IRON AND STEEL CORROSION PERTEMUAN 11 Matakuliah : S0372 – Kimia Teknik Sipil Tahun : Ganjil 2007/2008
IRON AND STEEL CORROSION Antecedent. Influence of pH. Corrosion in Ground Water. Grey Cast Iron.
ANTECEDENT • Iron if there are any in aggresive electrolyte environment hence will be dissolve like equation of the following reaction: Fe Fe2+ + 2e Amount of dissolve will as according to current of katodik. Its reaction of this him take place without many polarization because its reaction very quickly. Speed of reaction of corrosion determined by reaction of katodik which [is] its reaction shall be as follows: 2H+ + 2e H2
Its exit gas of H2 will very quickly if there is in sour environment. Become iron very dissolve in acid with reaction, the following: Fe + 2H+ Fe2+ + H2 But on the contrary will react very tardy if in environment of basa: Fe3+ + 3OH- Fe (OH )3 The forming of Fe(OH)3 represent sediment which forming endue covert till corrosion will be pursued, till NaOH can be barreled by steel.
INFLUENCE OF PH • At pH 4. In situation of sour environment as according to equilibrium diagram of E-Ph (iron – H2O) will not be formed by passive oxide. Reaction : 2H+ + 2e H2 Will walk very quickly because in sour environment (giving high current strength and low tension). Become will give two reaction: Fe Fe2+ + 2e (very quickly) 2H+ + 2e H2 (very slowly) Fe + 2H+ Fe2+ + H2
At addition of oxygen will be formed by reaction: 2H+ + 2e H2 Will be quickened by polarization and with addition of oxygen hence its reaction: O2 + 4H+ + 4e 2H2O Forming of gas of H2 will be able to change, its result will give current of korosi big. • 4 < pH < 8 (seeing diagram of E - pH of Fe – H2O) When there no O2, under is potential of “P” hence there will be current of corrosion very big, because next to nothing polarization.
8 < pH < 11 In environment which is basa mean reaction of anodik will very tardy caused by in small potential area. Potential of “P” pasiva will be keep handy in reaction of alkalis because in alkali environment will be difficult once incidence gas of H2. With oxygen in environment of alkalin will easy to happened pasiva.
CORROSION IN GROUND WATER • Pregnant ground water some element in number which vary consist of CO3-, HCO3-, Cl-,SO4, and SiO2and kation Ca2+, Na+, K+ and pittanceMg2+ dan Al3+. • Concentration water which enough is8 or 12oF to form sediment of yg functioning and flimsy as enduing is covert, colour of sediment is turning white or turn white brass. • If water concentration only 8oF, hence formed sediment do not become to endue impervious and homogeneous covert. If just a few is of acid contents or happened oxygen cell hence will happened cell of corrosion. • If water concentration is high, for example 30 – 40oF, hence chalk sediment will be thick. I too thick hence will harm at usage of heat, because that sediment have the character of isolator till pursue transfer of heat. As a result will happened too hot temperature, which in a moment will happened explosion like at steaming kettle by boiler stone. danger of corrosion will start if boiler stone sediment arise also iron rust of yg tan.
GREY CAST IRON • Grey cast iron if hitting ground water and also its land;ground will happened corrosion which its event very loo like with steel. But for time old ones resilience of corrosion grey cast iron is better than steel, especially if that cast iron do not get mechanic style. • This matter because of rate and graphite The high, till its product of him will fastening each other till form to endue impervious covert. • But this coat don't have high strength till grey cast iron will remain to hold up corrosion during functioning as object which not accept style like at coverr, etcetera.