Corrosion behaviours of typical metals in molten hydrate salt of Na₂HPO₄·12H₂O – Na₂SO₄·10H₂O for thermal energy storage

Abstract

The corrosion behaviours of stainless steel 316 (SS-316), copper (Cu) and aluminium 1060 (Al-1060) in composite molten hydrate salt phase change materials (PCM) composed of Na 2 HPO 4 •12H 2 O -Na 2 SO 4 •10H 2 O have been studied by electrochemical measurements in conjunction with SEM and XRD. The corrosion resistance of the metals in the composite molten PCM follows SS-316 > Cu > Al-1060. The role of the composite molten PCM affecting the corrosion resistance and its mechanisms of the metals are as following: the experiments reveal that SS-316 readily gets passivated in the composite molten PCM medium with no considerable corrosion; Cu suffers an initial rapid anodic dissolution which is further impeded by the formation of spot-like salt precipitates on the substrate, mainly consisting of CuHPO 4 •nH 2 O, CuSO 4 •nH 2 O and Cu 2 O; Al-1060 shows a continuous large dissolution in the studied PCM with a porous corroded surface morphology due to the complete destruction of passive film under the attack of OHions.

Corrosion behavior of aluminum in molten hydrated salt phase change materials for thermal energy storage

“…With the decrease of environmental pH, the corrosion potential of Al-1060 (E corr ) shifts to more noble values, which is consistent with the previous OCP observation, indicating the changes of Al-1060 surface film. However, the course of each polarization curve is similar during the whole anodic process which is in reasonable agreement with the Al-1060 behavior reported previously [19].…”

“…From figure 6(d), the corroded surface morphology of specimen subjected to PCM medium at pH 9 exhibits flake-like deposits and dense-micro pores. As discussed in previous work [19], these surface discontinuities characterize the continuous aluminum/oxide dissolution in alkaline environment, mainly through equations (13)- (15). Elemental analysis of corroded surface shows that the ratio of Al to O atoms reaches over 3 in the case of pH 9 which is indicative of a weak protection of surface oxides and a large area of exposed aluminum substrate.…”

“…The studied inorganic hydrated salt PCM composite was prepared in chemical purity of Na 2 HPO 4 ·12H 2 O and Na 2 SO 4 ·10H 2 O with a composition ratio (wt%) of 9:1, which has been reported in our pervious works as an efficient thermal energy storage medium [17][18][19]. The prepared PCM was mix and melted at 60°C by keeping magnetic stirring in a thermostatic water bath.…”

“…By contrast with acidic dissolution, the electrochemical process in alkaline medium contains consecutive formation and breakdown of the aluminum oxide film, in which aluminate ions rather than aluminium ions participate. 4 2− anions, also make essential contributions in the generating of surface oxide layer during the aluminum dissolution process, in which the precipitation of AlPO 4 ·H 2 O is highly likely [19,39]. The corrosion process and product property can be consequently affected by the incorporation of phosphorus through the following.…”

“…In previous studies, the thermal property of Na 2 HPO 4 ·12H 2 O-Na 2 SO 4 ·10H 2 O composite PCM was evaluated by combing with solar heating system [17,18] and its preferential compatibility with conventional container materials has been studied via electrochemical methods [19]. Results show that the thermal converting efficiency of domestic solar heating system was significantly enhanced up to 66.4% with the addition of studied PCM, while corrosion analysis indicated a severe dissolution that aluminum container could suffer when in contact with this PCM basically induced by the hydrolysis of the major specie of studied PCM, i.e., H 2 PO 4 others for PCM storage.…”