The Effect of Vanadate, Phosphate, Fluoride Compounds on the Aqueous Corrosion of Magnesium Alloy AZ31 in Dilute Chloride Solutions

Abstract

The anodic polarization response of magnesium alloy AZ31 was first characterized during exposure to aerated 0.1 M NaCl solutions with millimolar additions of NaVO3, Na3PO4, Na2HPO4, NaF and various pairings to assess their ability to inhibit corrosion kinetics and retard localized corrosion. Each of the candidate inhibitors reduced the corrosion rate of the alloy to some degree. A Na3PO4–NaVO3 pair produced a good inhibiting effect decreasing the corrosion rate to about 10−7 A/cm2, which was two orders of magnitude lower than the uninhibited control case. A Bliss Independence assessment indicated that this inhibitor pair acted synergistically. A Na2HPO4–NaVO3 pair reduced the corrosion rate to 10−6 A/cm2 but was not assessed to be acting synergistically. The NaVO3–NaF pair did not reduce the corrosion rate significantly compared to the control case and was an antagonistic pairing. SEM imaging showed film formation due to exposure, which appears to be the origin of the observed inhibition. The resistance to localized corrosion was assessed as the difference in the breakdown potential and the corrosion potential, with larger values indicating a lower probability of localized corrosion during free corrosion exposures. The effects of the inhibitors on this characteristic were mixed, but each of the inhibitor pairs yielded potential differences in excess of 100 mV. A conceptual conversion coating process based on a mixture of vanadate and phosphate compounds were demonstrated. A fluoride-bearing formulation produced coatings whose total impedance was increased by a factor of two compared to an uncoated control. A fluoride-free formulation produced coatings whose corrosion resistance was increased by more than a factor of three.

Corrosion Protective Film Formation on Mg Alloy AZ31 by Exposure to Dilute Selenite Solutions

“…Exposure in a higher concentration (5 mM and 10 mM) versus a lower concentration (1 mM) resulted in an increment of 0.14 V of breakdown potential. The breakdown potential and the corrosion potential may be assessed as the resistance to localized corrosion, with larger values indicating a lower probability of localized corrosion during free corrosion exposures [ 15 ]. Therefore, the coating in a high selenite concentration bath benefited the resistance to localized corrosion.…”

“…Compared with other conventional Mg coatings in a similar testing environment, the Se-based protective film has better performance than vanadate, stannate and Ce 3+ , has similar performance with PO 4 3− and Ca 2+ , and can be comparable to chromate [ 12 , 15 , 19 , 26 , 27 , 28 , 29 , 30 , 31 , 46 , 47 ]. Since the study of the selenite-based protective film was blank before.…”

“…Unfortunately, Mg is a very active metal and vulnerable to corrosion attacks when exposed to aqueous environments or humid air [ 1 , 8 , 9 ]. A number of approaches like alloying, coating, surface modification, and chemical inhibition have been addressed in the R&D community recently [ 1 , 2 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Among these approaches, the coating provides corrosion protection without affecting mechanical properties the way alloying approaches do.…”

Advances in the Modification of Silane-Based Sol-Gel Coating to Improve the Corrosion Resistance of Magnesium Alloys

“…The external methods include surface treatment and coating processes (electroplating, chemical conversion coating, electrochemical conversion coating, laser/ion or electron beam treatment, sol-gel coating, and organic coating, etc.) [ 6 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Among them, the surface coating method may be the most simple and direct method.…”

Na2SnO3 functions as outstanding magnesium alloy passivator by synergistic effect with trace carboxymethyl chitosan for Mg–air batteries for standby protection

“…[25][26][27][28] For example, Feng et al investigated the effect of the addition of NaVO 3 , Na 3 PO 4 , Na 2 HPO 4 , NaF, and various pairings of these inorganic salts on the polarization response of AZ31 exposed to 0.1 M NaCl solution, and the result suggested that all of these inorganic salts and their pairing reduced the corrosion rate of AZ31 except for NaF, in which the 10 mM Na 3 PO 4 -4 mM NaVO 3 inhibitor pair exhibited the best inhibition effect, decreasing the corrosion rate of AZ31 by nearly two orders. 29 Zhou et al suggested that the maximum inhibition efficiency of AZ31 in 3.5% NaCl reached 81.5% at a Na 3 PO 4 concentration of 1.0 g L À1 , while the Na 3 PO 4 concentration keeps on increasing; the inhibition efficiency decreases due to a remarkable shedding of the product layers. 30 For Mg-anode batteries, finding an electrolyte additive that not only activates the anode surface passivated layer but also reduce the self-corrosion rate of the Mg anode during the nodischarging periods is important.…”