Effect of Disulfite on the Solubility of Molybdate and Passivation of Iron in Concentrated LiBr Solution

Abstract

[Introduction]Absorption refrigerator utilizes highly corrosive concentrated LiBr solution as an absorbent for water vaper. In order to inhibit corrosion of carbon steel, LiOH and key inhibitor of Li2MoO4 are added into the absorbent. LiNO3 is also used as a strong passivator in order to accelerate passivation of steel especially at a startup stage. Because of the limited solubility of Li2MoO4 in the absorbent, its concentration diminishes during long-term operation not only by consumption by the reaction with steel but also by precipitation at lower temperature part of refrigerator, i.e. in an absorber. In this research, concentration change of molybdate in the absorbent is studied at the absorber temperature (40ºC) and additives that inhibit the precipitation of Li2MoO4 are searched. The effect of precipitation inhibitor on the passivation of iron are also evaluated through electrochemical measurements. [Experimental]The composition of the test solution is 17.3 mol kg-1 LiBr + 0.1 mol kg-1 LiOH with initial concentration of Li2MoO4 of 3.0×10-3 mol kg-1, where Li2MoO4 is supersaturated at 40ºC. The change in concentration of Li2MoO4 over time is measured at 40ºC. Several oxoacid salts are tested as a precipitation inhibitor for Li2MoO4. Electrochemical measurements are carried out for iron in the deaerated test solution at 155ºC in order to examine the effect of precipitation inhibitor (sodium disulfite) on the passivation of iron. [Results and discussion]The concentration of molybdate in the test solution (initially 3.0×10-3 mol kg-1) reduced gradually at 40ºC and became almost constant around 5.0×10-4 mol kg-1. The addition of sodium carbonate, sodium nitrate, sodium borate, sodium phosphate was not effective at all, while sodium sulfate delayed the concentration decrease of molybdate. Sodium disulfite and sodium sulfite (1.0×10-2 mol kg-1) showed noticeable effect: sodium disulfite, in particular, kept the molybdate concentration above 2 × 10-3 mol kg-1 at 40ºC for long time. The effect of disulfite was kept even after the test solution experienced heating up to 155 ºC with additional passivator of lithium nitrate. The time variation of corrosion potential of iron in the test solution (17.3 mol kg-1 LiBr + 0.1 mol kg-1 LiOH+ 3.0×10-3 mol kg-1 Li2MoO4) at 155 ºC indicated that passivation of iron was delayed somehow by the addition of 1.0×10-2 mol kg-1 disulfite: it took about 10 h to reach passive region, while 5 h was needed in the absence of disulfite. But the delay was made up for by the further addition of nitrate passivator: the corrosion potential shifted into passive region soon after temperature reached 155 ºC irrespectively of the presence of disulfite. Polarization curves for iron also indicated harmlessness of disulfite against passivation behavior of iron. The results suggested usefulness of disufite salt as a precipitation inhibitor for Li2MoO4 with no harm in passivation of steel.