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Abstract Al-based amorphous/nanocrystalline coating with a low porosity (0.42%) and a high amorphous content (80.3%) was fabricated by high velocity air-fuel spraying on aluminum alloy 2024. This thin amorphous/nanocrystalline coating with α-Al nanocrystals precipitated in amorphous matrix displays good short-term corrosion resistance, while the corrosion resistance decreases rapidly with increased immersion time. Sealing treatment can effectively close the connecting pores and reduce the porosity defects of coating, thereby improving the corrosion resistance. Among the three selected sealants, stearic acid exhibits the best sealing effect on Al-based amorphous/nanocrystalline coating due to its good impermeability and hydrophobicity, followed by potassium dichromate and nickel acetate. The sealing mechanisms of the three sealants are discussed in terms of reactions at the coating/sealant interface and hydrophobicity of stearic acid. Highlights • HVAF Al-based amorphous/nanocrystalline coating with 0.42% porosity was prepared. • The coating displays good short-term but poor long-term corrosion resistance. • Sealing treatment enhances the long-term corrosion resistance of coating. • Stearic acid exhibits best effect due to good impermeability and hydrophobicity. [ABSTRACT FROM AUTHOR]

Understanding the corrosion behavior of Fe-based amorphous coatings under wet-dry cyclic conditions is of pretty importance for coating applications in marine environments. In this paper, the corrosion evolutions of the coatings under wet-dry cyclic conditions were investigated by electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Accurate charge transfer resistance (R t) was obtained through fitting EIS results using a modified transmission line equivalent circuit model and a landscape map of R t was constructed for a panoramic investigation on coating corrosion. The results show that the corrosion rate of the coating changes cyclically under wet-dry cyclic conditions. Corrosion within a single wet-dry cycle can be presented as four stages: an initial decrease of corrosion rate due to passivation or repassivation when the coating is wetted, then a slow increase due to concentrated aggressive ions with solution evaporation, followed by a rapid increment associated with fast oxygen transport under ultrathin solution film and finally a stable stage as solution dries out. It is deduced that an oxygen concentration gradient can form between the inside and the outside of coating pores when the solution film over the coating is very thin, which further promotes the localized corrosion in the pores. According to the landscape map of R t , the coating maintains its high corrosion resistance for the initial 33 wet-dry cycles. But its corrosion resistance decreases significantly in the 35th wet-dry cycle accompanied by the attack of localized corrosion. After 65 wet dry cycles, the coating fails due to the penetration of localized corrosion. The interparticle oxide layers are defective, which enhances the localized corrosion process. Image 1064 • Accurate charge transfer resistance has been obtained for wet-dry corrosion of coatings. • Corrosion rate of coatings changes cyclically under wet-dry cyclic conditions. • Oxygen concentration gradient under thin solution film promotes localized corrosion in pores. [ABSTRACT FROM AUTHOR]

The influence of the size and types of nanophases on the corrosion behavior for Al 86 Ni 6 Y 4.5 Co 2 La 1.5 amorphous-nanocrystalline coatings in a chloride-containing environment were systematically studied. Results revealed that when the two coatings contained only α-Al nanophase with different size, it was the combined effects of the carrier density and the thickness for the passive film that determined the stability of the passive film. Moreover, with the presence of Al-TM and Al-RE nanophases, the origin of localized corrosion on the coating was ascribed to the weak passive film stability on Al-RE nanophase, which was preferentially dissolved by the interaction with chloride ions. [Display omitted] • Corrosion initiation of Al-based ANCs was observed by quasi in-situ TEM. • Cl− induced rapid thinning occurs for the passive film on larger α-Al nanophase. • Origin of localized corrosion for multi-phases contained coating was identified. • The unstable passive film on YAl 3 phase was the origin of localized corrosion. [ABSTRACT FROM AUTHOR]