Emprego da dissolução eletrolítica no monitoramento eletroquímico imediato de cromo, níquel e manganês em aços inoxidáveis

Abstract

Chemical analysis of the stainless steels is usually carried out using spectrometric techniques after sample preparation using acid digestion. This approach, in addition to having a high cost, is not considered eco-friendly, given the excessive use of reagents. In this context, new analytical approaches using electrolytic dissolution with immediate electrochemical monitoring of chromium, nickel and manganese in stainless steel samples were developed and successfully applied to seven different samples supplied by the Aperam company. Firstly, an unmodified gold electrode and a solution composed of HNO3 0.6 mol L-1 and H2O2 180.0 mmol L -1 were used for the electrolytic dissolution of the stainless steels and the quantification of chromium using Linear Scanning Voltammetry (LSV). Optimizations of the experimental conditions returned a sensitive method, with detection and quantification limits (LoD and LoQ) equal to 0.08 and 0.25 µmol L-1 respectively. In addition, the application of an electrical charge equal to 30 mC and an anodic current density equal to 0.8 mA cm-2 provided the highest dissolution efficiency for the steel samples, resulting in a repeatable process with relative standard deviation (RPD) equal to 3% performed in just 75 s. Similarly, the quantification of nickel content in stainless steels using the electrolytic dissolution and Square Wave Voltammetry with Adsorptive Redissolution (SWVAR) was carried out employing a new sensor based on a composite material made up of graphite and dimethylglyoxime (80:20 % m/m) immobilized on 3D printed supports (3Ds-MC/DMG). The electrode was properly characterized and all the voltammetric conditions were evaluated, obtaining LoD equal to 0.03 and LoQ equal to 0.09 µmol L-1 in a 0.1 mol L-1 ammonium buffer solution, pH = 9. Due to the alkaline nature of the electrolyte support, the electrolytic dissolutions of the stainless steels were carried out in small disposable reactors, where a 0.2 mol L-1 HCl solution, an electrical charge equal to 2.0 C and an anodic current density of 15.4 mA cm-2 provided higher efficiencies and greater repeatability (RSD = 3.3%). Finally, a fully 3D-printed sensor using acrylonitrile butadiene styrene thermoplastic (ABS) as a support and a conductive filament based on polylactic acid (PLA) and graphite (3Ds-PLA/GRA) was used to determine the manganese content in stainless steel samples by Differential Pulse Voltammetry with Cathodic Redissolution (DPVCR) in 0.1 mol L-1 BR buffer solution, pH = 4. The sensor was properly characterized and the voltammetric conditions were optimized, giving the method LoD and LoQ equal to 0.028 and 0.084 µmol L-1 respectively. Electrolytic dissolution was carried out directly in the voltammetric cell, with the most efficient conditions equal to 1.1 C of electrical charge and 15.4 mA cm-2 of anodic current density, giving repeatability with a RSD of just . 3.3%. In general, the chromium, nickel and manganese contents obtained for the seven stainless steel samples analyzed are statistically equal to the reference values (95% reliability), indicating the adequate accuracy of the methods developed. Furthermore, these methods proved to be sustainable and could easily be implemented in steelmaking laboratories.