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Title

EFFECT OF SURFACE NANOCRYSTALLIZATION ON INHIBITION BEHAVIOR OF MIXED OXYANION INHIBITORS ON PROTECTION OF MILD STEEL IN SEA WATER

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Abstract

 SOMEPROPERTIES OF MATERIALS ARE CLOSELY RELATED TO ITS SURFACE STRUCTURE, SO THAT SURFACE RESTRUCTURING MAY CHANGE SOME PROPERTIES LIKE CORROSION RESISTANCE, WEAR AND ETC [1]. IT CAN ALSO AFFECT THE FORMATION OF PASSIVE FILM ON THE SURFACE [2]. THE CORROSION BEHAVIOR OF AN ALLOY IN AGGRESSIVE MEDIA IS RELATED TO STRUCTURAL PROPERTIES OF METAL SURFACE AND CORROSIVITY OF ITS SURROUNDING ENVIRONMENTS. THE USE OF A CORROSION INHIBITOR IS ONE OF THE MOST USEFUL WAYS IN DECREASING THE AGGRESSIVENESS ACTION OF THE ENVIRONMENT ON METALS AND ALLOYS CORROSION. SODIUM MOLYBDATE HAS A GOOD PERFORMANCE AS ANODIC INHIBITOR THAT IS INCORPORATED INTO THE METAL SURFACE FORMING A PROTECTIVE FILM. THE PRESENCE OF AN OXIDE LAYER IS ESSENTIAL FOR THE CORROSION INHIBITION ACTION OF MOLYBDATE [3] AND IN ORDER TO ACCELERATE AND STABILIZE THE OXIDIZED SURFACE AN OXIDIZING COMPOUND SUCH AS SODIUM PHOSPHATE IS NECESSARY. PHOSPHATE IONS IN SOLUTION REACT WITH METAL CATIONS RELEASED FROM THE SURFACE AND PRECIPITATE AS A FILM WITH BARRIER PROPERTIES AT ANODIC AREAS [4]. THE AIM OF THIS WORK IS TO INVESTIGATE THE EFFECT OF SURFACE NANOCRYSTALLIZATION ON INHIBITION BEHAVIOR OF MIXED OXYANION INHIBITORS (SODIUM MOLYBDATE, SODIUM PHOSPHATE) ON PROTECTION OF MILD STEEL IN ARTIFICIAL SEA WATER. NEAR SURFACE SEVERE PLASTIC DEFORMATION (NS-SPD) PROCESS BY WIRE-BRUSHING TREATMENT WERE USED TO CREATE NANOCRYSTALLINE SURFACE [1]. X-RAY DIFFRACTION, WEIGHT LOSS, ELECTROCHEMICAL POLARIZATION AND IMPEDANCE WERE USED IN THIS INVESTIGATION. THE OBTAINED RESULTS DEMONSTRATE THAT NC-SPD CAN LEAD TO FORM NANOCRYSTALLINE SURFACE. THIS SURFACE TREATMENT WAS CHANGED CORROSION AND INHIBITION BEHAVIOR BY INCREASING PREFERENTIAL SITES TO ADSORPTION OF INHIBITOR AND CORROSIVE IONS. WIRE BRUSHING PROCESS DECREASES THE CORROSION RESISTANCE SLIGHTLY WHICH IS DUE TO INCREASING SURFACE ROUGHNESS. IT IS EASIER FOR ELECTRONS IN THE VICINITY OF A PEAK TO ESCAPE THAN THOSE IN A VALLEY, SO THAT THE PEAK WOULD BE CORRODED PREFERENTIALLY. THE ELECTRON WORK FUNCTION (EWF) FLUCTUATION INCREASED WITH AN INCREASE IN ROUGHNESS AND MAY ALSO PROMOTE THE FORMATION OF CORROSION CELL THAT COULD FURTHER ACCELERATE CORROSION OF A ROUGH SURFACE [5]. PRESENCE OF MIXED INHIBITOR IN CORROSIVE MEDIA DECREASES THE CORROSIVITY OF SURROUNDING ENVIRONMENTS OF MILD STEEL BY FORMING A PROTECTIVE FILM ON THE SURFACE OF ALLOY, WHICH DELAYS THE ANODIC REACTIONS. NS-SPD INCREASES THE HIGH ENERGY SITES ON THE SURFACE OF MILD STEEL WHICH MAKES MORE INTENSE COMPETITION BETWEEN INHIBITOR AND CORROSIVE IONS TO OCCUPY PREFERENTIAL SITES. THE RESULTS OF POLARIZATION AND IMPEDANCE CURVES WERE SHOWN THAT MIXED INHIBITOR FOR WIRE-BRUSHED SURFACE HAS HIGHER EFFICIENCY THAN THAT OF NONTREATED SURFACE. THIS SURFACE HAS MORE PREFERENTIAL SITES FOR ADSORPTION OF INHIBITOR. THEREFORE, THE INHIBITOR COVERAGE ON SURFACE IS DONE FASTER THAN THAT OF NONTREATED SURFACE, WHICH PROMOTES THE PASSIVATION CONDITION OF SURFACE BY INHIBITORS. EVENTUALLY IT MAY BE CONCLUDED THAT SURFACE STRUCTURE HAS AN IMPORTANT ROLE IN CORROSION AND INHIBITION BEHAVIOR. NANOCRYSTALLIZED SURFACE DONE BY NS-SPD TREATMENT DIMINISHES CORROSION RESISTANCE SLIGHTLY BY INCREASING SURFACE ROUGHNESS. NEVERTHELESS SUCH TREATMENT PROMOTES THE INHIBITION EFFICIENCY OF OXYANIONS BY INCREASING THE ADSORPTION OF THESE INHIBITORS ON THE SURFACE.

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