Research On Corrosion Inhibition Of Triazinedithiols Towards Aluminum Alloy In Hydrochloric Acid

Acid pickling is usually used in industry for removing rust and dirt on metal surface.Inhibitors are added in the system to decrease unnecessary loss of metal in pickling processand excessive consumption of acid solution. However, inhibitors, especially chromate andphosphoric acids, used in industrial production in recent years, are toxic and have caused highattention of environmental protection departments, thus the application of them wereprohibited in infrastructure construction and marine engineering. Therefore, the research ofefficient and environmental-friendly inhibitor has become a hotspot of corrosion sciencenowadays. Triazinedithiols are rich in heteroatoms, such as S and N, which can be consideredas promising potential environmental-friendly inhibitors due to their strong chemical activityand nontoxic property.The corrosion inhibition abilities of four triazinedithiol compounds (6-N,N-diallylamino-1,3,5-triazine-2,4-dithiol monosodium, DAN;6-N,N-dibutylamine-1,3,5-triazine-2,4-dithiolmonosodium, DBN;6-N,N-bis(6-methylheptyl)amine-1,3,5-triazine-2,4-dithiolmono-sodium,iso-DON6-(N-allyl-1,1,2,2-tetrahydroperfluorodecyl)amino-1,3,5-triazine-2,4-dithiol mono-sodium, AF17N)) toward aluminum were studied in the research. Weight loss, electrochemicalmeasurements, scanning electron microscopy and adsorption isotherm were used to study theinhibition efficiency and mechanism of these four compounds on AA5052aluminum alloycorrosion in1mol·L-1HCl solution. The effect of inhibitor concentration, etching time andtemperature on corrosion inhibition efficiency was discussed. The results are as follows:(1) The inhibition ability order of four triazinedithiol compounds was DAN≈DBN＞iso-DON＞＞AF17N. The inhibition efficiency was high just in low concentration(C=0.03mmol/L) of DAN and DBN (91.67%and84.25%, respectively), and increased withthe increasing of concentration. With the increasing of temperature, the inhibition effects ofDAN and DBN were decreased. The corrosion inhibition effects of DAN and DBN increaseda little at first and then decreased with the increasing of immersion time.(2) The corrosion inhibition efficiency of AF17N was the lowest with maximum valuelower than40%at20℃.The maximum value of inhibition efficiency of iso-DON decreasedfrom87.64%to30.20%when the temperature increased from20℃to30℃. Iso-DON andAF17N were flocculated to some extent in acid solution, and the ion concentrations of them decreased, which supposed to the interaction between the-S-of triazine ring and H+in acidsolution to form–SH, and the spatial effect of long branch chain in molecule structure(iso-DON:-CH2(C2H5)CH(CH2)3CH3; AF17N:-CH2CH2(CF2)7CF3). Therefore, it is not fit touse iso-DON and AF17N as corrosion inhibitor in acid solution.(3) The open circuit potential decreased from-0.738~-0.740V to-0.744~-0.752V byadding DAN and DBN. The results of polarization curves indicated that the corrosion currentdensity decreased distinctly with DAN and DBN, and the inhibition efficiency increased withthe increasing of concentration, which reached97.77%and98.83%at1mmol·L-1. Thecathode Tafel slop increased with increasing DAN and DBN concentrations, and the corrosionpotential remain unchanged. It was considered that DAN and DBN were mixed inhibitor. Theresult of EIS indicated that the size of capacitive loop and inductive loop became larger withthe increasing of DAN and DBN concentrations.(4) Adsorption model of DAN on AA5052aluminum alloy surface at differenttemperature was proved to be Langmuir isotherm. Adsorption model of DBN on AA5052aluminum alloy surface was Langmuir isotherm at30~35℃, Frumkin isotherm at40℃,Freundlich isotherm at50℃. Both Frumkin isotherm and Freundlich isotherm were found at45℃. Standard adsorption enthalpy ΔHθ, apparent activation energy Ea, standard adsorptionentropy ΔSθand standard adsorption free energy ΔGθall increased with increasingtemperature, adsorption mechanism turned from chemisorption to physical and chemicaladsorption, monolayer adsorption film turned to multilayer, the homogeneity of adsorptionactive site on metal surface might be change.(5) The result of quantum chemical calculation indicated that DBN has better corrosioninhibition ability, because its highest occupied molecular orbital energy EHOMOand energy gapΔE between highest occupied molecular orbital energy and lowest unoccupied molecularorbital energy were higher.