As curvas de polarização obtidas para o aço na solução de NaCl em diferentes sobrepotenciais catódicos iniciais podem resultar em diferentes interpretações, isto porque na região catódica podem haver mais de uma reação de redução que geram íons OH- que aumentam o pH local. A presença das hidroxilas
interfere nos parâmetros de Tafel através da formação dos hidróxidos de ferro que modificam e bloqueiam a superfície.
Os parâmetros de Tafel obtidos das curvas de polarização do aço imerso na solução tampão borato (pH 6,7) praticamente não mudam com o sobrepotencial catódico inicial aplicado, pois o efeito tampão neutralizam as hidroxilas gerada pelas reações de redução de oxigênio e água.
Os inibidores voláteis de corrosão foram dissolvidos nas soluções de NaCl e tampão borato (TB) + NaCl (pH 6,7) e avaliados por técnicas de curvas de polarização, micropolarização linear e espectroscopia de impedância eletroquímica. Os resultados obtidos pelas três técnicas eletroquímicas são concordantes, no entanto existem diferenças nos resultados obtidos entre os inibidores dissolvidos na solução de NaCl e na solução tampão (pH 6,7).
A solubilização dos inibidores na solução de NaCl 0,1 mol L-1 modificam o pH
da solução, provocando também alterações na superfície do eletrodo. A adição das aminas causam um aumento no valor de pH para valores bastante elevados (pH > 11), que pode ocasionar numa espontânea passivação na superfície pela formação de hidróxidos de ferro. Já com a adição do ácido caprílico, o pH da solução diminui tornando a superfície isenta de hidróxidos, no entanto a concentração do ácido e o pH da solução não permitem a passivação do aço. Nas soluções contendo sais de caprilatos observou-se que as variações nos valores de pH foram pequenas e o aço é passivado após 3 horas de imersão independentemente do sal de caprilato. Além disso, as curvas de polarização mostraram que na medida em que ocorre a dissolução do ferro maior será a proteção do aço, já que maior será a formação do caprilato de ferro.
Na solução de TB + NaCl 0,1mol L-1 (pH 6,7), as aminas que possuem caráter alcalino estão protonadas tornando-as ineficientes. No caso dos sais de caprilatos, a passivação do aço pode ser influenciada por dois fatores: as variações de pH local são pequenas devido ao efeito tampão; e/ou a presença das aminas induzem a
formação do caprilato de ferro insolúveis ou a adsorção destas espécies bloqueando superfície do eletrodo.
As eficiências dos inibidores obtidas nos testes eletroquímicos convencionais não são recomendadas para avaliar os VCIs, pois nestas condições o contato do oxigênio é muito menor do que na condição de vapor e o pH da solução pode modificar a superfície do eletrodo impedindo a interação Ferro-inibidor.
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