Developing and testing a new generation of protective coatings for outdoor bronze sculpture

Outdoor bronze sculpture is vulnerable to acid rain-induced corrosion and the present protection schemes utilized by conservators do not provide adequate protection under many circumstances. To replace the current most common clear bronze protection systems, wax or Incralac^® with a top coat of wax, work is underway to develop different options for conservators that include new longer-lasting, more durable systems having improved corrosion protection. The use of improved matrix binders based on fluorocarbon polymers, the use of nano-sized TiO₂ for UV absorption, and the extended use of UVAs and HALS are under examination in a search for improved UV resistance and longer lived corrosion protection in clear bronze coatings. Advanced spectroscopic and electrochemical methods are being used to characterize new coatings candidates with respect to UV resistance and corrosion resistance.     

Protection of ancient and historic bronzes by triazole derivatives

Ancient and historic tin bronzes can be substantially affected by corrosion when submitted to non adequate storage conditions or more aggressive environments such as, for example, the marked increase of air pollution and of acidity of rainwater in urban medium recently affecting outdoor monuments. To protect them, some triazole compounds were tested as corrosion inhibitors of bronze covered with patina layer. As in the case of cultural artefacts, each specimen is unique, an accurate comparison of inhibiting effect of different molecules is therefore hard to realise. To overcome this difficulty, on the basis of a global understanding of the nature of natural bronze patinas, artificial patinas were synthesised on a contemporary bronze (Cu-Sn-Pb) having a similar composition to that of bronze coins dating of the post Roman Empire (5th-6th century AD). Surface characterization and electrochemical experiments were carried out both on contemporary bronze covered with artificial patina and on three ancient bronze coins discovered in Morocco. All triazole derivatives used, benzo-triazole (BTA), amino-triazole (ATA) and bi-triazole (BiTA), exhibited fairly good protective properties on the synthetic bronze. The BTA is efficient in an artificial patina isolated of the substrate bronze and an old bronze specimen covered with patina layer, but this substance is considered as toxic. The BiTA has shown only a slight inhibiting effect on the ancient bronze coin. The ATA is the most effective at 0.1 mM concentration, and the initial treatment of bronze at a higher concentration makes inhibiting effect lower. The ATA is therefore considered the most promising candidate to be applied to protect antique bronze artefacts covered with natural patina layer.     

A smart anticorrosion coating based on hollow silica nanocapsules with inorganic salt in shells

This work presents the synthesis of corrosion inhibitor [1H-benzotriazole (BTA)]-loaded hollow silica nanocapsules with magnesium hydroxide precipitate in the shells (HSNs-M/BTA) through inverse microemulsion (W/O) polymerization. TEM and Brunauer–Emmett–Teller (BET) measurements indicate that the silica nanocapsules possess voids in the inner part and mesoporous on the surface. The actual loading capacity of BTA is 287.17 mg (BTA)/1 g (HSNs-M/BTA). The results of UV absorption spectra show that the release of BTA can be triggered by the changing of pH in the corrosion solution. The anticorrosive SiO _x /ZrO _x coatings embedded with hydrophobically modified HSNs-M have a better waterproof performance since the water contact angle can reach 140°. In comparison to the passive SiO _x /ZrO _x coatings with or without BTA, the enhanced corrosion protection performance of this developed anticorrosive system was observed by both electrochemical impedance spectroscopy and Tafel analysis. The fabrication of nanocapsules with corrosion inhibitors is promising as an intelligent element in protective coatings in the future.     

Corrosion protection of metal surfaces by atmospheric pressure plasma jet treatment

In this paper the corrosion protection of silicon-organic coatings applied by an atmospheric pressure plasma jet system was studied on copper (Cu) and aluminium (Al). The influence of a plasma pre-treatment on the defect density and the adhesion of the subsequently deposited plasmapolymer coating are presented. The quality of the coating was studied via visualization of defects by means of silver precipitation. Use of a reductive gas mixture for the plasma pre-treatment led to a significant reduction of the defect density. Additionally, the implementation of chromate-free corrosion inhibitors in the plasmapolymer layer enables an active corrosion protection of Al surfaces.     

Investigation of the corrosion protection of chemically and electrochemically formed patinas on recent bronze

In humid atmospheres, copper and bronze passivate, with the formation of an oxidized layer (patina). Patinas may also be created by chemical patination procedures, which are frequently used to achieve special visual effects. However, unless these patinas are effectively protected, corrosion may be initiated when they are exposed to a polluted atmosphere. In the present study the stability of the green nitrate and the green chloride type of patinas, and of electrochemically formed patina, was investigated, as well as suitable protection methods. Two inhibitors, dissolved in different solutions, were used: benzotriazole and the less hazardous imidazole type of inhibitor, and waxes. The dissolving of untreated and pre-treated surfaces, in test solutions representing urban rain (pH 5.0), was investigated by means of potentiodynamic techniques. The microstructure of the patinas and the corrosion products was investigated by SEM/EDX. It was found that both investigated inhibitors inhibited the corrosion of electrochemically formed patina, and of the green chloride type of patina, but were ineffective in the case of the green nitrate type of patina. The difference between the performance of the investigated inhibitors when brushed onto patinated bronze, and when the bronze is immersed in a solution containing the dissolved inhibitor, was determined.     

Electrospun protective self‐healing coatings for light alloys: A better understanding of the intrinsic potential of the technology

Polymeric coating systems exhibit high potentiality to provide an effective barrier against corrosion of metallic surfaces. However, these coatings can lose their protective characteristics because of their high susceptibility to damage. Thus, the addition of corrosion inhibitors is desirable and considered as an alternative route for active corrosion protection. In the present work, eco‐friendly electrospun coatings of poly(vinyl alcohol) (PVA) loaded with cerium salts have been deposited onto aluminium 6082 alloy. Two different precursors of cerium (III) (i.e., cerium nitrate and cerium acetylacetonate) were added to the electrospinning solutions and the effectiveness of the resulting nanofibrous coatings was evaluated for the healing of generated defects. The microstructural features of the electrospun coatings have been investigated by scanning electron microscopy, infraredspectroscopy, and thermal analysis. Tensile tests were performed to assess the mechanical properties of the different fibrous coatings. The electrochemical behavior of both intact and damaged coatings was evaluated in 3 wt % NaCl solution by means of electrochemical impedance spectroscopy. All the deposited PVA coatings loaded with cerium(III) salts showed remarkable corrosion resistance. In the case of artificially damaged coatings, a self‐healing effect, which stops the development of the corrosion process and provides a significant recovery of the corrosion resistance, has been observed only for coatings loaded with cerium III acetylacetonate. The release of cerium from damaged PVA fibers has been demonstrated by means of inductively coupled plasma mass spectrometry. The observed self‐healing effect has been ascribed to the formation of cerium hydroxide on the defective zone, which hindered the corrosion process. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42728.     

An electrochemical and Raman spectroscopic investigation of synergetic effects in the corrosion inhibition of copper

The synergetic inhibition of copper corrosion by benzotriazole (BTA) and benzylamine (BZA) in chloride and cyanide media is assessed by voltammetric and ac impedance measurements. BZA enhances the performance of the strong inhibitor BTA by accelerating the growth of a protective surface layer; used alone, BZA is ineffective as an inhibitor. The competitive adsorption of aggressive anions and inhibitors is studied by surface enhanced Raman scattering (SERS). The dramatic increase in corrosion caused by low concentrations of cyanide ions is shown to be due to the displacement of BTA from the copper electrode surface; comparisons are made with the behaviour of mercaptobenzoxazole (MBO).     

Protection of bronze covered with patina by innoxious organic substances

Cultural bronze artefacts are exposed in indoor or outdoor environment. They often suffer of a substantial alteration due to an increasing atmospheric pollution. In this work, we propose the use of some innoxious compounds as corrosion inhibitors of bronze objects covered with patina. The bronze used was Cu-6Sn (in wt.%). This composition was selected after a preliminary work on several archaeological bronzes found in Transylvania, Romania, dated from the Late Neolithic to Roman periods. First, an artificial patina was formed on Cu-6Sn bronze under potential regulation, in 0.2 g L⁻¹ NaHCO₃ + 0.2 g L⁻¹ Na₂SO₄ aqueous solution (pH 8), during 4 days. A pale blue to green patina was obtained and characterized using EDS and Raman spectroscopy. Then, four innoxious organic substances were examined as corrosion inhibitors: 5 mM 4-methyl-1-(p-tolyl)-imidazole (TMI), 10 mM 1-phenyl 4-methyl-imidazole (PMI), 1 mM 2-mercapto 5-R-acetylamino-1,3,4-thiadiazole (MAcT), 1 mM 2-mercapto 5-R-amino-1,3,4-thiadiazole (MAT), and for comparison 1 mM benzotriazole (BTA). The impedance spectra collected showed, for all of them, three depressed capacitive loops. On the basis of these capacitance values, these loops were allocated to the surface film with ionic conduction, the double layer capacitance with the charge transfer resistance, and the oxidation-reduction process involving the surface patina. TMI and MAcT were found to be efficient inhibitors though their performances are significantly lower than that of BTA.     

Effect of benzotriazole (BTA) addition on Polypyrrole film formation on copper and its corrosion protection

Benzotriazole (BTA) was added in a conducting Polypyrrole (PPy) film prepared on copper in oxalic acid aqueous solution containing pyrrole monomer to improve corrosion protection by the PPy film and reduce copper corrosion. When BTA was added in the preparation solution, the copper surface was covered by a BTA–Cu complex layer before the anodic polymerization of PPy was started. On the copper surface with the BTA layer, the initial dissolution of copper was inhibited and the PPy polymerization-deposition was started immediately after the anodic current was imposed. The PPy film thus formed was doped with oxalic ions and ionized BTA and was homogeneous in thickness and strongly adhesive. The PPy film containing BTA protected the copper from corrosion in 3.5 wt.% NaCl solution. In 400 h of immersion, copper dissolution was inhibited with 80% protection efficiency relative to that of bare copper.     

Eco‐friendly,acrylic resin‐modified potassium silicate as water‐based vehicle for anticorrosive zinc‐rich primers

Potassium silicate binder of zinc‐rich coating was modified by adding water‐based acrylic resin. Several series of coatings containing 5, 10, and 15 wt % of acrylic and acrylic/styrene binders were added to potassium silicate. The coatings were applied on steel and the corrosion resistance of coatings was evaluated by conventional methods such as electrochemical impedance spectroscopy, corrosion potential, salt spray, and scanning electron microscopy. The results indicated that the modification of silicate binder with acrylic and acrylic/styrene led to shortening the curing time, improved corrosion protection, better dispersion of zinc particles, and enhanced salt spray resistance of resultant coatings. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40370.     

Corrosion protective bi‐layered composites of polyaniline and poly(o‐anisidine) on low carbon steel

Bi‐layered composites of polyaniline (PANI) and poly(o‐anisidine) (POA) were investigated for corrosion protection of low carbon steel (LCS). In this work, homopolymers and bi‐layers of PANI and POA were electropolymerized on LCS from an aqueous salicylate solution by using cyclic voltammetry. These coatings were characterized by cyclic voltammetry, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Corrosion tests were carried out in aqueous 3% NaCl solution for LCS coated with PANI, POA, bi‐layered POA/PANI (POA on top of the PANI) or PANI/POA (PANI on top of the POA) composites using open circuit potential (OCP) measurements, potentiodynamic polarization technique, and electrochemical impedance spectroscopy (EIS). The single layer of PANI and POA protected the LCS in 3% NaCl for 8 and 16 h, respectively. The bi‐layered composite coatings provide effective protection to LCS for a longer time than a single layered PANI or POA coating. However, the corrosion protection offered to LCS depends on the deposition order of polymer layers in the composite. The PANI/POA composite provides better protection to LCS against corrosion than POA/PANI coating. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Adaptive Polymeric Coatings with Self‐Reporting and Self‐Healing Dual Functions from Porous Core–Shell Nanostructures

In biological system, early detection and treatment at the same moment is highly required. For synthetic materials, it is demanding to develop materials that possess self‐reporting of early damage and self‐healing simultaneously. This dual function is achieved in this work by introducing an intelligent pH‐responsive coatings based on poly(divinylbenzene)‐graft‐poly(divinylbenzene‐co‐methacrylic acid) (PDVB‐graft‐P(DVB‐co‐AA)) core–shell microspheres as smart components of the polymer coatings for corrosion protection. The key component, synthesized PDVB‐graft‐P(DVB‐co‐AA) core–shell microspheres are porous and pH responsive. The porosity allows for encapsulation of the corrosion inhibitor of benzotriazole and the fluorescent probe, coumarin. Both loading capacities can be up to about 15 wt%. The polymeric coatings doped with the synthesized microspheres can adapt immediately to the varied variation in pH value from the electrochemical corrosion reaction and release active molecules on demand onto the damaged cracks of the coatings on metal surfaces. It leads simultaneously to the dual functions of self‐healing and self‐reporting. The corrosion area can be self‐reported in 6 h, while the substrate can be protected at least for 1 month in 3.5 wt% NaCl solution. These pH‐responsive materials with self‐reporting and self‐healing dual functions are highly expected to have a bright future due to their smart, long‐lasting, recyclable, and multifunctional properties.     

Reliability improvement of conductive adhesives on tin (Sn) surfaces

Effective organic corrosion inhibitors are introduced into a typical electrically conductive adhesive (ECA) formulation and evaluated. With the incorporation of small amounts of these additives, lower bulk resistivity and highly stable contact resistance on tin (Sn) surfaces were achieved. Contact angle, grazing angle Fourier Transfer Infrared (FT-IR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS) characterizations showed the presence of coatings of the corrosion inhibitors on the metal surfaces. A barrier passivation layer formed on Sn surfaces with the effective corrosion inhibitors. X-ray diffraction analysis confirmed that such a passivation layer protected the Sn surface and prevented oxidation and corrosion under elevated temperature and humidity environment.     

Preparation of transparent nanocomposites from UV‐ and thermo‐curable epoxyacrylate and graphene oxide for the application of corrosion protection coatings

In this study, we report UV‐ and thermo‐curable epoxyacrylate/graphene oxide (EA/GO) nanocomposites that present good transparency, excellent pencil hardness and promising improvement in corrosion protection. A dual‐curable EA oligomer with one terminal epoxide and one double bond at the other end was synthesized by reaction of diglycidyl ether of bisphenol‐A and acrylic acid. After mixing EA and GO with the curing agents and reactive diluent followed by UV cure and thermo‐cure, the resulting EA/GO films on a glass slide with GO loading up to 3 phr exhibited over 86% light transmittance. Furthermore, the pencil hardness was enhanced from 3H for EA to 6H for the EA/GO composite at 2 phr GO loading. The corrosion protection of the EA/GO coatings was evaluated by a potentiodynamic polarization technique and electrochemical impedance spectra. The corrosion potential (E_corr) of the EA/GO‐coated steel increased with increasing GO loading. Meanwhile, Nyquist and Bode plots indicated that the higher the GO content in the EA/GO coating was, the higher was the coating resistance and also the charge transfer resistance after immersion in salt solution. All these results proved that the GO had positive effects on enhancement of the corrosion resistance. The improved corrosion protection by the EA/GO coatings was mainly due to the enhanced hydrophobicity, the deviation of electron transfer and the increased tortuosity of the diffusion path. The improved corrosion protection and hardness together with the useful dual‐curability make the EA/GO nanocomposite a competitive candidate for corrosion protection coatings. © 2019 Society of Chemical Industry     

Corrosion behavior of titania films coated by liquid‐phase deposition on AISI304 stainless steel substrates

Fouling deposition and localized corrosion on the heat‐transfer surfaces of the stainless steel equipments often simultaneously exist, which can introduce additional thermal resistance to heat‐transfer and damage heat‐transfer surfaces. It is a good anticorrosion way to coat a barrier layer of certain materials on the metal surface. In this article, the TiO₂ coatings with nanoscale thicknesses were obtained by liquid‐phase deposition method on the substrates of AISI304 stainless steel (ASS). The coating thickness, surface roughness, surface morphology, crystal phase, and chemical element were characterized with the film thickness measuring instrument, roughmeter, atomic force microscopy, field emission scanning electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy analyzer, respectively. Corrosion behavior of the TiO₂ coatings was evaluated by potentiodynamic polarization, cyclic voltammograms scanning, and electrochemical impedance spectroscopy tests with the mixed corrosion solution composed of 3.5 wt. % NaCl and 0.05 M NaOH. It is shown that the TiO₂ coating is composed of the nanoparticles with smooth, crack‐free, dense, and uniform surface topography; the roughness of coating surface increases slightly compared with that of the polished ASS substrate. The anatase‐phase TiO₂ coatings are obtained when sintering temperature being varied from 573.15 to 923.15 K and exhibit better anticorrosion behavior compared with ASS surfaces. The corrosion current density decreases and the polarization resistance increases with the increase of the coating thickness. The corrosion resistance of the TiO₂ coatings deteriorates with the increase of the corrosion time. The capacitance and the resistance of the corrosion product layer between the interface of the ASS substrate and the TiO₂ coating are found after the corrosion time of 240 h. A corrosion model was introduced, and a possible new explanation on the anticorrosion mechanisms of the TiO₂ coating was also analyzed. The corrosion mechanism of the TiO₂ coating might comply with the multistage corrosion process. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1907–1920, 2012     

Recent developments in the volatile corrosion inhibitor (VCI) coatings for metal: a review

Corrosion is a crucial worldwide problem that strongly affects metals. Out of the several ways to prevent corrosion, volatile corrosion inhibitors (VCIs) are predominantly used as a method of temporary protection. These compounds have the ability to vaporize and condense on the surface of the ferrous or nonferrous material and make the substrate less susceptible to corrosion but work only in a confined space. VCI compounds form a monomolecular layer over the metal surface, thereby preventing the electrochemical reactions on the metal surface which lead to corrosion. This review article outlines the use of volatile corrosion inhibitors (VCIs) as a temporary corrosion protection technique, their working mechanisms and the various compounds used as VCI. It also anchors on the latest research works which have been carried out on VCI coatings along with a glimpse of the works that were carried out in the past. The various factors affecting the volatile corrosion inhibitors along with the merits of VCI coatings are discussed in this paper. Formulations for both strippable and permanent VCI coatings are also reported with the various testing methods. Lastly, an overview of the recent developments of VCI coatings along with the various green VCI is given.     

Inhibiting effects of benzotriazole on the corrosion of α-Al-bronze in saline water

The inhibiting effect of benzotriazole (BTA) on the corrosion of -Al-bronze (Cu-7% Al) in 3.4% NaCl was studied. BTA showed good inhibition effects from short up to extended periods of time (about six weeks). The morphologies of the alloy surface were monitored after various periods of corrosion in the absence and presence of BTA, using SEM. The corrosion products were identified by X-ray diffraction. Corrosive attack occurs very early in the absence of BTA, leading to general and pitting corrosion. BTA was found to have a stronger inhibiting effect on the anodic dissolution of copper than on the cathodic reduction of oxygen. The current-potential relation is divided into two regions: region I within which BTA has a strong effect on the charge transfer kinetics, and a limiting current region where BTA has no significant inhibiting effect. It is also shown that the interaction of BTA with a Cu₂0-covered alloy surface is faster than on reduced alloy surfaces, although the protection efficiency on the latter is slightly better than on the former.     

A new class of corrosion inhibitors for waterborne coatings: 4-methyl-γ-oxo-benzene-butanoic acid complexes

Amine and transition metal based complexes with 4-methyl-γ-oxo-benzene-butanoic acid represent a new class of corrosion inhibitors specifically designed for long-term corrosion protection in waterborne coatings. Today, corrosion protection in waterborne technology is typically achieved using traditional anticorrosive pigments initially developed for use in solventborne coatings. Regulations concerning heavy metals and limitations regarding the compatibility and performance of such materials in waterborne coatings have created a need for novel approaches. Mechanistic aspects are discussed for the 4-methyl-γ-oxo-benzene-butanoic complexes based on electrochemical solution experiments (electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV)) and investigations performed on coated substrates. Exposure results underline the high efficiency of such complexes for both long-term corrosion protection and weld seam rust control in waterborne coatings.     

Synthesis of poly(aniline‐co‐o‐toluidine) coatings and their corrosion‐protection performance on low‐carbon steel

Strongly adherent poly(aniline‐co‐o‐toluidine) coatings were synthesized on low‐carbon‐steel substrates by the electrochemical copolymerization of aniline with o‐toluidine with sodium tartrate as the supporting electrolyte. These coatings were characterized with cyclic voltammetry, ultraviolet–visible absorption spectroscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The formation of the copolymer with the mixture of monomers in the aqueous sodium tartrate solution was ascertained by a critical comparison of the results obtained from the polymerizations of the individual monomers, aniline and o‐toluidine. The optical absorption spectrum of the copolymer was drastically different from the spectra of the respective homopolymers, polyaniline and poly(o‐toluidine). The extent of the corrosion protection offered by poly(aniline‐co‐o‐toluidine) coatings to low‐carbon steel was investigated in aqueous 3% NaCl solutions by open‐circuit‐potential measurements and a potentiodynamic polarization technique. The results of the potentiodynamic polarization measurements showed that the poly(aniline‐co‐o‐toluidine) coatings provided more effective corrosion protection to low‐carbon steel than the respective homopolymers. The corrosion rate depended on the feed ratio of o‐toluidine used for the synthesis of the copolymer coatings. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103:1868–1878, 2007