Influence of temperature on the corrosion behavior of tin in NaCl solution : master thesis

Praćenjem vremenske promjene potencijala otvorenog strujnog kruga, primjenom polarizacijskih metoda te elektrokemijske impedancijske spektroskopije proučavano je korozijsko ponašanje kositra u 0.5 M NaCl otopini kod različitih temperatura (10, 20, 30 i 40 o C) i pH vrijednosti otopine (3.0, 5.5 i 8.5). Oštećenja na površini metala određena su optičkim mikroskopom. Najveću otpornost prema koroziji kositar pokazuje u NaCl otopini pri 20 o C i pH = 8.5. Pri ovim eksperimentalnim uvjetima zabilježena je najveća vrijednost polarizacijskog otpora, R p , zatim najpozitivnija vrijednost korozijskog potencijala, E kor , i potencijala depasivacije, E dp te najmanja vrijednost korozijske, i kor , i pasivacijske struje, i pas . Također smanjenjem temperature i porastom pH vrijednosti NaCl otopine smanjuju se lokalna oštećenja površine uzorka. Korozijska otpornost kositra u NaCl otopini posljedica je formiranja površinskog oksidnog sloja čija svojstva ovise o eksperimentalnim uvjetima (temperatura, pH vrijednost, vrijeme izlaganja). Naime, impedancijska mjerenja provedena na potencijalu otvorenog strujnog kruga pokazuju da je otpor oksidnog filma na kositru kod svih ispitivanih temperatura dosta visok (reda veličine nekoliko desetina k cm 2 ) te da raste s vremenom izlaganja, naročito u otopinama nižih temperatura. Paralelno s navedenim, vremenom lagano raste i debljina oksidnog filma (brzina porasta debljine oksida ne ovisi o temperaturi otopine). Rezultati svjetlosne mikroskopije potvrđuju nepovoljan utjecaj temperature na zaštitna svojstva površinskog filma na kositru pri prolongiranom izlaganju NaCl otopini.The corrosion behaviour of tin in 0.5 M NaCl solution at different temperatures (10, 20, 30 and 40 o C) and solution pH values (3.0, 5.5 and 8.5) was investigated by observing the variation of the open circuit potential with time, applying polarisation methods and electrochemical impedance spectroscopy. Damage on the metal surface was determined using an optical microscope. Tin exhibits the highest corrosion resistance in NaCl solution at 20 o C and pH = 8.5. Under these experimental conditions, the highest value of polarisation resistance, R p , was recorded, followed by the most positive value of corrosion potential, E kor , and depassivation potential, E dp , and the lowest value of corrosion, i kor , and passivation current, i pas . Lowering the temperature and increasing the pH of the NaCl solution reduces local damage to the surface of the sample. The corrosion resistance of tin in NaCl solution is a consequence of the formation of a surface oxide layer, the properties of which depend on the experimental conditions (temperature, pH, exposure time). Indeed, open circuit impedance measurements show that the resistance of the oxide film on tin is quite high (of the order of a few tens k cm 2 ) and that it increases with exposure time, especially in lower temperature solutions. In parallel, the thickness of the oxide film also increases slightly with time (the rate of increase of the oxide thickness does not depend on the temperature of the solution). The results of light microscopy confirm the unfavourable influence of temperature on the protective properties of the surface film on tin during prolonged exposure to NaCl solution