中国腐蚀与防护学报, 2017, 37(3): 279-286
doi: 10.11902/1005.4537.2016.023
7020铝合金在3.5%NaCl溶液中的点蚀行为

Pitting Corrosion of 7020 Aluminum Alloy in 3.5%NaCl Solution
戴芸1,2,3, 刘胜胆1,2,3,, 邓运来1,2,3, 张新明1,2,3

摘要:

采用浸泡实验与电化学循环极化曲线测试研究了7020铝合金在3.5% (质量分数) NaCl溶液中的点蚀行为,并结合金相显微镜 (OM)、扫描电镜 (SEM) 及扫描透射电镜 (STEM) 的微观组织观察结果对相关机理进行了分析和探讨。结果表明:7020铝合金的最大点蚀深度随时间变化的曲线为S型,呈缓慢增长-快速增长-保持稳定的过程。合金中α-AlFeSiMn相在点蚀浸泡过程中充当阴极,且发生了去合金化,周围的Al基体充当阳极而被腐蚀,含MnCr的弥散相则伴随Al基体的腐蚀而脱落。浸泡后期点蚀敏感性降低,表面的腐蚀产物可起到一定的保护作用。

关键词: 7020铝合金 ; 点蚀 ; 腐蚀动力学 ; 第二相 ; 循环极化曲线

Abstract:

The pitting corrosion behavior of 7020 aluminum alloy in 3.5%(mass fraction) NaCl solution was investigated by immersion test and cyclic polarization curve, while the corrosion morphology of the alloy was characterized by means of optical microscopy (OM), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The results show that the curve of maximum depth of corrosion pits versus time exhibits ''S''-like shape. α-AlFeSiMn phase may act as a local cathode, thereby de-alloying occurred around particles of α-AlFeSiMn phase, i.e. Al matrix nearby the particles of α-AlFeSiMn phase was dissolved due to its anode nature, while Mn-and Cr-containing precipitates in the matrix may fall off along with the dissolved Al. Pitting susceptibility reduces in the later stage of corrosion due to that the formed corrosion products may act as a protective barrier for the alloy to some extent.

Key words: 7020 aluminum alloy ; pitting corrosion ; corrosion dynamics ; second phase ; cyclicpolarization curve

Al-Zn-Mg系合金具有强度高、焊接性能优良、耐腐蚀性能优越且挤压性能良好等优点,作为结构材料被广泛应用于轨道交通等领域[1]。在使用过程中这种合金常常会发生局部点蚀[2],从而降低结构件的可靠性和寿命。因此深入研究Al-Zn-Mg合金的点蚀行为,探明其腐蚀机理具有重要意义。

Grilli等[3]认为Al-Cu-Fe-Mn第二相是2219铝合金的腐蚀起始位置,且点蚀过程中常作为阴极,Al基体作为阳极而被腐蚀,腐蚀产物Al(OH)3会逐渐积聚在第二相颗粒周围,而Fe则会从Al-Cu-Fe-Mn相中慢慢溶解,长时间的点蚀过程会使第二相被腐蚀产物完全覆盖。Wang等[4]发现影响7A60铝合金发生严重点蚀的主要第二相是MgZn2粒子、Al2MgCu和Mg2Si相,而Al7Cu2Fe颗粒对点蚀性能的影响不大。Sameljuk等[5]研究了铸态Al-Zn-Mg合金的点蚀性能,发现其在3% (质量分数) NaCl溶液中会发生点蚀,主要是富铜相引发了周围含Zn、Mg析出相的溶解。以上研究均说明 (亚) 微米级第二相是影响合金点蚀性能的关键因素。针对7020铝合金,也已进行了相关研究。Ismael[6]认为含Zn的金属间化合物在7020-T6铝合金中作为阳极并控制腐蚀过程,且人工时效会增加Zn含量并减缓腐蚀速率。Al Sammarraie等[7]发现7020铝合金中不同温度下的点蚀主要与GP区、以及GP区长大形成的MgxZny或Al2Mg3Zn3等第二相有关,且析出相的密度越大,点蚀敏感性越高。然而,关于7020铝合金长时间的点蚀过程及其微观组织变化尚未见报道。

本文通过浸泡实验和循环阳极极化曲线研究了7020铝合金在3.5% (质量分数) NaCl溶液中不同时间的点蚀行为,通过扫描电子显微镜 (SEM) 观察了点蚀浸泡前后的表面形貌,用扫描透射电镜 (STEM) 观察了试样内部微观组织,探讨了其点蚀机理。

1 实验方法
1.1 实验材料

实验材料为10 mm厚的7020-T5铝合金挤压型材,其化学成分 (质量分数,%) 为:Si 0.084,Fe 0.31,Cu 0.17,Mn 0.31,Mg 1.27,Zn 4.45,Cr 0.22,Zr 0.13,Al 余量。

1.2 溶液浸泡实验

浸泡实验样品尺寸为15 mm×15 mm×10 mm,经除油、清洗干燥后,在3.5%NaCl溶液中连续浸泡,溶液温度通过恒温水浴控制在 (20±1) ℃,分别浸泡不同时间后取出。采用MX 3000金相显微镜观察了点蚀后试样横截面形貌,同时用FEI-Quanta 200型SEM对试样的初始形貌与浸泡后的表面形貌进行观察,并对一些典型第二相进行能谱 (EDS,Genesis60s) 分析。

1.3 扫描透射电镜

STEM样品先经机械研磨减薄至厚度约为80 μm,再冲成直径为3 mm的小圆片,采用MTP-1A型双喷电解减薄仪对圆片试样进行减薄、穿孔。双喷电解液为30%HNO3+70%CH3OH (体积分数),使用液氮将温度控制在-20 ℃以下,电流为50~70 mA,电压为10~20 V。采用FEI Tecnai G2 F20 型STEM (加速电压为200 kV)在高角环形暗场像 (HAADF) 模式下观察试样中的微观组织,并用EDS分析第二相化学成分。

1.4 循环阳极极化曲线测试

循环极化曲线能确定合金的各个电化学参数,从而预测合金点蚀倾向。其特征参数为自腐蚀电位Ecorr、击穿电位Epit、保护电位Erp以及自腐蚀电流密度Icorr[8]

在本实验中,将浸泡不同时间的样品进行循环阳极极化曲线测试。先将样品取出后与Cu线相连,留一面作为工作面,其工作面积为10 mm×10 mm,并用无水乙醇超声波清洗后干燥,非工作面用松香石蜡密封后进行电化学测试。用IM6EX型电化学综合测试仪,根据ASTM G59-97 (2003) 标准采用动电位扫描法测定极化曲线闭合环。在3.5%NaCl溶液中,采用三电极体系,7020铝合金样品为工作电极,Pt片为辅助电极,饱和甘汞电极为参比电极,测定开路电位后,先进行阴极极化,再进行阳极极化。扫描范围为Eop-1.5 V~+1 V~-1 V,扫描速率为2 mV/s。

2 结果与讨论
2.1 初始状态微观组织

图1为7020铝合金初始状态的SEM像和第二相EDS分析结果。可观察到两种形貌的析出相,一种是不规则形状的亮色第二相粒子,大部分沿挤压方向呈条带状分布,尺寸约为1~5 μm,如图1a中所示。对典型粒子进行EDS分析可见,其中主要包含Al、Fe、Mn和Si,如图1b所示,推测为含FeMnSi的初生相。该区域还能观察到很多尺寸更小 (亚微米级) 的弥散亮色第二相粒子,由于尺寸太小,无法进行EDS分析,因此采用STEM进一步观察和分析,典型结果如图2所示。

图1 7020铝合金的SEM像和第二相EDS分析结果

Fig.1 SEM image (a) and EDS result (b) of second phase in 7020 aluminum alloy

图2 7020铝合金的HADDF像及弥散相EDS分析结果

Fig.2 HAADF image (a) and EDS result (b) of dispersed phase in 7020 aluminum alloy

图2a中可观察到,晶粒内部分布着较多尺寸约100~300 nm的亮色第二相。对典型粒子进行EDS分析,结果如图2b所示,可见除了Al基体外,第二相中含有Mn、Cr、Zn及Cu,推测为含Mn、Cr的弥散相。

因此推测7020铝合金中存在的主要第二相可能为尺寸较大的含FeSiMn的初生相,由文献[9]可知这种相为α-AlFeSiMn相;以及尺寸较小的含MnCr弥散相。

2.2 点蚀动力学分析

采用断面金相法测量了试样浸泡不同时间后截面的最大腐蚀深度,见图3。可知,浸泡168 h后的最大腐蚀深度为5 μm,且仅能观察到细小的点蚀坑,如图3a所示;浸泡到504 h后,点蚀坑深度达到46 μm,如图3b所示;浸泡840 h后,可观察到点蚀坑边缘出现了沿晶腐蚀形貌;腐蚀1176 h后,仍能观察到类似特征,且最大腐蚀深度持续增大,如图3c和d所示。

图3 7020铝合金浸泡不同时间后截面最大腐蚀深度

Fig.3 OM images of 7020 aluminum alloy after immersion for 168 h (a), 504 h (b), 840 h (c) and 1176 h (d)

图4为7020铝合金浸泡1176 h后的截面SEM像。可以明显的观察到,腐蚀1176 h后合金呈现典型的网状结构,表现为沿晶腐蚀,如图4a所示。在高倍照片中可见腐蚀裂纹细长,尖端无明显腐蚀产物,如图4b所示,说明在腐蚀后期点蚀尖端是沿晶界发生了择优腐蚀。

图4 7020铝合金浸泡1176 h后的截面SEM像

Fig.4 Cross section of 7020 aluminum alloy after immersion for 1176 h (a) and the magnified image of the square area in Fig.4a

图5为最大点蚀深度随浸泡时间的变化曲线。在0~1176 h的浸泡中,最大点蚀深度随时间的变化呈S型曲线,336 h之前点蚀深度增长缓慢,至840 h点蚀深度迅速增加,而后速率降低,整体呈缓慢增长-快速增长-保持稳定的过程。胡艳玲等[10]认为Sigmoidal (Boltzman) 曲线可较好的反映这种趋势,其函数关系式如下:

d = d 0 - d max 1 + e ( t - t 0 ) / d t + d max (1)

其中,d为随时间t变化的最大腐蚀深度,d0dmax分别是最初和最大的腐蚀深度,t0是腐蚀深度中位数所对应的时间值,dt是与初始状态有关的常数。

图5 以最大点蚀深度d表征的腐蚀动力学图

Fig.5 Corrosion dynamics curve of maximum pitting corrosion depth vs time

图5中的拟合曲线R2=0.99781,拟合后的函数关系式如下:

d = - 126.57786 1 + e ( t - 567.57854 ) / 107.97737 + 126.24785 (2)

但是如果完全按S曲线拟合,当时间趋向无穷大时,最大点蚀深度趋向于常数,这与实际不符[10]。7020铝合金型材有着较强的沿挤压方向的晶粒变形与取向,且在晶界有强化相析出并产生无沉淀带 (如图2),因而能够形成沿挤压方向的晶界的阳极优先溶解通道。因此在点蚀后期进入晶间腐蚀阶段,且达到一定的腐蚀深度时,腐蚀便开始优先沿平行于表面的阳极溶解通道发展,此时沿深度方向的腐蚀速率减慢。

2.3点蚀形貌与微观组织

为了更好的研究点蚀过程,采用SEM观察了试样浸泡不同时间后的表面形貌,见图6。可观察到7020铝合金在3.5%NaCl溶液中的腐蚀形式以点蚀及点蚀的发展为主,随着时间的增加腐蚀程度逐渐加深,蚀点向四周扩散,腐蚀产物不断增厚且覆盖不均匀。试样浸泡168 h后,表面呈条纹状,并沿挤压方向分布;还可观察到少量蚀孔,其中分布着许多亮色第二相颗粒,且与基体之间存在黑色的沟壑,如图6a所示。浸泡504 h后,表面蚀孔变多且呈条带状,腐蚀区域和未腐蚀区域交替分布,部分腐蚀产物已经开裂,如图6b所示。浸泡840 h后,点蚀面积进一步增大,且部分蚀孔已经显著扩大并相互连接,且缝隙较宽,如图6c所示。浸泡1176 h后,可观察到许多大块开裂的腐蚀产物堆积在表面,如图6d所示。

图6 7020铝合金浸泡不同时间后的SEM像

Fig.6 SEM images of 7020 aluminum alloy after immersion for 168 h (a), 504 h (b), 840 h (c) and 1176 h (d)

正常情况下Al在水溶液中即会氧化,形成电阻很大的Al2O3H2O氧化膜[11];但在氯化物溶液中,由于Cl-的存在,在活性较高的局部位置 (如晶界和第二相等处),进行的不是成膜反应而是阳极溶解反应:

AlOH + C l - AlOHCl + e (3)

AlOHCl + C l - AlOHC l 2 + e (4)

发生阳极溶解反应后,钝化膜开始局部破裂,形成蚀孔,并发生如下反应[6]

Al A l 3 + + 3 e (5)

A l 3 + + 3 H 2 O Al ( OH ) 3 + 3 H + (6)

A l 3 + + 4 C l - AlC l 4 - (7)

AlC l 4 - + 3 H 2 O Al ( OH ) 3 + 3 H + + 4 C l - (8)

因此裸露的Al表面快速电离,蚀孔内部Al3+浓度逐渐增加,Cl-不断向孔内迁移导致孔内Cl-浓度升高。同时,蚀孔内H+浓度升高、pH值降低,蚀孔内溶液酸化,水解产生的H+和孔内Cl-促使蚀孔中的Al继续溶解,发生自催化反应,腐蚀不断发展,点蚀迅速扩展[12]

随着点蚀程度增加,腐蚀产物Al(OH)3在蚀孔处堆积,这阻碍了蚀孔内外的介质交换,形成了闭塞电池[13]。也就是说,蚀孔内部溶解的金属离子不易向外扩散,溶解氧不易向内扩散,造成蚀孔内部积累过多的正电荷,结果有更多的Cl-进入维持电中性;蚀孔内部Al的氯化物水解产生更多的H+和Cl-,促使溶解进一步加快,蚀孔面积不断变大,堆积的腐蚀产物也不断变厚,并逐渐覆盖蚀孔,腐蚀速率减慢。

图7 7020铝合金浸泡不同时间后的SEM像和EDS分析结果

Fig.7 SEM images (a, c) and EDS results (b, d) of 7020 aluminum alloy after immersion for 168 h (a, b) and 1176 h (c, d)

图7为7020铝合金浸泡不同时间后的SEM像和EDS分析结果。由图可观察到,样品浸泡不同时间后均可观察到尺寸不一的蚀孔,其中第二相稳定存在,到腐蚀后期第二相密度大大减少。根据EDS分析结果可知,α-AlFeSiMn相周围的蚀坑尺寸较大。对样品中的第二相成分含量进行统计,结果见表1。可见随着浸泡时间延长,Al,Fe和Mn的含量逐渐减小,因此推测α-AlFeSiMn初生相在点蚀的过程中常常充当阴极,且发生了去合金化[14,15],其中Fe和Mn被溶解,周围基体发生腐蚀。而腐蚀过程中只要阴极相存在,其周围的基体溶解就不会停止,只有阴极相从基体中掉落后,反应才会结束[5]。腐蚀后期第二相的密度减少,反应速率也进一步减慢。

由于含MnCr弥散相尺寸太小,无法进行EDS分析,但能观察到随浸泡时间的延长基本未被腐蚀,周围的Al基体发生开裂形成蚀孔,一些蚀孔中未见含MnCr弥散相,可能是随着基体的腐蚀而发生了脱落。

表1 7020铝合金在不同浸泡时间后第二相的化学成分
Table 1 Chemical compositions of the second phase in 7020 aluminum alloy after immersion for different time(mass fraction / %)
Time / h O Al Si Fe Mn
0 --- 68.73~85.74 4.60~5.43 16.39~20.90 3.93~4.19
168 6.71~8.19 55.02~61.11 5.23~6.14 14.75~19.28 3.23~3.41
1176 10.10~17.13 47.42~49.75 3.09~4.47 11.43~13.25 2.64~3.29

表1 7020铝合金在不同浸泡时间后第二相的化学成分

Table 1 Chemical compositions of the second phase in 7020 aluminum alloy after immersion for different time(mass fraction / %)

2.4 循环阳极极化曲线

图8为7020铝合金浸泡不同时间后的循环极化曲线,可观察到每条曲线都有一段稳态钝化区和一个滞后环,说明试样会在溶液中发生点蚀[5]表2为极化曲线所测得的各电化学腐蚀参数。一般Ecorr越正,合金的抗腐蚀性能越好[16]。铝合金耐点蚀能力还与表面钝化膜的完整和破损后的自修复能力有关,当电位高于Epit时,电流随电压升高开始快速增大,说明钝化膜发生了破裂,铝合金表面发生点蚀,因此Epit越大,铝合金的耐点蚀能力越强。而击穿电位与保护电位之差即|Epit-Erp|值越大,钝化膜破坏的越严重,蚀孔的发展趋势越大,腐蚀越严重[17,18]

图8 7020铝合金浸泡不同时间后的极化曲线

Fig.8 Polarization curves of 7020 aluminum alloy after immersion for different time

图8和表2的数据可知,随着腐蚀时间延长,Ecorr,Epit以及|Epit-Erp|均呈下降趋势,即耐蚀性能下降,然而浸泡到1176 h时,Epit和|Epit-Erp|突然上升,可能是由于长时间表面附着的腐蚀产物沉积在破损的钝化膜表面,对合金有一定的保护作用[19,20]。但电极电位仅能从热力学角度解释点蚀发展。从动力学角度来说,腐蚀电流密度Icorr越大,腐蚀速率越快。因此,Icorr在样品浸泡至168 h时有所增大,浸泡至840 h时显著增大,而后少量下降,腐蚀速率先快后慢,因此到浸泡后期试样表面氧化膜破坏速度变慢,点蚀敏感性减小。

表2 7020铝合金浸泡不同时间后的极化曲线特征参数
Table 2 Polarization characteristics for 7020 aluminum alloy after immersion for different time
Timeh EcorrmV EpitmV ErpmV |Epit-Erp|mV IcorrμAcm-2
0 -647 -572 -527 50 0.74
168 -967 -596 -546 50 15.60
504 -922 -622 -577 45 2.16
840 -974 -689 -663 26 137.00
1176 -1122 -642 -534 108 104.00

表2 7020铝合金浸泡不同时间后的极化曲线特征参数

Table 2 Polarization characteristics for 7020 aluminum alloy after immersion for different time

3 结论

(1) 7020铝合金点蚀的最大腐蚀深度随时间的延长逐渐加深,呈现S型变化,包含缓慢增长-快速增长-保持稳定的过程,腐蚀后期沿深度方向的腐蚀速率减慢。

(2) 7020铝合金中α-AlFeSiMn相在浸泡过程中充当阴极,且发生了去合金化,周围的Al基体充当阳极而被腐蚀;含MnCr弥散相则伴随着Al基体的腐蚀而脱落。

(3) 随着腐蚀时间的延长,7020铝合金自腐蚀电位、击穿电位以及击穿电位与保护电位之差均逐渐减小,耐蚀性能下降;而腐蚀电流密度逐渐增大,腐蚀速率逐渐加快。而到了腐蚀后期,表面附着的腐蚀产物能起到一定的保护作用。

The authors have declared that no competing interests exist.

参考文献

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文章选取了我国某类型高速列车常用的A7N01S-T5铝合金材料,进行了恒载荷应力腐蚀试验,对A7N01S-T5材料的应力腐蚀机理、应力腐蚀强度因子K1SCC和裂纹生长机制进行了研究.结果表明,A7N01S-T5铝合金的应力腐蚀破坏敏感性很高,其临界应力场强度因子K<sub>1</sub>SCC为5.2 MPa·m1/2,仅为断裂韧度K<sub>1</sub>C的0.15,对应的临界应力门槛值б为123MPa,仅为拉伸强度б<sub>b</sub> 0.27.裂纹断口面有明显的舌状凸起和凹坑的存在.A7N01S-T5铝合金的应力腐蚀裂纹以沿晶开裂为主,形貌呈树枝状,同时相邻二次裂纹之间有竞争生长的关系.
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The pitting corrosion behaviors of 7A60 aluminum alloy in the retrogression and re-aging (RRA) temper were investigated by electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) techniques, and the microstructure and the second phase content of the alloy were observed and determined by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results show that there exist two different corrosion stages for 7A60 alloy in 3.5% NaCl solution, and the corrosion process can be detected by the appearance of EIS spectrum with two capacitive time constants and the wavelet fractal dimension D extracted from EN. SEM and EDS results also demonstrate that severe pitting corrosion in 7A60 alloy is mainly caused by electrochemical active MgZn2 particles, secondly by Al2MgCu and Mg2Si. Al7Cu2Fe particles make little contribution to the pitting corrosion of 7A60 alloy.
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The corrosion behaviour of Al–5Zn–3Mg–0.6Cu–0.8Zr–0.25Cr–0.15Ni–0.15Ti alloys, produced by traditional and powder technologies, with similar thermo-mechanical treatments, in 3% sodium chloride solution, has been examined by electrochemical methods, scanning electron microscopy, transmission electron microscopy and X-ray microanalysis. The alloys reveal similar precipitation but of different shape, size and distribution; further, both alloys experience localized corrosion. Copper-rich precipitates initiate the dissolution of surrounding particles, enriched in Zn and Mg. As a result, the surface is enriched with other alloying elements after a full polarisation run. Cast material has lower corrosion properties because of the higher heterogeneity of the structure. The structure heterogeneity of the cast material involves a more non-uniform distribution of the precipitates, larger Zn- and Mg-rich particles, and depletion of the matrix and areas around the precipitates by alloying elements compared with the powder material.
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Effects of pH solution and chloride (Cl) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.00302wt% and 5.502wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting and corrosion potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH02=022) and alkaline chloride solutions (pH02=0212) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH02=026).
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The corrosion anisotropy of 7050鈥揟7451 Al alloy thick plate in NaCl solution was investigated by immersion tests, slow strain rate testing (SSRT) technique, potentiodynamic and anode polarization measurements, optical microscropy (OM) and scanning electron microscopy (SEM) observations. The results show that the thick plate exhibits severe corrosion anisotropy due to the microstructure anisotropy. The observations of immersion surfaces together with the analysis of polarization curves reveal that the differences of the corrosion morphologies on various sections in this material are mainly related to the area fraction of the remnant second phase, and higher area fraction displays worst corrosion resistance. The stress corrosion cracking (SCC) susceptibility of different directions relative to the rolling direction is assessed by SSRT technique, ranked in the order: S direction >L direction >T direction. The result show that the smaller the grain aspect ratio, the better the corrosion resistance to SCC.
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Corrosion behaviour, residual stress, intermetallics and microstructure of friction stir welded aerospace Al–Zn–Mg and Al–Zn–Mg–0.10Sc–0.10Zr (wt.%) alloys were investigated comparatively and in detail. The thermo-mechanically affected zones adjacent to weld nugget are most susceptible to corrosion in two weld joints. In addition to the enrichment of coarse Si and Fe intermetallics, wide precipitate free zones (in Al–Zn–Mg weld joint) and high tensile residual stress are the key factors. Secondary Al3ScxZr161x nanoparticles can effectively improve stress corrosion cracking resistance and decrease exfoliation corrosion rates of Al–Zn–Mg weld joints by restraining the formation of precipitate free zones and refining grains.
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The corrosion behaviour of cast and heat-treated Al–6%Zn–1%Mg and Al–6%Zn–1%Mg–1%Ag alloys and metal matrix composites (MMCs) were investigated using dynamic polarisation techniques, followed by scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The addition of silver and the incorporation of continuous Altex fibres into the alloys have a positive effect on the materials’ corrosion resistance. It was observed that due to the presence of second phase particles and microsegregation, pitting occurs preferentially at the grain boundary and fibre/matrix interface regions in the cast alloys and composites, respectively.
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The corrosion behavior of commercial Al alloys was studied in neutral 0.6 mol L 611 NaCl by using single-cycle polarization. Qualitative interpretation of pitting scans in both deaerated and naturally aerated NaCl solution, with the aid of corrosion morphology characterization, allowed for inference of the features of localized corrosion as a function of Al substrate nature. Electrochemical characteristic parameters such as pitting ( E pit), repassivation or protection ( E prot) potentials and pit transition potential ( E ptp) were determined. The validity of the difference Δ E = | E pit 61 E prot| as criterion for susceptibility to localized corrosion of aluminium alloys is discussed.
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Initiation of localized corrosion upon high strength aluminum alloys is often associated with cathodic intermetallic particles within the alloy. Electrochemical measurements and metallurgical characterization have been made to clarify and quantify the physical properties of Al 7Cu 2Fe particles in AA7075-T651. Prior studies regarding either the stereology or electrochemical properties of Al 7Cu 2Fe are scarce. Quantitative microscopy revealed a significant population of Al 7Cu 2Fe in the alloy; comprising up to 65% of the constituent particle population and typically at a size of 1.7 ± 1.0 μm. It was determined that Al 7Cu 2Fe may serve as a local cathode in the evolution of localized corrosion of AA7075-T651 and is capable of sustaining oxygen reduction reactions at rates of several hundreds of μA/cm 2 over a range of potentials typical of the open circuit potential (OCP) of AA7075-T651 in NaCl solution of various concentrations and pH. The presence of Al 7Cu 2Fe leads to the development of pitting at the particle–matrix interface.
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[20] Wloka J, Hack T, Virtanen S.Influence of temper and surface condition on the exfoliation behaviour of high strength Al-Zn-Mg-Cu alloys[J]. Corros. Sci., 2007, 49: 1437
Exfoliation experiments were performed on two different 7000 series aluminium alloys in two different tempers and two different surface conditions according to ASTM G34 standard (EXCO test). The results of this experiment were evaluated with regard to surface condition, temper and alloy composition. The results after an EXCO test were evaluated according to the specification in the standard (in comparison to reference pictures), by visually ranking the samples, and by metallographic investigation. According to the ASTM standard, almost no changes could be seen with regard to the temper. Ranking and metallography revealed that AA7010 generally is less susceptible to exfoliation compared to AA7349. The differences between the temper conditions are not significant. It could be elaborated that the higher zinc content of AA7349 is detrimental for the exfoliation behaviour.
DOI:10.1016/j.corsci.2006.06.033      URL     [本文引用:1]
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关键词(key words)
7020铝合金
点蚀
腐蚀动力学
第二相
循环极化曲线

7020 aluminum alloy
pitting corrosion
corrosion dynamics
second phase
cyclicpolarization curve

作者
戴芸
刘胜胆
邓运来
张新明

DAI Yun
LIU Shengdan
DENG Yunlai
ZHANG Xinming