje elektrokemijske ravnotee biti potrebno formiranje jaega elektrinoga

dvojnog sloja (npr. morska voda / cink), kao i obrnuto (npr. morska voda / bakar). Jakost sedvojnog sloja moe opisati elektrodnim potencijalom, EM u V, te izmjeriti voltmetrom (V),


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koristei se usporedbom s referentnom elektrodom poznatog elektrodnog potencijala. Ulaboratorijima se najee koristi standardna zasiena kalomelova elektroda (SZKE).

Uspostavljena elektrokemijska ravnotea se remeti:(a) elektrolitikim i elektrinim spajanjem metala s drugim metalom,(b) lokalnim promjenama svojstava metala ili elektrolita.

Uslijed poremeaja ravnotee, uspostavlja se korozijski lanak (anoda | elektrolit | katoda) irazvija se korozija uz tenju:

1. ravnomjerne raspodjele elektrona u krutoj fazi metala elektroni gibaju izpodruja vee u podruje manje koncentracije,

2. zaustavljanja otapanja iona metala formiranjem dovoljno jakog dvojnog sloja opeg ili lokalnog.

Brzina korozije se moe opisati jakou korozijske struje, IM u A , te izmjeritiampermetrom (A). Tijekom razvoja korozije se u naelu mogu pratiti elektrodni potencijali i

jakosti korozijske struje, ali se racionalna mjerenja u praksi teko provode.Sa stajalita elektrokemije korozija je anodna oksidacija metala praena katodnom

redukcijom agensa korozije. U vodenim su sredinama najei agensi korozije otopljenemolekule kisika i uvijek prisutni ioni vodika, a razvoj korozije se moe opisati jednadbama:

anodna oksidacija metala:M n e M+n na primjer: 2 Fe 4 e 2 Fe+2

(oksidacija otputanje elektrona, anoda mjesto na kome se odvija oksidacija) katodna redukcija agensa korozije:

utroak otopljenog kisika oslobaanje vodika(O2)2 O + 4 e2 O2 redukcija

2 O2 + 2 H+2(OH) spajanje4 H+ + 4 e4 H redukcija

4 H 2 H2 spajanje

Pokazatelj koncentracije vodikovih iona:

pH= log c(H+)

i moe se zakljuiti kako u oba sluaja tijekom razvoja korozije raste pH elektrolita. Uneutralnim vodenim otopinama je c(H+)= 107mol H+/l pH= 7.

Termodinamika analiza korozije

Termodinamikim pristupom se utvruje mogunost razvoja korozije, a u analizi se estokoriste Pourbaixovi dijagrami elektrokemijske ravnotee:

voda eljezo bakar


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Razliito su oznaene oblasti:

korozije metal oksidira, te ioni metala prelaze u elektrolit

pasivnosti metal oksidira, a oksidi metala formiraju korozijski zatitni sloj

imuniteta metal ne oksidira

Na temelju Pourbaixovih ravnotenih dijagrama se ne moe pouzdano predvidjeti tijekelektrokemijskih i prateih kemijskih reakcija strogo uzevi, oni vrijede samo za ist metal iistu vodu. esto dodatak male koliine legirajueg elementa osnovnom metalu ili otapanjemale koliine kemijskog spoja u vodi rezultira znaajnim izmjenama veliina i oblika oblastiu dijagramima elektrokemijske ravnotee.

6.4.2 Zatita od korozijeZatita od korozije obuhvaa postupke spreavanja ili smanjivanja brzine korozije:

Pravilno konstruiranje

Pravilnim se konstruiranjem strojarskih dijelova/sustava izbjegavaju pojavenehomogenosti metala i/ili elektrolita.


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Izmjene svojstava metala

Izmjenama svojstava metala on se prilagoava zadanoj okolino.

Izmjene svojstava okoline

Izmjenama svojstava se okoline prilagoavaju zadanom metalu.

Formiranje izolacijskih slojeva

Formiranjem izolacijskih slojeva se korozijski neotporan metal elektrolitiki odvaja odokoline.

Elektrokemijska zatita

Elektrokemijskom zatitom se metal iz oblasti korozije (dijagrami elektrokemijskeravnotee) prevode u oblasti imuniteta (katodna zatita) ili pasivnosti (anodna zatita).

Katodna se zatita provodi sa: (a) rtvenom anodom i (b) izvorom struje i korozijskiotpornom anodom. Kako bi se smanjila potronja rtvene anode ili struje, tieni se metal

prevlai elektrino izolacijskim slojem polimera (nalii).

Kemijska korozija

Kemijska se korozija odvija direktnim sudarima atoma reaktanata kisika i metala.

Znaajne se brzina kemijske korozije metala dostiu tek pri visokim temperaturama(kaljenje, visokotemperaturna korozija u loitima kotlova).

6.4.3 Postupci zatite od korozije


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Prevlake

Plating and Related Processes

Conversion Coatings Physical Vapor Deposition

Chemical Vapor Deposition

Organic Coatings

Porcelain Enameling and Other Ceramic Coatings

Thermal and Mechanical Coating Processes

Formiranje prevlaka inenjerskih materijala

Metal products are almost always coated - by painting, plating, or other process Exceptions: stainless steel, brass

Nonmetallic materials are also sometimes coated

Plastic parts coated for a metallic appearance

Antireflection coatings applied to glass lenses

Coating and deposition processes are used in the fabrication of semiconductorchips and printed circuit boards

Osnovne svrhe formiranja prevlaka

Provide corrosion protection of the substrate

Enhance product appearance, e.g., provide color or texture

Increase wear resistance and/or reduce friction

Increase electrical conductivity or resistance

Prepare a metallic surface for subsequent processing

Rebuild surfaces worn or eroded during service

Platiranje

Coating of a thin metallic layer onto the surface of a substrate material

The substrate is usually metallic, although methods are available to plate plastic andceramic parts

Processes:

Electroplating (the most common plating process)

Electroforming

Electroless plating

Hot dipping


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Elektroplatiranje

Electrolytic process in which metal ions in an electrolyte solution are deposited onto a

cathode workpart Also called electrochemical plating

The anode is generally made of the plating metal and thus serves as the source of theplate metal

Direct current from an external power supply is passed between anode and cathode

The electrolyte is an aqueous solution of acids, bases, or salts

Figure 29.1 - Setup for electroplating

Jednadba elektroplatiranja

Faradays laws can be summarized:

V = C i t

Where: V= volume of metal plated, mm3 (in3);

C= plating constant which depends on electro- chemicalequivalent and density, mm3/amp-s;

i t(current x time) = electrical charge, amps-s

The value of C indicates the amount of plating material deposited onto the cathodicworkpart per electrical charge

Osnovni postupci elektroplatiranja

Barrel plating - performed in rotating barrels oriented either horizontally or at anoblique angle (35) - suited to plating many small parts in a batch

Rack platingracks made of heavy-gauge copper wire and formed into suitable shapesfor holding the parts and conducting current to them - used for parts that are too large,heavy, or complex for barrel plating

Strip plating a continuous strip is pulled through the plating solution by means of atake-up reel suited to high production

Uobiajeni metali prevlake


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Zinc - plated on steel products such as fasteners, wire goods, electric switch boxes, andsheetmetal parts as a sacrificial barrier to corrosion

Nickel - for corrosion resistance and decorative purposes on steel, brass, zinc diecastings, and other metals; also used as a base coat for chrome plate

Tin - widely used for corrosion protection in "tin cans" and other food containers

Copper- decorative coating on steel and zinc, either alone or alloyed as brass; also,applications in printed circuit boards

Chromium - decorative coating widely used in automotive, office furniture, andkitchen appliance applications; also one of the hardest electroplated coatings for wearresistance applications

Precious metals (gold, silver, platinum) - plated on jewelry; gold is also used forelectrical contacts

Elektroformiranje

Electrolytic deposition of metal onto a pattern until the required thickness is achieved, afterwhich the pattern is removed to leave the formed part

The process is virtually the same as electroplating but its purpose is different

Whereas typical plating thickness is only about 0.05 mm (0.002 in) or less,electroformed parts are often substantially thicker, so the production cycle is

proportionally longer

Proizvodi elektroformiranja

Metals used for electroforming: copper, nickel, and nickel cobalt alloys most common

Typical applications: fine molds and dies (e.g., for lenses) and plates for embossingand printing

Notable application: molds for compact disks (CDs) and video disks (DVDs)

Surface details imprinted onto a CD are measured in m or-in (1 m = 39.4-in).

These details are readily obtained in the mold by electroforming

Platiranje bez elektrinog izvoraPlating driven entirely by chemical reactions - no external source of electric current is

required

Deposition of metal onto a part surface occurs in an aqueous solution containing ionsof the desired plating metal - the workpart surface acts as a catalyst for the reaction inthe presence of a reducing agent

Metals that can be electroless plated are limited: nickel and certain of its alloys,copper, and gold

Notable application: copper for plating through-holes of printed circuit boards


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Potapanje u vruu kupku

Metal substrate (part) is immersed in a molten bath of a second metal; upon removal, the

second metal is coated onto the first Common substrate metals: steel and iron

Coating metals: zinc, aluminum, tin, and lead

Primary purpose is corrosion protection

Postupak potapanja u vruu kupku

Galvanizing- zinc (Zn) coated onto steel or iron

By far the most important hot dipping process

Aluminizing- coating of aluminum (Al) onto a substrate

Excellent corrosion protection, in some cases five times more effective thangalvanizing

Tinning - coating of tin (Sn) onto steel for food containers, dairy equipment, andsoldering applications

Terneplate - plating of lead-tin alloy onto steel

Konverzijske prevlake

A family of coating processes in which a thin film of oxide, phosphate, or chromate isformed on a metallic surface by chemical or electrochemical reaction

Immersion and spraying are the two common methods of exposing the metal surfaceto the reacting chemicals

Common metals treated: steel (including galvanized steel), zinc, and aluminum

Svrhe konverzijskih prevlaka

Corrosion protection

Preparation for painting

Wear reduction

Permits surface to better hold lubricants for metal forming processes

Increase electrical resistance of surface

Decorative finish

Dva tipa formiranja konverzijskih prevlaka

1. Chemical conversion coatings - chemical reaction only

Phosphate and chromate conversion coatings are the common treatments

2. Anodizing- oxide coating produced by electrochemical reaction

Anodize is a contraction ofanodic oxidize

Most commonly used on aluminum and its alloysKemijski formirane konverzijske prevlake


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Base metal is exposed to chemicals that form thin nonmetallic surface films

1. Phosphate coating - transforms base metal surface (e.g., steel, zinc) into phosphatefilm by exposure to phosphate salts and dilute phosphoric acid

Useful preparation for painting of automobiles

2. Chromate coating- transforms base metal (e.g., aluminum, copper, magnesium, zinc)into various forms of chromate films (sometimes colorful) using solutions of chromicacid, chromate salts, etc.

Anodizacija

Electrolytic treatment that produces a stable oxide layer on a metallic surface

Applications: aluminum and magnesium most common; also zinc, titanium, and othermetals

Dyes can be incorporated into the anodizing process to create a wide variety of colors Especially common in aluminum anodizing

Functions: primarily decorative; also corrosion protection

Organske prevlake

Polymers and resins (natural or synthetic) usually formulated to be applied as liquids thatdry or harden as thin surface films on substrate materials

Advantages: Wide variety of colors and textures possible

Capacity to protect the substrate surface

Low cost

Ease with which they can be applied

Komponente organske prevlake

1. Binders - give the coating its properties

2. Dyes or pigments - provide color to the coating3. Solvents - dissolve the polymers and resins and add proper fluidity to the liquid

4. Additives

Veziva

Polymers and resins that determine the solid state properties of the coating, such asstrength, physical properties, and adhesion to the substrate surface

The binder holds the ingredients in the coating during and after application to thesurface

Common binders in organic coatings:

Natural oils (used to produce oil-based paints)


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Resins of polyesters, polyurethanes, epoxies, acrylics, and cellulosics

Bojila i pigmenti

Provide color to the coating two types:1. Dyes - soluble chemicals that color coating liquid but do not conceal the surface

beneath when applied

Coatings are generally transparent or translucent

2. Pigments - solid particles of uniform, microscopic size dispersed in coating liquid butinsoluble in it

They not only color the coating; they also hide the surface below

Since pigments are particulate, they also tend to strengthen the coating

OtapalaLiquid substances used to dissolve the binder and certain other ingredients in the liquid

coating composition

Common solvents used in organic coatings:

Aliphatic and aromatic hydrocarbons

Alcohols

Esters

Ketones

Chlorinated solventsAditivi

Surfactants (to facilitate spreading on the surface)

Biocides andfungicides

Thickeners

Heat and lightstabilizers

Coalescing agents

Plasticizers

Defoamers

Catalysts to promote cross-linking

Postupci nanoenja tekuih organskih prevlaka

Brushing and rolling

Spraying

Dip coating (immersion)

Flow coating (showering)


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Suenje i stvrdnjavanje

Drying- process in which organic coating converts from liquid to solid

Many organic coatings dry by evaporation of their solventsCuring- in order to form a durable film on the substrate surface, a further conversion is

necessary

This involves a chemical change in the organic resin in which polymerization orcross-linking occurs to harden the coating

Primjer primjene automobil

Typical sequence applied to the sheetmetal car body in a mass production automobile:

1. Phosphate coat applied by dipping car body

2. Primer coat applied by dipping car body

3. Color paint coat applied by spray coating

4. Clear coat (for high gloss and added protection) applied by spraying

Emajliranje

Porcelain coatings are valued for:

Their beauty and color

Smoothness and ease of cleaning

Chemical inertness and general durability

Porcelain enameling is the name given to the technology of these ceramic coatingmaterials and the processes by which they are applied

Materijali i proizvodi

Porcelain = a ceramic made from kaolin, feldspar, and quartz

Substrates: steel, cast iron, and aluminum as a vitreous porcelain enamel

Products: sinks, bathtubs, lavatories, ranges, water heaters, washing machines,dishwashers, jet engine components, automotive mufflers, and electronic circuit

boards

Termiko trcanje

Spraying of molten coating materials onto a substrate, where they solidify and adhere tothe surface

Coating materials: pure metals and metal alloys; ceramics (oxides, carbides, andcertain glasses); other metallic compounds (sulfides, silicides); cermet composites;and certain plastics (epoxy, nylon, teflon, and others).

Substrates: metals, ceramics, glass, some plastics, wood, and paper

Heating technologies: oxyfuel flame, electric arc, and plasma arc


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Primjena

First applications were to rebuild worn areas on used machinery components and to

salvage workparts that had been machined undersize Also used in manufacturing as a coating process for corrosion resistance, high

temperature protection, wear resistance, electrical conductivity, electrical resistance,electromagnetic interference shielding

Tvrde prevlake

Surfacing technique in which alloys are applied as welded deposits to substrate metals

Distinguishing feature is that fusion occurs between coating and substrate, whereas thebond in thermal spraying is typically mechanical interlocking which does not stand upas well to abrasive wear

Especially suited to applications requiring good wear resistance

Applications: coating of new parts and repair of heavily worn, eroded, or corroded partsurfaces

Literatura1. Kolumbi Z., Tomac N.: Materijali podloge za diskusiju; Sveuilite u Rijeci;

Filozofski fakultet, Odsjek za politehniku, Rijeka 2005; http://www.ffri.hr/~zvonimir/.

2. Filetin T.: Svojstva i karakteristike materijala Katalog opisa; Sveuilite u Zagrebu,

Fakultet strojarstva i brodogradnje, Zagreb, 1994.3. Smoljan B., Osnove toplinske obrade elika, Sveuilite u Rijeci, Pedagoki fakultet,

Rijeka 1997.

4. Krumes D., Toplinska obradba, Strojarski fakultet, Slavonski Brod, 2000.
http://www.ffri.hr/~zvonimir/http://www.ffri.hr/~zvonimir/

5 Prilagodba materijala

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