Corrosion performance of Zn–Mg–Al coated steel in accelerated corrosion tests used in the automotive industry and field exposures

The corrosion performance of Zn–Mg(1–2%)–Al(1–2%) (ZMA) coatings has been compared to zinc–iron alloy (galvannealed, GA) and zinc–aluminum coating (Zn–5Al, Galfan) as well as to conventional zinc coatings produced by hot‐dip galvanization (HDG) and electrogalvanization (EG). For this purpose, cosmetic samples (painted and uncoated) and hem‐flange panels were produced. Their corrosion performance was compared in three different accelerated corrosion tests, as regularly used by the automotive industry, e.g., VDA621‐415, N‐VDA (VDA233‐102), and Volvo STD 423‐0014. As can be concluded from our results, the behavior of ZMA coatings was strongly dependent on the testing conditions as well as on the configuration of the samples. The advantageous effect of ZMA coating was more pronounced in open situations than in confined ones, irrespective of the testing conditions. ZMA coatings provided a significant improvement in comparison to conventional coatings in tests involving a significant salt load such as VDA621‐415 or neutral salt spray especially on cosmetic configurations. By contrast, the beneficial effect of ZMA coatings was less obvious in tests with lower salt load (VDA233‐102, Volvo STD423‐0014), particularly when considering cosmetic corrosion on painted samples and corrosion in confinement. Interestingly, no significant differences were observed between samples with varying Al and Mg content in the metallic coating (1–2% each). The results were compared to data from field exposure at stationary sites.     

Influence of climatic factors in cyclic accelerated corrosion test towards the development of a reliable and repeatable accelerated corrosion test for the automotive industry

Two designs of experiments made of nine accelerated tests each were used in order to study the influence of a selection of important climatic parameters such as the concentration of NaCl, the drying level, the basic humidity cycle, the frequency of salt spray and the temperature. The accelerated corrosion tests were all performed using automatic chambers. Different automotive materials were selected in order to study the resistance to cosmetic corrosion, perforation corrosion and bi‐metallic corrosion. For cosmetic materials, coated panels of cold rolled steel (CRS), zinc‐coated steel and aluminium alloys were chosen. Perforation corrosion was investigated using crevice panels of CRS, zinc‐coated steel or different grades of aluminium alloys. From the results, the influence of testing conditions on the cosmetic and perforation corrosion of different automotive materials was obtained. As an example, it was shown that an elevation of the temperature from 35 to 45 °C in the cyclic corrosion test increased the scribe creep on painted CRS and aluminium alloys while it has no significant effects on painted hot dip galvanized (HDG) panels. The results were also compared to that obtained after 2 years of exposure at a marine exposure site and on busses driving in area using de‐icing salt. It was shown that one of the tests performed gives a good correlation to field results.     

加速腐蚀实验研究碳钢的大气腐蚀行为

目的快速研究大气环境对钢制输电铁塔腐蚀的影响。方法采用室内加速腐蚀实验,研究碳钢试样在湿度和SO2浓度不同的气氛中的腐蚀行为。结果用极化曲线法测得的金属腐蚀速率与失重法测得的结果具有很高的一致性。相对湿度较低时,改变腐蚀性气体的浓度,对碳钢的腐蚀影响不显著;碳钢在高湿度的含SO2气氛中会发生严重腐蚀。结论降低湿度是防止或减缓碳钢在含SO2气氛中腐蚀的有效途径。     

Effect of porosity of phosphate coating on corrosion resistance of galvanized and phosphated steels Part II: Evaluation of corrosion resistance

The effect of porosity of phosphate coatings on the corrosion resistance of ungalvanized (UG), electrogalvanized, and hot dip galvanized steels is evaluated in this study. The corrosion resistance of phosphatized and painted steel is related to the integrity and continuity of phosphate and paint layers, and pores in the phosphate layer affect the corrosion resistance of material. The porosity of the phosphate coating was evaluated by using the cathodic polarization electrochemical test. To evaluate the corrosion resistance of the phosphatized and painted steels, they were submitted to accelerated corrosion tests. As was expected, the creepback from the scribe increased with the increase in porosity. This behavior was evident for UG steel, but less evident for galvanized steels due to cathodic protection and/or barrier effect of the zinc coating.     

Corrosion performance of Zn–Al–Mg coatings in open and confined zones in conditions simulating automotive applications

Panels coated by hot dipping with zinc (HDG), Zn–5Al (Galfan) and Zn–1.5Al–1.5Mg coatings at different thicknesses were phosphated and painted on an industrial line. Crevice panels with non‐painted bare parts modelling conditions in hem flanges, reference panels with open surfaces and formed non‐painted panels were exposed to a cyclic accelerated automotive test. Zn–Al–Mg coatings with the thickness of 10 µm provided similar or even better protection than HDG and Galfan at 20 µm in both confined and open configurations. In comparison to 10‐µm HDG, the Zn–Al–Mg coating delayed red rust appearance in crevices by a factor of 2 and the maximal depth of corrosion in the steel substrate was by 42% lower. Confined areas were more corroded than open surfaces. For HDG, the time to red rust appearance dropped by 50–75%, corrosion attack in steel was from 3.5 to 7 times deeper and mass gain was about 2.3 times higher in crevices than on open surfaces. Corrosion of Zn–Al–Mg may be more affected by local environmental conditions created by the crevice configuration than for HDG. Red rust appearance on formed panels of 20‐µm Galfan, 7‐, 10‐ and 14‐µm Zn–Al–Mg was delayed to 10‐µm HDG by a factor of 2.8, 3.5, 3.8 and >4.5, respectively. No adverse effect of forming was noticed. The results indicate that 2‐ to 3‐fold reduction of the coating thickness for Zn–Al–Mg alloy coatings in comparison to traditional HDG may be possible without compromising the corrosion performance.     

Effects of coolant chemistry on corrosion of 3003 aluminum alloy in automotive cooling system

In this work, effects of coolant chemistry, including concentrations of chloride ions and ethylene glycol and addition of various ions, on corrosion of 3003 Al alloy were investigated by electrochemical impedance spectroscopy measurements and scanning electron microscopy characterization. In chloride‐free, ethylene glycol–water solution, a layer of Al‐alcohol film is proposed to form on the electrode surface. With the increase of ethylene glycol concentration, more Al‐alcohol film is formed, resulting in the increase in film resistance and charge‐transfer resistance. In the presence of Cl^? ions, they would be involved in the film formation, decreasing the stability of the film. In 50% ethylene glycol–water solution, the threshold value of Cl^? concentration for pitting initiation is within the range of 100 ppm to 0.01 M. When the ethylene glycol concentration increases to 70%, the threshold Cl^? concentration for pitting is from 0.01 to 0.1 M. In 100% ethylene glycol, there is no pitting of 3003 Al alloy even at 0.1 M of Cl^?. Even a trace amount of impurity cation could affect significantly the corrosion behavior of 3003 Al alloy in ethylene glycol–water solution. Addition of Zn²⁺ is capable of increasing the corrosion resistance of Al alloy electrode, while Cu²⁺ ions containing in the solution would enhance corrosion, especially pitting corrosion, of Al alloy. The effect of Mg²⁺ on Al alloy corrosion is only slight.     

Combined corrosion and fatigue performance of joined materials for automotive applications

The aim of this study is to evaluate fatigue performance of joined assemblies (spot weld and/or adhesive bonding) in corrosive environment. Various assemblies have been tested in alternated and simultaneous fatigue‐corrosion modes. Adhesive joints are strongly affected by simultaneous fatigue‐corrosion with a large drop of the fatigue life compared to results in air. By alternating fatigue and corrosion, the reduction of fatigue life is important. For spot welding, fatigue life is decreased at higher load amplitudes and increased at lower amplitudes. These results are strongly linked to the opening of the gap near the spot weld at high load amplitudes. At low amplitudes, corrosion might limit the local stress at the notch root of the weld.     

430不锈钢室内加速腐蚀与大气暴露的相关性研究

目的研究室内腐蚀膏试验对430不锈钢的加速性,以及腐蚀膏试验与大气暴露腐蚀试验的相关性。方法采用室内腐蚀膏腐蚀试验对材料进行腐蚀,研究其腐蚀过程的电化学特性和机理,腐蚀产物形貌、组成和结构,以及腐蚀过程的动力学规律,并与材料大气环境暴露试验的结果对比,探讨腐蚀膏试验与大气暴露试验的相关性。最后计算出室内腐蚀膏试验对430不锈钢的加速性。结果 430不锈钢在腐蚀膏腐蚀试验中,腐蚀失重与时间方程为P=0.3998t1.0391,呈幂指数关系,在腐蚀初期腐蚀速度最大。腐蚀机理主要是点蚀。由于腐蚀膏中的Cl-对钝化膜的穿透作用,使得钝化膜在一定点能够维持高的电流密度,并使阳离子杂乱移动而活跃起来,当膜-溶液界面电场达到临界值,点蚀发生。腐蚀表面存在点蚀坑,且随着腐蚀时间的增加,点蚀区域增加,相邻点蚀相互连接,形成腐蚀面。腐蚀面可见红色锈斑,对腐蚀产物进行XPS分析,其主要成分为Fe OOH和Fe3O4。430不锈钢在大气环境的腐蚀过程中,其腐蚀机理也是点蚀,腐蚀失重与时间也呈幂指数关系,腐蚀产物和形貌结构与室内腐蚀膏试验相同。结论腐蚀膏试验对430不锈钢腐蚀具有加速性,且与其大气暴露试验具有相关性,可以作为模拟430不锈钢大气腐蚀的一种加速试验。     

镀锌板在不同加速腐蚀环境下的腐蚀行为研究

目的研究中性盐雾环境、中性盐雾加周浸环境和中性盐雾加湿热环境下镀锌板的腐蚀行为,明确不同加速腐蚀试验环境对镀锌板腐蚀行为的影响。方法对镀锌板进行中性盐雾、中性盐雾加周浸、中性盐雾加湿热等不同类型的加速腐蚀试验,通过失重法、SEM、XRD以及极化曲线等方法,对比不同加速腐蚀试验类型对镀锌板腐蚀行为的影响。结果在三种加速腐蚀试验环境下,镀锌板的腐蚀过程基本相同:镀锌层及白色锌锈腐蚀、锌层破裂、锌层完全脱落、基体碳钢腐蚀。腐蚀产物都以Zn(OH)_2、γ-FeOOH、ZnO、Fe_2O_3等为主。三种环境下镀锌板的腐蚀都不均匀,不同位置的腐蚀情况存在一定差异。其中,中性盐雾环境下材料腐蚀情况最为严重,中性盐雾加周浸环境次之,中性盐雾加湿热环境下最轻。结论三种加速环境都是主要通过Cl~-在锌表面的沉降和溶解作用增强介质的导电性能,加剧锌层的腐蚀。在中性盐雾环境中,Cl~-浓度最高,试样腐蚀最严重。在中性盐雾加周浸环境中,SO_4~(2-)与Cl~-协同作用,腐蚀也较严重。而中性盐雾加湿热环境中,Cl~-被稀释,腐蚀情况最轻微。     

Corrosion performance and mechanical properties of joined automotive materials

The automotive industry envisions that an optimized vehicle in terms of performance and cost can be achieved only by using different materials at different vehicle locations in order to utilize the functionality of the different materials to a full extent. Currently, steel and aluminum are the most important construction materials for the mass production of automotive structure. However, other materials such as magnesium alloys and stainless steel are also used. The use of dissymmetric assemblies of materials in the automotive industry has also led to the development of joining technologies other than spot welding and arc welding such as clinching, adhesive bonding, laser welding, and MIG brazing. However, and despite the development of these new joining technologies, there are still important gaps of knowledge with regards to the corrosion performance of different joint populations using dissymmetric and symmetric materials. Materials commonly used in the automotive industry including steel and aluminum‐based susbtrates were assembled with different combinations using various joining techniques in order to evaluate their corrosion performance as well their mechanical properties after cyclic accelerated corrosion tests. The results indicated a relationship between the corrosion inside the confined joint and the decrease of the mechanical properties of the assemblies.     

Long-term corrosion resistance of zinc-rich paint using functionalised multi-layer graphene-tripolyphosphate: in situ creation of zinc phosphate as corrosion inhibitor

ABSTRACT

This study investigates the effect of hybrid multilayer graphene-tripolyphosphate on long-term corrosion protection efficiency of zinc-rich paint. The hybrid nanomaterial as a corrosion resistance modifier was prepared trough a simple method by the addition of sodium tripolyphosphate aqueous solution to a dispersed multi-layer graphene in water/ethanol during stirring. The performance of the hybrid nanomaterial was studied by salt spray, electrochemical impedance spectroscopy, scanning electron microscopy, X-ray diffraction, cathodic delamination and pull-off adhesion test. The experimental results showed that in the presence of 1?wt-% of the hybrid nanomaterial, cathodic protection duration increased from 28 days to more than 50 days. The improvement in the barrier property and the cathodic delamination resistance was significant in the presence of the nanomaterial after two years long-term study. X-ray diffraction analysis showed that the formation of zinc phosphate was responsible for the higher corrosion protection efficiency.     

2A11铝合金/碳钢偶接件在强化自然环境条件下的腐蚀特性

采用户外周期喷淋试验,研究了2A11铝合金与Q235钢组成的偶接件在强化自然环境条件下的腐蚀行为与规律.结果表明:2A11铝合金的偶接部位发生严重腐蚀,腐蚀严重部位主要在距接触边缘5～10mm的范围内;铝合金的腐蚀质量损失随暴露时间的延长而不断增大;电偶腐蚀和缝隙腐蚀并存是铝合金偶接部位腐蚀严重的主要原因,大气环境中沉...     

A novel method to determine the flow accelerated corrosion rate in the elbow

Flow‐accelerated corrosion (FAC) is the most common failure in nuclear power plants. The FAC rate conforms to the experimental results in lab, which are calculated by some other FAC rate prediction models. However, when these models are employed to calculate the FAC rate of the elbow, the result is opposite to actual situation of failure of the elbow. In this paper, a new prediction model is proposed to calculate the flow accelerated corrosion (FAC) rate in the elbow, which combine the steady‐state mass transfer model of electrochemical theory and one‐dimensional galvanic corrosion model. Firstly, the distribution of velocity in the elbow is counted by FLUENT. Secondly, the free corrosion potential and the free corrosion current density are calculated by steady‐state mass transfer model. At last, the novel model is used to obtain the FAC rate of the elbow. The result will show that the FAC rate of the outward bend of the elbow is two‐orders than the inward bend of the elbow. The outward bend of elbow is the defective position, which accords with the statistical result of Kuen Ting et al.