INTERMEDIATE2 1 Corrosion

Transcription

1 INTERMEDIATE2 1 Corrosion CORROSION OF METALS Corrosion is a chmical raction which involvs th surfac of a mtal changing from an lmnt to a compound. Th mor ractiv th mtal th quickr it corrods. Many of th mor ractiv mtals, including, ar lost through corrosion. EXAMPLE 1 Aluminium corrods slowly in air : Al + -> Al 2 O 3 EXAMPLE 2 corrods slowly in moist air (Rusting). What happns during rusting? Both OXYGEN AND WATER ar rquird as th following xprimnt shows : OIL H O (prvnts accss 2 of Oxygn) O 2 H 2 O rust Silica gl (absorbs watr from th air) RUSTING NO RUSTING NO RUSTING This is what happns to th : is oxidisd in two stags : -> > 3+ + Th lctrons lost by th ar pickd up by th Oxygn and Watr which ar thrfor rducd in th procss : + 2 H 2 O + 4 -> 4 OH- Hydroxid ions ract with (III) ions to form th brown prcipitat, (III) hydroxid (rust?) : OH- -> 3+(OH-) 3

2 INTERMEDIATE2 2 Corrosion EXPERIMENT Plac an nail in rroxyl indicator solution. rroxyl indicator nail rroxyl indicator turns BLUE in th prsnc of 2+ ions. A BLUE colour appars round th showing that 2+ ar formd. rroxyl indicator turns PINK in th prsnc of OH- ions. A PINK colour should appar round th showing that OH- ar formd. Th is usually swampd by th intns blu colour and difficult to s. Salt (Sodium chlorid Na+Cl-) sprad on roads, incrass th corrosion rat of car bodywork. As corrods an ara of positiv charg builds up around its surfac du to th (II) ions. This rpls any additional (II) ions which try to form and thus slows down corrosion. Th chlorid ions, from th Sodium chlorid, diffus into th ara of positiv charg around th, canclling out th charg and allowing mor (II) ions to form.g. + - Na Cl

3 INTERMEDIATE2 3 Corrosion PREVENTING CORROSION Rusting can b prvntd by stopping th Oxygn and Watr from gtting to th using a barrir.g. oil, gras, paint, plastic or mtal : H Paint Plating with Tin is a common xampl of th 'barrir' mthod. It has on drawback. If th Tin layr cracks thn Oxygn and Watr can gt to th. A cll dvlops: lctrons flow from th most ractiv mtal () to th last ractiv mtal's ions (Tin (II)). This allows th to form ions and corrod. 2+ H Tin Corrosion is mor rapid than in an uncoatd pic of : th Tin(II) ions pull lctrons off th : -> Sn > Sn W can, howvr, larn from this disastr. W can dvlop an lctrical mthod of prvnting corrosion. Elctrons ar lost during th first stp in th corrosion of : -> If w forc ths lctrons back on to th w will rvrs th first stp >

4 INTERMEDIATE2 4 Corrosion This can b don in TWO ways : 1. Conncting th to th ngativ trminal of a battry: - + Hydrogn gas dvlops blu colour dvlops frroxyl indicator At th anod (+v) lctrons ar pumpd away from th, spding up th corrosion rat : -> and lading to th production of 2+ ions which turn th indicator blu. At th cathod (-v) lctrons ar pumpd on to th, slowing down th corrosion rat. NO (II) ions ar producd so NO blu colour appars. Instad a appars! Rmmbr that Watr, du to its ionisation, contains H+ ions. H 2 O H+ + OH- As th lctrons ar pumpd on to th th xcss lctrons 'spill ovr' into th solution whr thy ar pickd up by H+ ions in th Watr : 2 H > H 2 Rmoval of th Hydrogn ions from Watr lavs xcss hydroxid ions. Hydroxid ions turn rroxyl indicator pink.

5 INTERMEDIATE2 5 Corrosion 2. By conncting th to a mor ractiv mtal (.g. Magnsium) Elctrons flow from th mor ractiv mtal (Magnsium) to th lss ractiv mtal's ions ((II)) thus prvnting loss of lctrons by : Mg -> Mg > Th dirction of lctron flow can b shown with a cll : Magnsium Hydrogn gas V Hydrogn gas frroxyl indicator NO blu colour appars round th so no (II) ions ar bing producd. Excss lctrons ar capturd by Hydrogn ions in th Watr forming Hydrogn gas : 2 H > H 2 This lavs xcss OH- ions which turn th indicator pink. Magnsium scrap is connctd to oil piplins mad of to prvnt corrosion. Notic that, as a rsult of protcting th, th Magnsium loss lctrons, forms ions and corrods itslf. Th Magnsium thrfor sacrifics itslf to sav th (Sacrificial Protction). Galvanising (coating with Zinc) is th most common us of sacrificial protction. If th Zinc layr cracks: H Zinc th Oxygn and Watr can gt to th but corrosion still dos not occur bcaus lctrons flow from Zinc to prvnting loss of lctrons by. Bridg girdrs ar galvanisd.