Cyclodextrins as Carriers for Organic Corrosion Inhibitors in Organic Coatings

Abstract

Coatings are widely applied on metal surfaces for protection against corrosion. Once a coating is damaged and a defect is filled by aqueous solutions, delamination of the coating initiates further materials degradation. Cathodic delamination can be suppressed by adding active additives to the coating matrix. In this study, the impact was investigated of β-cyclodextrin inhibitor complex (β-CD [Inh]) additives in polyvinylbutyral model coatings on the cathodic delamination of coatings on zinc, using the scanning Kelvin probe (SKP). With the presence of the β-CD [Inh] in the coating, a decrease was observed of the delamination rate to 1/4 of that in the reference system without β-CD [Inh]. Release experiments using UV-Vis spectroscopy showed that in the presence of cyclodextrins, the released concentration of the inhibitor 2-mercaptobenzothiazole from the coating increased. The increase was most pronounced under alkaline conditions, where the released concentration within 24 h increased by a factor of 6 compared to a reference system without β-CD. Zinc and its alloys are industrial products for cathodic corrosion protection of steel components. They are hence used in a variety of applications, e.g. in metallic coatings.1 On the other hand, zinc may corrode itself actively, which is why a protection of the metal is needed. A straightforward way to protect non-noble metals such as zinc against corrosion is the application of organic coatings. 2-4

Electrochemical contrast switching between black and white appearance of gelatin-covered zinc

“…Our group has recently investigated the possibility to use the biopolymer chitosan as ingredient in passive corrosion protection systems on zinc to limit ion mobility along the interface [11]. The cyclic oligosaccharide β-cyclodextrin was used to enhance corrosion inhibitor availability by incorporation into a model coating on zinc and into a zinc oxide based pretreatment [12,13]. Aim of this work was to investigate the electrochemical properties of gelatin-modified zinc.…”

Inclusion of 5-Mercapto-1-Phenyl-Tetrazole into β-Cyclodextrin for Entrapment in Silane Coatings: An Improvement in Bronze Corrosion Protection

“…The use of the complex instead of pure BTA was intended to reduce the toxicity of the additives used for corrosion protection [19]. Moreover, complexes of β-CD and γ-CD with mercaptobenzothiazole (MBT), mercaptobenzimidazole (MBI), mercaptobenzimidazole sulfonate, and thiosalicylic acid [20][21][22][23] were incorporated in coatings and improved the corrosion resistance of Al alloys and zinc. In fact, these complexes represent a reservoir of corrosion inhibitor molecules that, at the onset of corrosion phenomena and in correspondence of regions of coating delamination, tend to adsorb on the metal surfaces, shifting the complexation equilibrium towards the release of further inhibitor molecules.…”

Cyclodextrin inhibits zinc corrosion by destabilizing point defect formation in the oxide layer

“…Control experiments in a recent work on encapsulating poorly water soluble organic corrosion inhibitors in the cyclic oligosaccharide β-cyclodextrin (β-CD) for incorporation into model polymer coatings on zinc led to the surprising result that the coating delamination rate reduced also in the presence of pure β-CD [ 8 ]. This observation served as a motivation to investigate whether β-CD acts as corrosion inhibitor on its own.…”

“…β-CD is not surface-active [ 10 ]. Although the use in food, medical, and pharmaceutical applications, including drug delivery, is well documented [ 11 ], application of CDs in corrosion protection is rare [ 8 , 12 – 14 ]. These applications mostly use CDs to solubilize hydrophobic corrosion inhibitors [ 8 , 12 – 13 ].…”