The improvement of anticorrosion properties of zinc-rich organic coating by incorporating surface-modified zinc particle

The corrosion behaviors of zinc-rich coating with various zinc contents, ranging from 0 to 60 volume percent, in thin organic coatings (below 5 μm) were characterized by electrochemical impedance spectroscopy (EIS), free corrosion potential (E_corr) measurement and cycle corrosion test (CCT). It was verified that both coatings with 60 volume percent of zinc powder and without zinc powder showed good corrosion resistance mainly due to the cathodic protection and barrier effect, respectively. On the other hand, coatings with an intermediate concentration (10–40 vol.%) of zinc powder was not successful in protecting a steel substrate efficiently. To improve anticorrosion property of zinc-rich coating, the surface modification of zinc particle was carried out with derivatives of phosphoric and phosphonic acid in the aqueous solution. The effects of the surface modification of zinc particle on corrosion resistance of the coating were investigated with scanning vibrating electrode technique (SVET) and X-ray photoelectron spectroscopy (XPS). The best anti-corrosion performance was achieved when the incorporated zinc particle was treated with phosphoric acid 2-ethylhexyl ester and calcium ion simultaneously, which induced the formation of alkyl-phosphate-calcium complex layer of 190 nm in thickness on zinc particles. Corrosion resistance was improved by the decreased zinc activity and the increased compatibility between the formed complex layer on zinc surface and polymer binder matrix.     

Effects of coating thickness and surface treatment on the corrosion protection of diglycidyl ether bisphenol-A based epoxy coated carbon steel

The corrosion resistance of carbon steel specimens was compared, using different surface treatment methods such as blasting and zinc phosphate treatment on carbon steel followed by application of different coating thickness. Specimen surface morphology was observed by SEM. The corrosion performance of epoxy coated carbon steel was examined by electrochemical impedance spectroscopy (EIS), along with immersion test in 0.5N-NaCl solution and NORSOK M 501 cyclic test, respectively. EIS results showed that higher thickness of epoxy coating and zinc phosphate treatment on carbon steel showed better corrosion protection than rest of the specimens. Specimens with no treatment and blasting treatment followed by epoxy coating, showed poor corrosion protection. Results of NORSOK M 501 cyclic test showed similar trend as those of EIS results however degradation of specimens was more severe than those from immersion test.     

Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism

Effect of zinc content in the epoxy zinc rich coating on the mechanism of corrosion protection was investigated using electrochemical impedance spectroscopy (EIS). The zinc content in the coating (on dry film) was varied from 40% to 90% in steps of 10%. Open circuit potential (OCP) measurements and salt spray tests were also carried out to generate supporting evidences and to deepen the understanding in the area of zinc rich coatings. The healing or bridging ability of these coatings was studied by making a scribe on the coating and monitoring the evolution of OCP. EIS was also utilized to screen the amount of zinc particles required to provide efficient galvanic protection and to study the effect of purity of Zn on corrosion protection performance. Both EIS and OCP measurements indicate that coatings containing 40% Zn (on dry film) provides very good barrier protection coming mainly from polymer whereas the one containing >80% Zn provides excellent galvanic protection to the steel substrate. When the zinc loading is between 60% and 70%, coatings neither show barrier protection nor galvanic protection.     

Zinc deposited polymer coatings for copper protection

Polypyrrole (PPy) and polyaniline (PAni) coatings were electrosynthesized on copper, by using cyclic voltammetry technique. Then, these coatings were modified with the deposition of zinc particles from aqueous zinc sulphate solution. The electrodeposition of zinc was achieved at a constant potential value of −1.20 V, in the amount of ∼0.75 mg/cm². The corrosion performance of zinc modified polymer coatings were investigated in 3.5% NaCl solution; by using the electrochemical impedance spectroscopy (EIS), and anodic polarization curves. The zinc particles improved the barrier property of polymer films, thanks to formation of voluminous zinc corrosion products within the pores of polymer coating. Also, the zinc particles provided cathodic protection to the substrate, where the polymer film played the role of conductance between zinc particles and copper.     

Improvement of the anticorrosion properties of polypyrrole by zinc phosphate pigment incorporation

A commercial zinc phosphate pigment was incorporated into polypyrrole (PPy) matrix during its electrochemical synthesis in order to improve the corrosion protection of polypyrrole on AISI 1010 steel. PPy/zinc phosphate composite films were synthesised in sodium salicylate medium with high current efficiency and containing 10% by weight of zinc and 4% by weight of phosphate. The influence of stirring and concentration of the electrolyte on the degree of pigment incorporation were investigated, as well as polymerisation time and applied current density. The morphology of the films was determined by scanning electron microscopy (SEM) and the distribution of pigment in the polymeric matrix was carried out by X-ray photoelectron spectroscopy (XPS). The PPy and PPy/zinc phosphate films were submitted to salt spray corrosion test, weight loss test and to electrochemical measurements like corrosion potential with time. In all tests, the composite films showed an enhancement in its protective action in comparison with PPy films.     

Performance of zinc phosphate coatings obtained by cathodic electrochemical treatment in accelerated corrosion tests

The formation of zinc phosphate coating by cathodic electrochemical treatment and evaluation of its corrosion resistance is addressed. The corrosion behaviour of cathodically phosphated mild steel substrate in 3.5% sodium chloride solution exhibits the stability of these coatings, which lasts for a week's time with no red rust formation. Salt spray test convincingly proves the white rust formation in the scribed region on the painted substrates and in most part of the surface on unpainted surface. The protective ability of the zinc corrosion product formed on the surface of the coated steel is evidenced by the decrease in the loss in weight due to corrosion of the uncoated mild steel, when it is galvanically coupled with cathodically phosphated mild steel. Potentiodynamic polarization curves reveal that E_corr shifts towards higher cathodic values (in the range of −1000 to −1100 mV versus SCE) compared to that of uncoated mild steel and conventionally phosphated mild steel substrates. The i_corr value is also very high for these coatings. EIS studies reveal that zinc dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the non-metallic nature of the coating is progressively increased due to the formation of zinc corrosion products, which in turn enables an increase in corrosion resistance with increase in immersion time. The zinc corrosion products formed may consist of zinc oxide and zinc hydroxychloride.     

The spatial distribution of Zn during galvanic corrosion of a Zn/steel couple

The spatial distribution of Zn²⁺ during galvanic corrosion of a model Zn/steel couple in 0.01 M NaCl was investigated using a scanning zinc disk electrode. The couple had a coplanar arrangement of a steel substrate with an electroplated zinc layer at the center. During galvanic corrosion, the marked changes in the Zn²⁺ concentration were confined to a thin solution layer ca. 1.0 mm thick above the couple surface. In this thin solution layer above the zinc layer, a higher concentration region of Zn²⁺ in the range of 5-18 mM extended around the zinc layer in the solution during galvanic corrosion. Conversely, above the steel surface distant from the zinc layer, the surface concentration of Zn²⁺ was almost zero during galvanic corrosion. On this surface, the precipitation of zinc corrosion products due to the hydrolysis reaction of Zn²⁺ was observed. The distribution of the Zn²⁺ concentration supported that Zn²⁺ acted as a buffer that suppressed the increased pH due to the cathodic reaction on the steel surface near the zinc layer and almost no corrosion products formed there. The spatial distribution of Zn²⁺ is discussed in relation to the distributions of potential and pH and the surface morphology of the galvanic couple.     

锌极化曲线的测定及应用实验研究

锌电极极化曲线的测定及应用实验研究很少,本实验采用恒电位法测定锌在不同出缓冲溶液中的阳极极化曲线。及在酸性介质磷酸溶液中和在酸性介质中加入缓蚀剂硫脲后的极化曲线。得到锌在不同介质中钝化电位、钝化电流等电化学参数,由此得出锌在pH9．18缓冲溶液中的钝化电位比在pH4．00和pH086中低。而钝化电位的范围更宽,说明锌在讲9．18溶液中更容易钝化。因为锌在此溶液中生成了致密的氢氧化膜,从而阻碍腐蚀。在酸性介质磷酸溶液中加入缓蚀剂硫脲后,阻碍了阳极过程,自腐电流下降。极化曲线的测量对化学电源、电镀、电冶金、电解、金属防腐蚀和电化学基础研究等有着重要的意义。     

耐液锌腐蚀材料的研究

实现热镀锌内加热技术的关键是解决耐锌蚀材料的问题。本文分析了当前解决液锌腐蚀的不同方法。研究了等离子喷涂耐液锌腐蚀陶瓷涂层,提出了等离子喷涂耐液锌腐蚀陶瓷涂层的设计方案。     

锌钙系磷化的最新应用研究

研究了3种新的锌钙系磷化工艺和配方，中温高耐蚀黑色磷化，磷化膜细致，耐蚀性优异，可用于钢铁的防锈磷化；钙改良的低温低锌磷化，钙的加入有利于镀锌板，铝板，钢板的复合件的磷化，可用于电泳涂装，中温耐热锌钙系磷化可用于需高温烘烤涂料涂装前的磷化处理，锌钙系磷化是磷化技术发展的一个方向。     

Influence of the alloying component on the protective ability of some zinc galvanic coatings

The composition of the corrosion products of pure Zn galvanic coatings as well as of some zinc alloys (Zn-Mn and Zn-Co) after treatment in selected free aerated model media (5% NaCl and 1N Na₂SO₄) is studied and discussed. X-ray diffraction and X-ray photoelectron spectroscopy investigations are used for this purpose. It is concluded that the corrosion products (zinc hydroxide chloride hydrate in 5% NaCl and zinc hydroxide sulfates hydrates in 1N Na₂SO₄) play a very important role for the improved protective ability of the zinc alloys toward the iron substrate, compared to the pure Zn coatings. Another result is that, for a given medium, the corrosion products are one and the same for both alloys independently of the fact that the alloying component is electrically more positive or negative than the zinc. Some suggestions about the models of the appearance of these products and their protective influence are also discussed.     

Zinc hypophosphite: a suitable additive for anticorrosive paints to promote pigments synergism

In this investigation, the synergism promoted by zinc hypophosphite in an anticorrosive pigment mixture is reported. This paper describes the anticorrosive behavior of a commercial pigment mixture containing zinc hypophosphite, reduced levels of zinc phosphate, and zinc oxide. The anticorrosive performance of the pigment mixture was assessed by electrochemical techniques (corrosion potential and linear polarization measurements) employing pigment suspensions. The behavior of each separate component of the mixture was also studied in the same way. The nature of the protective layer was investigated by scanning electron microscopy (SEM). In a second stage, the anticorrosive properties of the pigment were assessed by incorporating it into alkyd and epoxy paints that were evaluated by accelerated (salt spray and humidity tests) and electrochemical measurements. Experimental results showed that improved anticorrosion protection is achieved in paints with reduced zinc phosphate contents as a consequence of the synergistic interaction between zinc hypophosphite and the other components of the pigment mixture.     

Surface potential distribution over a zinc/steel galvanic couple corroding under thin layer of electrolyte

In this study, surface potential and surface pH changes over a zinc/steel galvanic couple corroding in artificial seawater (ASW) at 60 and 90% RH have been investigated. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test. The potential difference over the zinc and steel surface in 90% RH was very low (less than 200 mV) showing that whole steel surface was under galvanic protection. On the other hand, in 60% RH, after several days of corrosion the potential difference between the zinc coating and the steel surface was very high (more than 500 mV) and hence the galvanic protection was limited to interface region. The X-ray analysis of the sample corroded in 60% RH has shown that the zinc corrosion products were deposited on the steel surface near the interface, the same region has shown a low pH compared to than in other part of the steel surface. This led to conclude that with the progress of corrosion, the coating surface of zinc coated steel acidifies by the hydrolysis reaction of the dissolved zinc ions, and the iron surface showed the alkalinity by the oxygen reduction reaction. Moreover, the parts of the steel surface covered with zinc corrosion products had developed relatively less noble potential than other parts indicating that zinc corrosion products took a role to protect the base steel against corrosion. It was assumed that this behavior was related to a combination of the water absorbing capability of zinc corrosion products and adsorption of zinc ion on the steel surface due to low pH.     

The atmospheric corrosion of zinc: The effects of salt concentration, droplet size and droplet shape

The influence of droplet characteristics on the atmospheric corrosion of zinc was investigated using a multi-microelectrode approach. Arrays of close-packed zinc wires, 500 μm in diameter, were coupled such that the net anodic and cathodic current flowing through each electrode interface was measured as a function of time. Droplets in the range of 1–10 μL placed onto the array showed that electrochemical fields are immediately established and can remain unaltered for significant periods of time. The total charge passed was found to be almost identical for 0.6 M NaCl droplets of varying size when compensating for the electrolyte–zinc contact area, however significant differences in current exchange were observed when varying electrolyte concentration.In contrast to studies on other metals, the highest rates of zinc corrosion, as measured by the microelectrode array, did not occur during drying cycles. It was observed that the initial wetting and subsequent holding at high relative humidity led to the highest measured currents. Corrosion damage during drying cycles, however, appears to be enhanced for large droplets with high contact angle and increased electrolyte concentration. The importance of these observations to the modeling of atmospheric corrosion has been discussed and future developments of the technique are outlined.     

Modification of zinc powder to improve the corrosion resistance of weldable primers

This paper reports the modification of zinc powder to improve the corrosion resistance of weldable primers. These primers are thin zinc rich organic coatings applied by roll coating at the steel manufacturer. The automotive industry uses them to protect areas in the car body that become inaccessible after joining processes. In this work, with the objective of increasing the corrosion resistance of these systems, the zinc particles were chemically treated or simply replaced by powder of 55AlZn alloy. The rest of the formulation remained intact. The performance of the commercial and modified formulations was compared by SEM, SVET and EIS. The best results were obtained when the zinc powder was replaced by powder of the aluminium zinc alloy.     

The effects of zinc ions on the performance of epoxy coated mild steel under cathodic protection (CP)

This work aimed to address the issues that arise when cathodic protection is applied to a coated structure. The use of zinc chloride (ZnCl₂) as a corrosion inhibitor was suggested; the idea was to bridge the weakly cross linked areas on the coating by depositing a film of zinc compounds in order to seal them from the corrosive environment.     

Effect of Al on the galvanic ability of Zn-Al coating under thin layer of electrolyte

The effect of Al on the galvanic ability of Zn-Al coating has been studied under thin electrolyte layers by measuring surface potential and surface pH. The changes of surface potential and surface pH over Zn-Al/steel galvanic couple corroding in artificial sea water (ASW) were measured at 60% and 90% RH at 298 K. In the initial stage of corrosion, Zn-55Al coating has shown better galvanic protection ability than Zn-5Al coating in both 60% and 90% RH. However, Zn-5Al coating was better in long term corrosion. The better galvanic ability of Zn-55Al coating in the initial stage of corrosion was related to the observation of pH as low as low as 2 on its surface. The low pH value was due to hydrolysis of Zn²⁺ and Al³⁺ ions. The low pH value was further confirmed by observing evolution of gas due to H⁺ reduction on the Zn-55Al coating. With the progress of corrosion, the low pH region of coating layer extended towards the base steel. This helped expand the deposition of zinc corrosion products on the steel surface. The enhanced dissolution of zinc in Zn-55Al coating led to the formation of a barrier layer which limited the galvanic protection of remaining steel. This was not the case in Zn and Zn-5Al coating. The X-ray analyses of the corroded samples have shown the deposition of zinc corrosion products on the steel surface, which greatly depended on the RH value. The part of the steel surface covered with zinc corrosion products has shown relatively less noble potential than other part indicating that zinc corrosion products took a role to protect the base steel against corrosion. The results from surface potential and surface pH measurements were substantiated by the surface observation of the corroded sample during and after the corrosion test.     

脉冲电流对锌酸盐镀锌层的影响

研究了在锌酸盐镀锌体系中采用脉冲电流时，脉冲频率，脉冲的占空比、反向电流对镀层外观，耐蚀性的影响，发现在最佳脉冲条件下得到的镀锌层比直流电镀得到的镀层光亮，结晶细致，耐蚀性能有很大的提高。     

EIS investigation of zinc dissolution in aerated sulphate medium. Part II: zinc coatings

The kinetics of anodic dissolution and corrosion of zinc coatings deposited onto steel sheet either by electrodeposition or by hot dipping are investigated by electrochemical impedance spectroscopy in aerated sulfate medium. The results are compared with those obtained previously on pure bulk zinc and interpreted on the basis of the model derived in the first part of this paper. It is shown that zinc coatings are less sensitive to corrosion than pure bulk zinc and changes of their behavior with time are not identical. Important differences are observed between the various coatings in the respective contributions of the three parallel paths of the dissolution process. Three kinds of oxidation products were identified by Raman spectroscopy. A compact non-stoichiometric zinc oxide was formed by surface reaction on zinc. Above it, a thick and porous layer made of zinc hydroxi-sulfate and stoichiometric ZnO, was formed by precipitation from a local saturation of the solution. A strong correlation was evidenced between the oxidation products and the various paths of the reaction model. It was assumed that the impurities, initially present in the metal, may affect the interfacial reactions, the increase of the micro-roughness, and may also reinforce the protective properties of oxidation product layers. The differences between the various zinc coating behaviors result mainly from their impurities. Their crystal preferred orientations have no significant influence.     

Distribution of pH during galvanic corrosion of a Zn/steel couple

The spatial distribution of pH during galvanic corrosion of a model Zn/steel couple immersed in 0.01 M NaCl was investigated using a scanning tungsten pH electrode. The couple consisted of a steel substrate with a 3 μm-thick layer of zinc electroplated at its center. During galvanic corrosion, marked changes in pH occurred, which were confined to a thin layer of solution ca. 1.5 mm over the couple surface. As the surface was approached, the pH over the zinc layer decreased slightly from ca. 5.6 in the original bulk solution, while the pH over the steel surface distant from the zinc layer increased to ca. 11.5. The area of low pH extended not only over the zinc layer, but also over the steel surface adjacent to the zinc. Zinc corrosion products precipitated in the region of high pH. The pH distribution arising from the galvanic corrosion of the couple is discussed in terms of the potential distribution and the morphology of the precipitated zinc corrosion products.