Researches On Corrosion Mechanism Of Carbon Steel, Copper And Copper-Nickel Alloy For Lithium Bromide Refrigerator

Lithium bromide absorption chiller has received increasing attention in recent years owing to the virtues of high thermal efficiency, wide heat resources, low energy consumption and zero discharge. The exploitation of multiple cycles, which operates at higher LiBr concentration and operation temperature, has mainstreamed all over the world for the purposes of improving thermal efficiency and miniaturization. The lithium bromide solution with strong caustic can cause serious corrosion problems of carbon steel, copper and copper alloy, which become more serious in new type chiller since the LiBr concentration and the operation temperature are both increased. Those problems are desiderated to be solved in exploiting multiple cycles.In this paper, the effect of inhibitors such as LiOH, Na2Mo04, LiNO3 and BTA on corrosion resistance of carbon steel, copper and copper-nickel alloys in 55%LiBr65%LiBr solutions are studied by weight-loss tests and electrochemical measurements. The results indicate that the corrosion rates of carbon steel decreases to 8.39u m/a and 4387u m/a respectively after adding m100g/L BTA to the boiling 55%LiBr+0.07mol/L LiOH+150mg/L Na2MoO4 and the boiling 60%LiBr+ 0.07mol/L LiOH+150mg/L Na2MoO4 solutions, which the dissolution of the alloy is effectively inhibited. The reason is that Fe-BTA complex compound and the passive film composed of Fe-BTA, oxides of Fe and Mo form on the carbon steel surface in 55%LiBr and 60%LiBr solutions, respectively, which prevents Br" from absorbing on the metal surface. When the operating temperature is lower than 200癈, the dissolution of carbon steel is effectively inhibited either adding 0.07mol/L LiOH+150mg/L Na2Mo04+100mg/L LiNO3 or 0.07mol/L LiOH+150mg/L Na2MoO4+100 mg/L BTA in 55%LiBr-65%LiBr solutions. However, the corrosion rate of carbon steel increases sharply when the temperature is higher than 200C except that of in 55%LiBr solution. The adsorptions of BTA and Na2MoO4 on carbon steel surface can be well fitted to Langmuir adsorption isotherm in boiling 55%LiBr solution. The free energies of adsorption of BTA and Na2MoO4 are -45.07 kj/mol and -40.69kJ/mol, respectively.The corrosion data of copper and copper-nickel alloy are obtained in boiling 55%LiBr-65%LiBr solutions containing LiOH, Na2MoO4, BTA and LiN03. The corrosion rates of copper and copper-nickel alloy are decreased when adding LiOH, Na2MoO4 or BTAto boiling 55%LiBr~65%LiBr solutions individually. Their inhibition properties decrease with the increasing of LiBr concentration and boiling temperature. The effect of LiOH on corrosion behavior of copper-nickel alloy is similar to that of copper. Low LiOH concentration is of benefit to forming oxide film. While, high LiOH concentration results in dissolution of oxide films. The optimal concentrations of LiOH are O.100mol/L and 0.15mol/L for copper and copper-nickel alloy respectively. The corrosion rates of copper and cooper-nickel alloy have a slight change after the concentration of Na2MoO4 or BTA up to a certain value. The inhibition effect order of the four inhibitors on copper is LiOHBTA Na2Mo04>LiN03. As to the copper-nickel alloy, the inhibition effect sequence is BTA Na2MoO4>LiOH>LiNO3.In boiling 55%LiBr solution, the adsorptions of BTA and Na2Mo04 on copper surface can be well fitted to Langmuir adsorption isotherm. The free energies of adsorption of Na2MoO4 and BTA are -41.15 kJ/mol and -44.93kJ/mol, respectively.Neutral network model based on the experimental data is developed for the computation of corrosion rate relating to temperature, LiBr concentration, LiOH concentration, Na2Mo04 concentration, LiNO3 concentration and BTA concentration. The predicted results agree with the experimental data well. This indicates that the trained BP network can be used to satisfactorily predict corrosion rate of carbon steel in different concentration LiBr solutions containing various inhibitors at different temperatures. It provides a novel method for making corrosion monitoring and corrosion control of metals into realization in the intell...