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 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.     

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.     

双镀锌铝合金镀层的组织结构和耐蚀性

采用“双镀法”在钢板表面热浸镀不同铝含量的锌铝镀层(Zn-5Al、Zn-11Al、Zn-17Al和Zn-23Al),利用扫描电镜和能谱仪考察浸镀液中铝含量对镀层组织结构的影响,采用电化学测试、中性盐雾试验等手段评价镀层耐蚀性的变化。随着浸镀液中铝含量升高,镀层表面由片层状交替排布的富铝、富锌共晶组织向富铝枝晶网络结构转变,耐蚀性逐渐提高。但铝含量过高(质量分数大于17%)会导致大量脆性Fe-Al-Zn金属间化合物生成,合金层厚度明显增大。     

Protective mechanisms occurring on zinc coated steel cut-edges in immersion conditions

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn²⁺ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH)₂ film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH)₂ covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.     

汽车用钢板纳米晶镀锌层在氯化钠溶液中的腐蚀机理

将汽车用钢板置于碱性镀锌液中电镀,制备了纳米晶镀锌层.采用场发射扫描电子显微镜(FE-SEM)观察镀锌层在3.5%NaCl溶液中腐蚀前后的表面形貌,以能谱仪(EDS)研究腐蚀产物组成,运用塔菲尔(Tafel)曲线和电化学阻抗谱(EIS)考察纳米晶镀锌层的电化学腐蚀行为.结果表明,镀锌层由直径为80～100 nm的针状结...     

钛和铈对热浸锌镀层组织与耐蚀性能的影响

为了获得厚度适中、耐蚀性能好的热浸镀锌层,在锌浴中添加微量的Ti和Ce,分别在Zn-0.04%Ti、Zn-0.02%Ce和Zn-0.04%Ti-0.02%Ce镀浴中制得热浸锌合金镀层.采用金相显微分析以及电化学阻抗谱、电化学极化测试、中性盐雾试验,研究了Ti和Ce对镀层组织和耐蚀性能的影响.结果表明:在锌浴中添加0.0...     

机械镀铜及铜合金工艺及其耐蚀性研究

在机械镀锌和机械镀铝-锌工艺的基础上,通过调整活化剂和促进剂的种类及用量,研制开发了机械镀铜及铜合金工艺.利用该工艺在钢基表面上得到了铜以及铜-锌、铜-锌-铝、铜-镍等合金镀层.这些铜及铜合金镀层的耐蚀性能皆好于机械镀锌层,尤以铜-镍合金镀层的耐蚀性能最高,提高15倍左右;在铜-锌中加入2%～3%的铝可以进一步将铜-锌合金镀层的耐蚀性能提高2～3倍.     

机械镀锌-镍合金工艺的研究

在机械镀锌、锌-铝工艺的基础上,通过调整活化剂和促进剂的种类及用量,研制开发了机械镀锌-镍合金工艺.利用该工艺在钢基表面上得到了不同配比的锌-镍以及锌-铝-镍合金镀层.对这些镀层进行5%NaCl溶液喷雾加速腐蚀试验以及全浸加速腐蚀试验,并与机械镀锌层进行对比.结果表明,各种配比的锌-镍合金镀层的耐蚀性能皆好于机械镀锌层,并且随着镍含量的增加而提高;锌-铝-镍合金镀层的耐蚀性能比镀锌层提高7倍左右.     

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.     

Influence of rare earths addition on the corrosion behaviour of Zn-5%Al (Galfan) alloy in neutral aerated sodium sulphate solution

The effect of rare earth metal (Ce, Er, Y) additions on the corrosion behaviour of Zn-5Al Galfan alloy has been investigated. The corrosion resistance of Zn-5Al-1Ce, Zn-5Al-1Er and Zn-5Al-1Y alloys has been assessed by various electrochemical tests, such as corrosion potential measurements, polarization curves and electrochemical impedance spectroscopy. They have been performed in a 0.1 M Na₂SO₄ solution, at approximately neutral pH, without stirring and in contact with the air. For comparison, the electrochemical tests have also been carried out on the Zn-5Al alloy. Moreover, the surface morphology and nature of the corrosion products have been investigated. The results indicated that rare earths’ addition improves the corrosion behaviour of Galfan, the effect being more pronounced for the Er- and Y-containing alloys. The mechanism by which the corrosion resistance of Zn-5Al alloy is enhanced in presence of the rare earths is discussed.     

钢丝热浸双镀锌-铝稀土合金镀层的组织性能研究

通过热浸双镀工艺在不同钢丝基体上制得了锌-铝合金镀层,对镀层的形貌、耐蚀性能、显微硬度等进行了分析测试.研究表明,采用高碳钢为基体所得的镀层主要由粒状胞状组织组成;而低碳钢基体上的镀层则由层片状组织构成,同时前者具有较好的耐蚀性能.     

热浸Zn-0.05Al-0.2Sb合金镀层锌花耐蚀性研究

用金相显微观察、能谱分析(EDS).中性盐雾试验(NSS)和电化学阻抗谱(EIS)研究了热浸Zn-0.05Al-0.2Sb合金镀层表面的3种锌花:亮锌花,羽毛状锌花和暗锌花.结果表明,3种锌花表面都存在明显的Sb、Al偏析,按照亮锌花、羽毛状锌花和暗锌花的顺序,锌花的表面粗糙程度和表面Sb、Al偏析都依次增大,而耐盐雾腐蚀性能和EIS的低频阻抗依次降低.     

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.     

脉冲参数对镍镀层在NaCl溶液中耐蚀性的影响

采用扫描电镜观察镀层腐蚀前后的微观表面形貌,其腐蚀为小孔腐蚀,并分析镍镀层在NaCl溶液中的腐蚀机理.利用浸泡腐蚀试验及阳极极化曲线测试镍镀层的耐蚀性.利用阳极极化曲线测试结果作为正交试验指标,着重分析脉冲参数及pH值对镀层耐蚀性的影响规律.脉冲频率越大,脉冲平均电流密度在6 A/cm2左右,pH值为3.8时,镀层耐蚀性能最好.     

超塑锌铝合金铬酸盐钝化改进工艺

针对钝化试样表面的白雾现象,在原有工艺的基础上,增加了锌盐掩蔽工序,改进了锌铝合金铬酸盐钝化工艺。改进后的工艺操作简便、设备简单、流程稳定、产品质量可靠,不仅适用于锌铝合金制件室内装饰及防护,而且可作为锌铝合金涂覆的前处理。采用该工艺处理锌铝合金,其耐蚀性和外观得到改善,基体与涂层间的结合力增强。介绍了各工序的工艺条件和操作要点,对该工艺的使用推广和现场生产具有指导意义。     

机械镀Zn-Cr及Zn-Al-Cr合金工艺及耐蚀性

为了获得性能良好的含铬的锌合金镀层,在机械镀锌和机械镀Zn-Al合金工艺的基础上,通过调整活化剂和促进剂的种类及用量,开发了机械镀Zn-Cr及Zn-Al-Cr合金工艺。利用该工艺在钢基表面获得了Zn-Cr及Zn-Al-Cr合金镀层。对这些镀层进行了5%NaCl溶液喷雾腐蚀试验,并与机械镀锌层进行了对比。结果表明,Zn-Cr及Zn-Al-Cr合金镀层的耐蚀性能皆好于机械镀锌层。     

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.     

Influence of power ultrasound on the corrosion of aluminium and high speed steel

The influence of power ultrasound on the corrosion of aluminium and high speed steel in chloride medium has been investigated. Open circuit potential, polarization curves and electrochemical impedance spectroscopy were employed to study the corrosion process before, during and after ultrasonic irradiation as well as scanning electron microscopy of the corroded surfaces. Cavitation led to formation of pits, erosion and increased mass transport. The effect of ultrasound power, chloride concentration, horn tip–metal distance and of tip diameter were assessed. The principal effect of ultrasound on aluminium is destruction of the oxide film and on high speed steel removal of corrosion products and increased solution mass transport; the influence of ultrasound is greatest at low chloride concentrations. For both metals it was shown that for chloride ion concentrations up to 0.1 M the influence of ultrasound power is most evident, followed by the distance between probe tip and metal, then [Cl^–] and lastly the size of the probe tip. The potentialities of using ultrasound as a rapid diagnostic test of corrosion resistance are indicated.     

钢结构防腐配套方案的设计

论述了钢结构腐蚀机理和腐蚀防护措施。介绍了大气环境的6种腐蚀分类方法。对处于不同腐蚀环境中的钢结构涂装规范进行了详细说明。以两个成功的案例说明了锌粉底漆 环氧中间漆 丙烯酸聚氨酯面漆涂装配套体系具有较佳的防护性能。