The present paper studies the influence of different cooling media (water and cryogenic media) on microstructure, mechanical, and corrosion behavior of friction stir processing of AA2014. From the electron backscattered diffraction results, it was observed that the grain size in stir zone of air-cooled friction stir processing, dry ice-cooled friction stir processing, and underwater friction stir processing are 4.9 µm, 3.5 µm, and 0.9 µm respectively, and the fraction of high angle grain boundaries are more in underwater friction stir processing sample compared to other conditions. The ultra-fine grained structure (0.9 µm) was achieved in underwater friction stir processing due to uniform heat dissipation from the processing zone to the water. Mechanical properties such as hardness and strength were improved in underwater friction stir processing compared to other conditions. The fine precipitates formed in the underwater friction stir processing sample were distributed randomly at grain boundaries, and hence corrosion resistance was improved in underwater friction stir processing sample compared to other conditions.

Material flow has a significant impact on the joint properties and is one of the most challenging aspects to be understood in dissimilar friction stir welding. The present study emphasizes the role of process parameters on material flow, mechanical properties and corrosion behavior of dissimilar friction stir welds of AA5083-AA6061. Microstructural analysis revealed that the onion ring sub-layer width observed at the stir zone was substantially changed by varying process parameters. It was understood that the higher rotational speeds promote better intermixing and enhanced mechanical properties. The notch tensile strength values were in correlation with the intermixing of materials at the stir zone and the highest notch tensile strength value was obtained at 1400 rpm and 60 mm/min. A remarkable degree of material intermixing and fragmentation of intermetallics at higher rotational speeds resulted in better corrosion resistance.

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Dissimilar friction stir welding of AA5083-AA6061 alloys in different cooling environments (air, liquid nitrogen, and water) was successfully employed as an alternative method to enhance corrosion resistance and mechanical properties. The evolution of microstructure, corrosion behavior, and mechanical properties of friction stir welded joints were studied using optical microscopy (OM), electron backscattered diffraction, scanning electron microscope, electrochemical workstation, and universal testing machine. The results indicated that the width of the stir zone and grain size of heat-affected zones were reduced by the use of external cooling media. Electron backscattered diffraction results showed that the grain size in air-cooled friction stir welding, nitrogen-cooled friction stir welding and water-cooled friction stir welding were 7.6 µm, 4.5 µm, and 3.2 µm, respectively, and water-cooled friction stir welding joint developed a larger fraction of high-angle grain boundaries at stir zone. The intermetallics formed in the joints using cooling media were finer compared to that of the air-cooled samples. The corrosion behavior of the stir zone was impacted by the cooling environment while potentiodynamic polarization results revealed that the water-cooled friction stir welding joint showed excellent corrosion resistance due to the finer size of intermetallics. The minimum hardness values shifted to the stir zone in the case of nitrogen-cooled friction stir welding and water-cooled friction stir welding from the heat-affected zone location as in the air-cooled friction stir welding joint. For the joint made with water-cooled friction stir welding, maximum yield strength was obtained with a joint efficiency of 96% relative to AA5083 base material.

The presence of chloride ions exceeding its critical concentration in its surrounding is one of the major causes for reinforcing steel corrosion, and subsequently, reduction in the strength of the structure occurs. A common method of preventing such deterioration is to prevent chlorides from penetrating the structure to the level of the reinforcing steel bar by organic corrosion inhibitors. The electrochemical behaviour of mild steel in seawater with or without 4[{(4,6-diamino-1,3,5triazine-2-yl)imino}methyl]-2,5-cyclohexadiene-1-one, a new triazine inhibitor, is identified and synthesised by Insilco techniques in parts I and II of our previous study. Part III presents the evaluation of the new inhibitor in reinforced steel concrete specimens by current induced accelerated corrosion method and also its performance to prevent chloride penetration in concrete structures. Tests were conducted to Indian standards, ASTM standards and Council of Scientific and Industrial Research guidelines to control rebar corrosion. Results showed that the inhibitor efficiency is up to 85%, and also the rate of chloride penetration into the concrete specimens was decreased when the new inhibitor is used as admixture. In addition, the relationship between the molecular structure of these compounds and their inhibition efficiency has been investigated by ab initio quantum chemical calculations using Cerius 2 software program.