Corrosive Study On X70 Steel In Sand Contaminated By Zn（NO₃）₂ And Cu（NO₃）₂

Corrosive behavior and mechanism of X70 steel in sand contaminated by Zn(NO3)2 and Cu(NO3)2 were investigated by modern techniques such as macro images, scanning electron microscope(SEM), polarization curves(PC), electrochemical impedance spectroscopy(EIS) and energy dispersive spectrum(EDS).Corrosive macro and micro photoes of X70 steel showed that corrosive degree of X70 steel in Cu(NO3)2 sand is higher than that in Zn(NO3)2 sand. Corrosive degree become greater with the increase of Cu2+ content. Corrosive type of X70 steel in Zn(NO3)2 and Cu(NO3)2 sand are both local corrosion with lower content and uneven global corrosion with higher content.Energy dispersive spectrum(EDS) testing results showed that corrosive products of X70 steel in Zn(NO3)2 sand contains the inner layer of Fe compounds and outer layer of Zn compounds. However, corrosive products of X70 steel in Cu(NO3)2 sand contains the inner layer of Cu and outer layer of Fe compounds and Cu compounds.It can be concluded from the polarization curve(PC) that the corrosive rate of X70 steel is accelerated by both Zn(NO3)2 and Cu(NO3)2. Corrosive rate of X70 steel in Zn(NO3)2 sand increases with time, and the corrosive rate reaches the maximum in low contents. With increasing of Zn(NO3)2 content, the corrosive rate are increasing, decreasing and increasing at 7 days, and the peak of corrosive rate appears when the Zn(NO3)2 content is 0.2%. Corrosive rate became increase, decrease and tend to be stable at 35 days, and the peak of corrosive rate appears when the Zn(NO3)2 content is 0.1%. Corrosive rate of X70 steel in Cu(NO3)2 sand increases with increasing of Cu(NO3)2 content, but it dropped dramatically when the Cu(NO3)2 content greater than 1.0%. At the same time, the corrsive potential of X70 steel become positive in a large extent.It can be concluded from the electrochemical impedance spectroscopy(EIS) of x70 steel that two capacitive arcs both constitute the Nyquist graph in uncontaminated sand. For Zn(NO3)2 sand, the Nyquist graph of X70 steel consists of two capacitive arcs and a impedance arc in low frequent region at 7 days and the Nyquist graph consists of a capacitive arc in high frequent region and Warburg resistance in low frequent region at 35 days. However, for Cu(NO3)2 sand, the Nyquist graph of X70 steel showed two capacitive arcs when content is low, and Warburg appeared in low frequent region when content is high at 35 days.The corrosion mechanism analysis showed that the anodic process of X70 steel in Zn(NO3)2 sand is the iron oxidation reaction, and the cathodic process consist of oxygen reduction and hydrogen reduction reaction and H+ here is generated by Zn2+ hydrolysis. The anodic process of X70 steel in Cu(NO3)2 sand is the iron oxidation reaction, and the cathodic process consist of the reduction reaction of Cu2+, oxygen and hydrogen.