Preparation And Anti-corrosive Performance Of Phosphate-based Inhibitors And Chemical Conversion Coatings For Iron And Steels

Phosphate molecule contains hydrophilic group (phosphate) and hydrophobic group (alkyl) and can adsorb on the metal surface to form a thin film that protects the metal from corrosion to some degree. Among all kinds of organic phospahtes, phytic acid, as one environmentally friendly multi-complexing agent for metal ions, has been widely used as an inhibitor and a film former for constructing chemical conversion coatings in the field of metal corrosion and protection. In recent years, researchers have studied the anti-corrosive performance of phytic acid as an inhibitor and a type of chemical conversion coating material for aluminium alloy, magnesium alloy, copper and its alloy. So far, there have been only a few reports concerning how PA molecules or its ions adsorb on metal surfaces and in what way they form protective barrier films. And more, there is almost no report about how to modify the phytic acid molecule to improve the anti-corrosive performance of phytic acid-based chemical conversion coating on the metal surface. In this paper, we firstly investigated the inhibition efficiency and mechanism of several alkyl phosphates as the inhibitors for the corrosion of iron. Then, we selected phytic acid, a special phosphate, to investigate the inhibition efficiency and mechanism on the corrosion of iron under acidic and neutral conditions. Finally, we modified the phytic acid according to our design, and did some studies about the use of the modified phytic acid-based materials as chemical conversion coating materials and silane congting additives.The main contents of this thesis include:(1) Corrosion inhibition of iron in acidic solutions by monoalkyl phosphate esters with different chain lengthsThe corrosion inhibition efficiency of inhibitors is mainly influenced by their molecular structures, hydrophilic-hydrophobic properties and the molecular aggregation states at the metal/solution interface. In order to investigate the relationship among these factors, in this paper, choosing three monoalkyl phosphate esters with different chain lengths, mono-n-butyl phosphate (BP), mono-n-hexyl phosphate (HP) and mono-n-octyl phosphate (OP), as the corrosion inhibitors of iron, we investigated their inhibition effectiveness in 0.5 M H2SO4 solutions. BP, HP, and OP were synthesized by the direct esterification reaction of phosphorus pentoxide with 1-butanol,1-hexanol, and 1-octanol. According to the conductivity measurements results, HP and OP exhibit some surfactant characteristic behavior and their CMC values are 166 ppm and 68 ppm, respectively. In contrast to HP and OP, BP does not show any characteristic behavior of a surfactant. The polarization curves and EIS results show that the inhibition efficiency of HP or OP with the longer alkyl chain is much higher than that of BP with the shorter alkyl chain under similar condition. The reason is that the alkyl chain of BP is so short that could not form stable monolayer film through van der Waals interactions among the inhibitor molecules. HP and OP behave like a surfactant and are able to form ordered molecular aggregates at the iron/solution interface. The XPS results give evidence that BP, HP and OP adsorb on the iron surface to form a corrosion inhibition film. The adsorption of BP, HP, and OP all obey the Langmuir adsorption isotherm, and the adsorption type belongs to chemisorption. But the HP and OP molecules adsorb on the iron surface more easily than BP molecules.(2) Influence of phytic acid on the corrosion behavior of iron under acidic and neutral conditionsPhytic acid as one infrequent multi-complexing agent could chelate with almost all metal ions of bivalence or higher valence to form stable chelates. In this paper, we studied the influence of PA and its anions on the iron corrosion in 0.5 M H2SO4 and 3.5 wt%Na2SO4 solutions. The results exhibited that PA can effectively inhibit the corrosion of iron as a mixed type inhibitor in H2SO4 solution and the inhibition efficiency may reach a maximum (82.96%) in the case of 1.0 x 10"4 M. The XPS results reveled that PA adsorbs on the iron surface mainly in the form of undissociated molecules and its adsorption obeys the Langmuir adsorption isotherm and the adsorption type is considered as chemisorption. But the behavior of phytic acid is quite different in 3.5 wt%Na2SO4 compared with that in 0.5 M H2SO4. Under this condition, phytate ions shows the strong complexible ability to the Fe (II) ions produced during anodic dissolution, and the resulting chelates can deposit on the surface of iron electrode, leading to the formation of protective barrier layers. Because the layer is loose and porous, the corrosion performance is so weak. In this case, phytate works as a film former rather than a corrosion inhibitor.(3) Excellent anti-corrosive pretreatment layer on iron substrate based on three-dimensional porous phytic acid/silane hybridAlthough phytic acid chemical conversion coating reveals many advantages, a major disadvantage is the lower anti-corrosion ability. In order to overcome the disadvantage of phytic acid thin films, we designed and synthesized an environmentally friendly film-forming material, PA/silane (PAS) hybrid, for the first time by the condensation reaction of PA with methyltrihydroxysilane, which was produced by the hydrolysis of methyltriethoxysilane (MTES) in aqueous solution. Two kinds of PAS-based pretreatment layers, namely NaBrO3-free and NaBrO3-doped PAS layers, were fabricated on iron substrates using the dip-coating method. SEM and AFM observations showed that for the NaBrO3-free PAS layer, the layer was uniform when the layer formation time was 60 min, but for NaBrO3-doped PAS layer, the layer could be very compact and uniform when the layer formation time was only 5 min. X-ray photoelectron spectroscopic (XPS) characterization demonstrated that the two PAS layers bound to the iron surface via the formation of-P-O-Fe bond. The highest protection efficiencies for the NaBrO3-free and NaBrO3-doped PAS layers were 85.7% and 95.7%, respectively. Moreover, the formation time of the chemical conversion coating decreased to only 5 min with NaBrO3 doping.(4) Effect of amorphous phytic acid nanoparticle on the anti-corrosive performance and stability of sol-gel coating coated on cold rolled steelDue to the advantages of non toxicity and good adhesion, organosilane coating is becoming a hot spot of research. However, the anti-corrosive performance of this kind of coating originatiates from its blocking effect to corrosive substances in corrosive media. Once the coating is damaged somewhat, the coaintg will not protect the metal substrate from corrosion any more. In this paper, we exploited the powerful chelating ability of PA to improve the anti-corrosive performance and stability of the silane coating. We designed and synthesized amorphous PA nanoparticles via a condensation reaction of PA with aminopropyltriethoxysilane. SEM and EDS results show that the as-obtained nanoparticles are amorphous and the sizes are around 70 nm and the main composition elements of the nanoparticles are C, N, O, Si, P. The X-ray photoelectron spectroscopic (XPS) and fourier transform infrared (FTIR) spectroscopic characterizations demonstrated that the synthesis mechanism of nanoparticles is due to the formation of many Si-O-P bonds between the PA and aminopropyltriethoxysilane and thus the surface of the nanoparticles contains abundant phosphate groups. The bare silane coating and the PA nanoparticles doped silane coating were fabricated on cold rolled steelsubstrates by using silane sol-gel systems. The electrochemistry tests results demonstrate that the PE values of the bare silane coating and PA nanoparticle doped coating were around 97.2% and 99.7, respectively. But after the coatings were immersed in 3.5 wt% NaCl solution for 48 hours, the PE value of the bare coating was decreased to 73.7% and the PE value of the PA nanoparticle doped coating was still remained at 98.2%.