Corrosion resistance of amorphous AlPO₄ coating in salty atmospheres

Abstract

The objective of the present study was to introduce a cost‐effective and environmentally friendly coating to improve the corrosion resistance of the structures located in salt water. The coating solution, based on amorphous aluminum phosphate composition, was synthesized by sol–gel process and applied to AISI 304 stainless steel by dip coating technique. X‐ray diffraction, scanning electron microscopy, and energy dispersive X‐ray spectroscopy analyses were employed to investigate the phase composition and morphology of the coating. Corrosion behavior of the uncoated and coated samples was investigated using standard salt spray test, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. Salt spray test results for the bare substrate revealed a corrosion rate of six‐time greater than that of the coated surface after 168 hr exposure time. Electrochemical test results declared that the amorphous AlPO4 coating decreased the corrosion current density of the AISI 304 stainless steel by 10 orders of magnitude. Furthermore, according to the corresponding EIS measurements, the coated surface exhibited a superior anti‐corrosion performance than uncoated sample. Overall, the results declared that the amorphous AlPO4 coating could be a good choice for surface protection of stainless steel against electrochemical corrosion in salty environments.

Role of Silicoaluminophosphate as a Corrosion Inhibiting Nanocontainer and Filler in Environmentally Benign Bio-Based Hybrid Coatings

“…The decrease in the corrosion rate might be due to two plausible causes: (i) the synergistic interaction between silicon (Si), aluminum (Al), phosphate (PO 4 ), and molybdenum (Mo) ions within the SAPO-5 framework not only facilitates the formation of a protective passive oxide layer but also, through the presence of Mo, leads to the reduction of MoO 3 to MoO 2 in the electrolyte solution. This results in the formation of an insoluble metal oxide layer on the metal substrate, effectively shielding it from further oxidation. , (ii) In neutral, alkaline, and aeration solutions, anodic and cathodic reactions occur as follows: Fe → Fe 2 + + 2 e − ( a n o d i c r e a c t i o n ) 1 2 O 2 + 2 H + + 2 e − → 2 OH − ( c a t h o d i c r e a c t i o n ) The liberation of OH – in the cathodic site makes the electrolyte solution alkaline, which helps MoO 3 to pop out of the SAPO-5 nanocontainer due the larger electrostatic repulsive interaction between the MoO 3 and the SAPO-5 . Further, the release inhibitor MoO 3 reacts with the hydroxyl ions and gets reduced according to …”

“…This results in the formation of an insoluble metal oxide layer on the metal substrate, effectively shielding it from further oxidation. 35,71 (ii) In neutral, alkaline, and aeration solutions, anodic and cathodic reactions occur as follows:…”

“…The advantage of using zeolite is that it offers nontoxicity and can act as a defensive lining against the corrosive species, thereby inhibiting the release of harmful entities . Moreover, the improved performances from these types of materials comes from the higher electronegativity of the phosphate (PO 4 3– ) polyanion and its counter cation aluminum (Al), which offers an impeding behavior towards the flow of electrolytes. , Apart from the phosphate and aluminum, the silicon group also imparts good characteristics by providing optical transparency and thermal stability to the polymer matrix . Formerly, chromium and its derivatives are recognized as one of the promising materials for reducing corrosion behavior.…”