Cover Picture: Materials and Corrosion. 12/2020

Cover: Optical microscopy of anodes self-corrosion after 139 days in the deep sea at a depth of 1420 m: (a) Al-Zn-In-Mg-Ti-Ga-Mn, corrosion pits with two kinds of distinct surface morphologies: (i) deep pits (pit 1: typically 300 μm deep) and (ii) broad/shallow pits with rough surface features (pit 2: typically 75 μm deep). (b) Al-Zn-In-Si, only large corrosion pit features and the diameter is about 2.5 mm. More detailed information can be found in: Shaohua Xing, Jian Hou, Mingxian Sun, Weimin Guo, Likun Xu, Weili Li, Julian Wharton, In situ study of the deep sea electrochemical performance of aluminium-based galvanic anodes, Materials and Corrosion 2020 , 71, 1946.     

图像分维对腐蚀坑分布特征的表征

采用图像分维探讨了腐蚀图像特征与表面腐蚀坑分布特征的关系.统计碳钢土壤腐蚀试片表面测量数据表明,蚀坑直径和深度分布在一定尺度范围内遵循幂函数方程,即蚀坑直径或深度与相应出现概率(个数)在双对数坐标存在线性区,其线性区斜率代表坑直径或深度的分布分维,反映腐蚀表面的粗糙程度.同时,根据盒计数法原理提出计算图像二维和三维分维的方法.大量实验数据证实,腐蚀图像二维和三维分维分别与坑直径和坑深度分布分维线性相关,并在数值上近似相等.这为根据腐蚀图像特征确定腐蚀表面坑分布规律提供了简便快捷的方法.     

图像灰度与腐蚀特征的相关性研究

根据实验数据研究腐蚀图象灰度和实际腐蚀深度的相关性,发现对孤立、规则圆孔的图像,经双向扫描校准后,其局部平均灰度和孔深存在很好线形关系,相关系数达0.95;对实际腐蚀图像,其局部平均灰度和相应位置的深度仍在0.95置信度下线性相关。但复杂深坑可能导致灰度值偏高（亮）,而且,坑深和灰度统计分布相关曲线表现为斜率不同（以坑深0.045cm为转折点）的两段直线,综上所述,基于图像灰度的腐蚀形貌诊断更适合腐蚀发展初期。此外,图像灰度和实际腐蚀参数的相关性研究表明,图像灰度极差是平均腐蚀速度及最大坑深的良好表征,它们在0.95置信度下线性相关。     

In situ pH responsive fluorescent probing of localized iron corrosion

A fast and versatile method has been demonstrated to detect the early stage of iron corrosion in situ, with minimum external perturbation, by using a pH sensitive fluorescence dye. The decreasing fluorescence intensity inside corrosion pits induced by hydrogen ions produced during iron corrosion is used as a signal indicator. Identification of corrosion pits (with resolution of <1 μm) and quantification of corrosion rate based on local hydrogen concentration were achieved. The developed technique has been applied to show that surface roughness, under a constant current condition, influences the corrosion of iron. Corrosion occurs preferentially adjacent to local topographic features.     

Pitting corrosion behaviour of AZ31 magnesium in tropical marine atmosphere

Corrosion behaviour of AZ31 magnesium during the initial six exposure periods in a tropical marine atmosphere is investigated. The results reveal that corrosion process of magnesium is dominated by pitting corrosion which consists of initiation of new pits, propagation of small scale pits and coalescence of neighbouring pits. There exists a critical depth above which the pits cease to grow down, resulting in the fluctuation of the mean pit depth. Different exposure conditions are found to be crucial for the different pit characters. Pits on the skyward surface are in deep-hole shape, while the groundward surface is covered with shallow dish pits.     

A novel accelerated corrosion test for exhaust systems by means of power ultrasound

Cycling corrosion tests have been performed to simulate corrosion conditions in the cold end of an automotive exhaust system. A middle range 1.4512 (AISI 409) stainless steel is submitted to a conventional dip dry test (DDT) parallel to a similar test but including an additional external stress thanks to an ultrasonic transducer. This new ultrasonic test (so called UST) is expected to reduce the diagnostic time of the corrosion test by the combined action of the chemical corrosion process and of mechanical degradations. Both corrosion tests are performed in two different media in order to simulate internal corrosion due to exhaust gas condensate and external cosmetic corrosion, greatly enhanced by road salt during winter. It respectively concerns a synthetic gas condensate, the composition of which is derived from what is obtained from motor gasoline combustion, and NaCl solution. In both electrolytes the stainless steel suffers from pitting corrosion. Samples are examined by optical micrography and surface profilometry. The degradation state is quantified according to three parameters: maximum pit depth, average pit diameter and porosity rate. The efficiency of each test is then related to the time of immersion. As expected, use of ultrasound allows pits growth to be achieved from the beginning of the exposure time, so that the maximum pit depth recorded after 180 h of immersion is twice than with the classical dip dry test. It seems that it does not modify the pit initiation mechanism but only increases growth kinetics.     

The kinetics of pit growth on alloy 600 in chloride solutions at high temperatures

Alloy 600 tube specimens were subjected to corrosion in de-aerated 0.1 M NaCl solution at 100–250°C at applied potentials more positive than the respective pit nucleation potentials, and in de-aerated 200 ppm CuCl₂ solution at 150 and 280°C under open circuit conditions. In NaCl, the pits grew in depth proportionally to t^0.3, t^0.5 and t^0.7 (t = time) at 100, 150 and 250°C, respectively. In CuCl₂ at both 150 and 280°C, the pit deepening rate was proportional to t^0.4. In both the solutions studied, the density of pits observed on the surface of specimens after the same time of corrosion at 100°C was less than at higher temperatures, whereas the pit depth was greater at 100°C than at higher temperatures. The morphology of pits was temperature dependent. The corrosion products accumulated on pit bottoms were enriched in Cr, Ti and S, and impoverished in Fe and Ni.     

Morphological characteristics and geometric prediction model of pits in complex corrosion environment

For aircraft serving in coastal environments, the aluminum alloy skin is exposed to high temperature, humidity, and salt environment for a long time. Pitting and exfoliation will affect its normal service life. In this article, static salt spray experiments and electrochemical tests were carried out on 2198-T8 Al–Li alloy in nine different corrosive environments. The effects of salt concentration, temperature, and humidity on the corrosion behavior were analyzed and compared. The relationship between corrosion environment parameters and pits was established. The results show that, under the same corrosion environment, the length, width, and depth of the pits all follow the log-normal distribution; compared with the salt concentration and humidity, the increase in temperature has the most obvious promotion on the corrosion rate. Based on the frequency statistics of pit size and electrochemical test results, the pit morphology was simplified to a semiellipsoid geometric model, and the pit size in different corrosive environments was predicted. The predicted size data all fall within the 1.5-fold dispersion region.     

Long-term external pitting and corrosion of buried cast iron water pipes

Improved understanding of the development with time of pitting and corrosion of the external surfaces of buried cast iron water pipes is crucial for the management of water supply distribution networks. The present study reports observations of surface topography of cast iron pipes exposed between 34 and 129 years in clay soils. Pit depths, surface areas and widths associated with deeper pits are reported, including as functions of exposure period. Pitting appears to progress in a step-wise manner as also earlier reported for the corrosion of steels. While there is a relation between pit depth and lateral size of pits through the amalgamation of pits to form plateaus into which new pits tend to form, the predominant progression of material loss through corrosion and pitting is into the metal rather than laterally. The practical implications of these observations are discussed.     

Novel images of the evolution of stress corrosion cracks from corrosion pits

SCC in turbine disc steels exposed to simulated steam-condensate tends to nucleate preferentially from corrosion pit precursors. The evolution of these cracks is not straightforward and not well understood. In this work, unique three-dimensional X-ray microtomographic images have confirmed that cracks develop predominantly at the shoulder of the pit, near the pit/surface interface, for specimens stressed to 50–90% σ_0.2. In support of this observation, FEA of model pits indicate that strain is a maximum on the pit wall just below the pit mouth. Implications of these observations for the pit-to-crack transition and predictive-modelling of crack nucleation and growth are discussed.     

Pitting corrosion in low carbon steel influenced by remanent magnetization

A new type of experiment is performed using a closed magnetic circuit configuration in order to study the influence of the magnetic field on pitting corrosion in low carbon pipeline steel. The statistical assessment of the pitting corrosion data shows that the magnetic field in the material under test reduces the extreme pit depths and also the average depth of the pit population. It is proposed that reduction in pit depth under the influence of remanent magnetization can be explained based on the behavior of the paramagnetic corrosion products under the influence of the magnetic field gradient produced inside pits.     

Pit growth behaviour of aluminium under galvanostatic control

The pit growth process on (1 0 0) aluminium under anodic pulse current in a mixed solution of 1 M HCl and 0.1 M H₂SO₄ at 30 °C has been evaluated using potential transient measurements and pit size distributions obtained by scanning electron microscopy. Sustained pit growth is observed for all pits during the initial anodic potential rise before reaching a steady-state etch potential, whereas a substantial fraction of the pits passivate at the steady-state etch potential. The pit growth rate during the initial potential rise is 3.4 μm s⁻¹, which is similar to that at the steady-state etch potential. The growth rates of active pits are potential-independent.     

A numerical study of damage caused by combined pitting corrosion and axial stress in steel pipes

Localized pipe wall damage accounts for many failures. Numerical modelling of pipes under increasing axial load and constant internal pressure when there is corrosion pits on the exterior surface of the pipe is reported herein. It is shown that for the assumed ideal elastic–plastic material the shape and volume of the plastic field depend on pit depth and its geometry. Pipe wall fracture around a pit can be associated with a critical plastic section. The results reported herein should be relevant for estimating of the risk of perforation and of loss of contents for steel pipes under different loading.     

Influences of NH4 + ions and thin electrolyte layers (TEL) thickness on the corrosion behavior of AZ91D magnesium alloy

In this study, the influences of NH₄ ⁺ ions and the thin electrolyte layer (TEL) thickness on the corrosion behavior of the AZ9D magnesium alloy in NH₄⁺‐containing environments were investigated by electrochemical measurements and surface characterization. The experimental results indicate that NH₄⁺ greatly accelerates the corrosion of AZ91D magnesium alloy whether in a bulk solution or in a TEL. As the TEL thickness decreasing, the corrosion resistance of the AZ91D magnesium alloy is strengthened. According to the corrosion morphology, electrochemical analysis, and characterization analysis of corrosion products, the corrosion mechanism of AZ91D under TELs with different thicknesses is divided into three stages: (a) uniformly distributed corrosion pits with deep depth and large size when TEL thickness is higher than or equal to 500 μm; (b) slightly corrosion with randomly distributed corrosion pits and the accumulation of small amount of corrosion product when TEL thickness is between 100 μm and 200 μm; (c) no corrosion pit with only deposition of corrosion product when TEL thickness is lower than or equal to 100 μm. Among which, the roles of NH₄⁺, TEL thickness, and corrosion product are emphasized and discussed in the corrosion process of AZ91D magnesium alloy under TEL with different thicknesses.     

表面形貌对冷作模具钢摩擦磨损性能的影响

目的 为了研究表面形貌对冷作模具钢表面摩擦系数及磨损性能的影响。方法 用打磨和化学腐蚀法分别获得了光滑表面形貌和带有腐蚀坑的表面形貌,通过采用扫描电镜和激光共聚焦显微镜观察和测量表面形貌的分布和大小,研究了腐蚀时间对表面形貌的影响。通过对腐蚀前后的试样进行摩擦磨损试验,研究分析了表面形貌对冷作模具钢表面摩擦系数及磨损性能的影响。结果 试样NO.1、NO.3表面的腐蚀坑比NO.2、NO.4明显。试样NO.0的摩擦系数随着摩擦时间的增加不断增大,而试样NO.1、NO.2、NO.4的摩擦系数相对稳定,无明显增大趋势,摩擦系数最小的试样为NO.4,其摩擦系数相对NO.0降低了52.26%。试样NO.0的磨痕相对其他试样较规则,且磨痕上有鳞状物,而其他试样则没有,试样NO.0的磨痕两侧峰值较大,其他试样的磨痕曲线不是很明显。结论 采用浸泡腐蚀可以得到具有一定腐蚀坑的表面形貌,腐蚀坑的大小、深度与腐蚀时间有关,腐蚀坑的深度对摩擦系数有非常大的影响,腐蚀坑可以减少磨屑导致的二次磨损。合理的表面形貌可以有效地改善模具的磨损情况。     

Structure–property quantification of corrosion pitting under immersion and salt-spray environments on an extruded AZ61 magnesium alloy

Extruded AZ61 magnesium coupons were exposed to immersion and cyclical salt spray environments over 60 h in order to characterize their corrosion rates. The characteristics of general corrosion, pitting corrosion, and intergranular corrosion were quantified at various intervals. General corrosion was more prevalent on the immersion surface. In addition, more pits formed on the immersion surface due the continuous exposure to water and chloride ions. However, the pits on the salt spray surface showed larger surface areas, larger volumes, and covered more area on the micrographs as compared to the pits on the immersion surface, due to the dried pit debris that trapped chloride ions within the pits.     

Die Bedeutung der metastabilen Lochkorrosion bei hochlegierten Stählen

The importance of metastable pitting corrosion in the case of high alloy steels A suitable computer-aided experimental method allows during potentiostatic tests to recognize and quantitatively treat current transients. The current transients result from metastable pitting phenomena below the pit propagation potential. It is possible under certain conditions to detect metastable pitting which in the SEM appears in the form of small (approximately 1 μm), in most cases hemispherical pits. A detailed study of metastable pitting has brought about fundamental knowledge about the mechanism of pit initiation, stable pit growth and repassivation in pitting and crevice corrosion processes.     

Pit to crack transition in stress corrosion cracking of a steam turbine disc steel

Long term exposure tests have been carried out on a 3 NiCrMoV steam turbine disc steel in the form of cylindrical tensile test specimens self-loaded to 90% of σ_0.2 and exposed to three environmental conditions, viz. deaerated pure water, aerated pure water, and aerated water containing 1.5 ppm of chloride ion. Pitting occurred in all environments but the density and depth of pits in the chloride-containing medium was markedly greater. No cracking was observed in deaerated pure water but cracks initiated in aerated water between 13 and 19 months and in less than 7 months in aerated 1.5 ppm Cl⁻ solution. The probability of a crack initiating from a pit of specific depth in aerated solution could be described well by a Weibull function. Profiling of pits and cracks in the disc steel tested in aerated 1.5 ppm Cl⁻ solution showed that there while there were many cracks with a depth greater than that of the corresponding pit the depth of some cracks was smaller than that of the corresponding pit, suggesting that cracks do not necessarily initiate from the bottom of the pits. The growth rate of short cracks emerging from pits appeared greater than that of long cracks in fracture mechanics specimens.     

An alternate explanation for the abrupt current increase at the pitting potential

A critical transition potential is shown to be associated with the abrupt increase in the number of metastable pits and the onset of spatial clustering of pits amongst nearest neighbor defects in both commercial and model stainless steels. This critical transition potential is correlated with the parameter, E_pit commonly observed in near-neutral NaCl solutions when stainless steels form pits. The explosive growth of pit sites, the onset of clustering of pits, and E_pit all depend on the diffusion length associated with aggressive corrosion products formed at pits and thus on the stirring rate. E_pit is observed to increase significantly and pit sites remain random to high potentials in both experiments and modeling when the diffusion length is decreased below the nearest neighbor distance (NND) associated with surface defects such as non-metallic inclusions that form a random array of susceptible sites across the otherwise passive surface. This phenomenon is confirmed by experiments and modeling using model stainless steel alloys with decreased inclusions densities and, thus, increased defect NNDs. If the NND is greater than the lateral diffusion length, then E_pit will increase, clustering of pit sites will decrease and explosive growth in the number of pits will be suppressed at a given applied point.     

On the detection of corrosion pit interactions using two-dimensional spectral analysis

A statistical methodology for detecting pits interactions based on a two-dimensional spectral analysis is presented. This method can be used as a tool for the exploratory analysis of spatial point patterns and can be advanced as an alternative of classical methods based on distance. One of the major advantages of the spectral analysis approach over the use of classical methods is its ability to reveal more details about the spatial structure like the scale for which pits corrosion can be considered as independent. Furthermore, directional components of pattern can be investigated.The method is validated in a first time using numerical simulations on random, regular and aggregated structures. The density of pits, used in the numerical simulations, corresponds to that assessed from a corroded aluminium sheet. In a second time, this method is applied to verify the independence of the corrosion pits observed on the aforementioned aluminium sheet before applying the Gumbel theory to determine the maximum pit depth. Indeed, the property of independence is a prerequisite of the Gumbel theory which is one of the most frequently used in the field of safety and reliability.