2025 Volume 66 Issue 2 Pages 165-170
Hydrogen embrittlement affected by three types of plating (low-P and high-P types electroless Ni-P plating and electrolytic zinc plating) was investigated by means of slow-strain-rate three-point bending test on three aluminum alloys (2017-T3, 6061-T6 and 7075-T651). Hydrogen generated by the Ni-P and zinc plating was absorbed in the aluminum alloy substrates, and the trap sites in the aluminum alloy substrates for the absorbed hydrogen differed between Ni-P and zinc plating. Hydrogen embrittlement was able to be evaluated by the three-point bending tests on plated aluminum alloys. Zinc plating did not cause embrittlement for all the alloys, but Ni-P plating induced embrittlement only for the 6061-T6 aluminum alloy. The result of embrittlement of the Ni-P plated 6061-T6 aluminum alloy corresponds to the highest amount of hydrogen desorbed below 240°C than the other alloys.
This Paper was Originally Published in Japanese in J. JILM 73 (2023) 196–200.
Fig. 5 Load-displacement curves of the three alloy substrates untreated and plated in the three ways, subjected to the three-point bending test.
Hydrogen embrittlement is a phenomenon in which the strength of a metal decreases, or its fracture characteristics indicate brittle behavior owing to the absorption of hydrogen into the metal, leading to crack initiation or early fracture. As the strength of steel increases, which is a core material, the susceptibility to hydrogen embrittlement increases. In high-strength steels exceeding 1500 MPa, even a small amount of hydrogen in the order of ppb can cause cracking and lead to serious accidents [1]. Conversely, aluminum alloys which have been increasingly applied as lightweight materials, mainly in automobiles and other transportation equipment, except for 7000 series alloys, have been reported to have excellent resistance to hydrogen embrittlement [2, 3]. In particular, the 6061-T6 alloy is one of the few structural metals approved for use as a liner material for 70-MPa high-pressure hydrogen storage tanks owing to its excellent hydrogen embrittlement resistance in wet and hydrogen environments [4].
Meanwhile, during plating, metal ions in the solution are reduced to form a film. However, the hydrogen ions are also simultaneously reduced to generate hydrogen that is then absorbed by the metal substrate, thus inducing hydrogen embrittlement, which is widely known to occur owing to zinc and cadmium plating on high-strength steels [5]. The authors performed three-point bending tests to investigate the hydrogen embrittlement behavior of electroless Ni-P plating with different phosphorus contents on high-strength steel and reported that electroless Ni-P plating also induces hydrogen embrittlement [6]. Because hydrogen absorbed into steel substrates by plating induces hydrogen embrittlement, the mechanical properties of aluminum alloys are also expected to be affected by them. The authors previously reported that high-phosphorus electroless Ni-P plating reduced the fatigue strengths of 2017-T4, 6061-T6, and 7075-T6511 aluminum alloys, which are mainly used for structural materials, and this factor was attributed to the hydrogen absorbed in these substrates by the plating [7–9]. However, because aluminum alloys, except the 7000 series alloys, demonstrate excellent resistance to hydrogen embrittlement, there are limited studies regarding the effect of plating on the hydrogen embrittlement of aluminum alloys. Although slow-strain-rate-technique (SSRT) tensile test is widely applied as an evaluation method for hydrogen embrittlement [2], the concentration of stress is critical for evaluating hydrogen embrittlement [10]. We have reported that the three-point bending test, in which the highest stress is generated at the load section, is useful for the evaluation of hydrogen embrittlement [11].
In this study, low-phosphorus and high-phosphorus electroless Ni-P plating were performed on commercial aluminum alloys used as structural materials, and zinc electroplating was also applied to steel materials to induce hydrogen embrittlement. Three-point bending tests were performed on the specimens before and after plating to determine the bending properties and clarify the effect of various types of plating on the hydrogen embrittlement of the structural aluminum alloys; furthermore, the mechanism of hydrogen embrittlement was discussed.
Three various commercially available aluminum alloy sheets with different thicknesses were used for the three-point bending tests: 2017-T3 aluminum alloy sheet (2 mm thick, hereinafter referred to as 2017-T3 alloy), 6061-T6 aluminum alloy sheet (2 mm thick, hereinafter referred to as 6061-T6 alloy), and 7075-T651 aluminum alloy sheet (3 mm thick, hereinafter referred to as 7075-T651 alloy). The chemical composition of each alloy is shown in Table 1, and the tensile properties are shown in Table 2. For the three-point bending test, a semicircular shape with a radius of 2 mm was machined at the base of the notch shown in Fig. 1 to induce a stress concentration at the site of maximum tensile stress. Two types of electroless Ni-P plating with different phosphorus contents (hereafter referred to as Ni-P plating) were performed on the machined specimens. Commercially available plating solutions (low phosphorus (P) (KHN-HK) and high phosphorus (P) (KRB-HK); manufactured by Uyemura & Co., Ltd.) were used for Ni-P plating. Zinc (Zn) electroplating was also evaluated. Tables 3 and 4 list the conditions for Ni-P and Zn plating, respectively. The average thickness of the plating film was adjusted to 10 µm, respectively. Aluminum alloys immediately form an oxide film, and normal pretreatment does not provide adhesion of the plating film. Therefore, a “double zincate treatment” (hereinafter referred to as zincate treatment) was applied to the plating pretreatment to improve the adhesion of the plating film [12]. Masking was not performed on the specimens during the zincate treatment and subsequent plating process. Therefore, the plating film covered the entire surface, except for the contact area with electrode. The surface of the specimens was observed before and after plating, and a thermal desorption analysis (TDA) was performed to investigate the effects of plating on the hydrogen absorption. A semiconductor hydrogen sensor with a detection sensitivity of 5 ppb was used for the hydrogen measurement (SGHA-P2; Nissha F.I.S. Co., Ltd.), which was capable of measuring the hydrogen content once every two minutes. The temperature increased at a rate of 200°C/h. Note, hydrogen diffuses over-time after plating, resulting in hydrogen desorption through the coating, thus decreasing the amount of hydrogen [6]. In the experiment, all the specimens were maintained in desiccators for more than 14 days after plating because the hydrogen content of each electroless Ni-P-plated specimen became nearly constant after 14 days from the end of plating [6].
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Shape and dimension of the specimen for three-point bending test.
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A microautograph (MST-I II type HS/HR) was used for the three-point bending tests. A load was applied at the center between the supports with a distance of 30 mm and a crosshead displacement speed of 0.05 mm/min. The load and displacement were measured until the load began to decrease, and the effect of plating was examined.
Figure 2 presents the SEM images of the untreated specimen surface, Ni-P-plated coating, and Zn-plated coating for the 2017-T3 alloy. Machining scratches owing to specimen preparation were observed in the untreated specimen. Smooth films were obtained for both low- and high-P Ni-P plating; furthermore, the waviness characteristic of the electroless Ni-P plating was also observed. The effect of the phosphorus content on the surface morphology of the films was insignificant. Zn plating was confirmed to demonstrate dense angular deposits derived from the hcp structure [13], although not clearly observable at this magnification. No pits or other defects were observed in any of the plating films, and the surfaces of the specimens were completely covered.
Secondary electron images showing surface appearance of the untreated and plated 2017-T3 aluminum alloy specimens. (a) untreated; (b) electroless-plated with low-P type Ni-P; (c) electroless-plated with high-P type Ni-P; (d) electrolytically zinc-plated.
Figure 3 shows the results of the thermal desorption hydrogen analysis (TDA) for the untreated, Ni-P plated, and Zn-plated specimens. The hydrogen desorption behavior of the untreated specimens depended on the type of alloys used. The 2017-T3 and 6061-T6 alloys showed slight hydrogen desorption up to 420°C, whereas that above 420°C apparently might correspond to the desorption of molecular hydrogen strongly trapped in the micropores and other features [14, 15]. Hydrogen desorption in the 7075-T651 alloy was observed at approximately 300°C. These peaks might correspond to the desorption of hydrogen trapped at the grain boundary [14] or the precipitate/aluminum interface [14, 16], as indicated for the plated specimens described below.
TDA results of the untreated and plated specimens of the three alloys.
Regarding the effect of plating, the hydrogen desorption behavior differed depending on the type of alloys and plating; however, hydrogen desorption was observed in the region where hydrogen was not desorbed in the untreated specimens for all the alloys, confirming hydrogen uptake by plating. A desorption peak near 150°C was observed for all the alloys, including for both the high- and low-P Ni-P plating. The height of the desorption peak near 150°C was higher for the high-P Ni-P plating than for the low-P Ni-P plating in the 6061-T6 alloy; conversely, the peak height of the low-P Ni-P plating slightly exceeded that of the high-P Ni-P plating in the 2017-T3 and 7075-T651 alloys. Fukumuro et al. compared the hydrogen desorption behavior of electroless Ni-P plating on a copper substrate with almost no hydrogen solubility and a 7075-T651 alloy substrate. The results demonstrated that the majority of the hydrogen desorption peaks in the 7075-T651 alloy due to the electroless Ni-P plating were owing to the release of hydrogen trapped in the alloy [17]. It has been reported that the interstitial hydrogen in aluminum has a peak near 140°C [14]. Therefore, most of the hydrogen incorporated by Ni-P plating is assumed to be present in the interstitial space. A shoulder was observed near 200–240°C, where hydrogen desorption was attributed to the hydrogen present as vacancy-hydrogen clusters [17]. Furthermore, hydrogen desorption near 300°C was presumably owing to the desorption of hydrogen trapped at the grain boundary and at the interface between the precipitate and aluminum matrix [14, 16].
Conversely, for Zn plating, the hydrogen desorption peaks for all the alloys were significantly different from those of the Ni-P plated specimens, and the hydrogen desorption peaks appeared in the region above 300°C. The desorption peak of the 7075-T651 alloy was smaller than that of the other alloys, similar to the results obtained for Ni-P plating. It has been shown that the desorption peak of hydrogen trapped in the Zn plating film appears around 350°C [18]. In both Ni-P and Zn plating, hydrogen ions in the aqueous solution were reduced by electrons to generate hydrogen, which was incorporated into the aluminum alloy in atomic form. However, the significant difference in the hydrogen trapping sites between the Ni-P and Zn plating suggests that the hydrogen permeability of the plating film differs between Ni-P and Zn plating, which will be investigated in future studies.
3.3 Three-point bending testFigure 4 presents a three-point bending test on a 6061-T6 alloy with Ni-P plating (high-P type). As shown in this example, cracks occurred at the notch in all the specimens.
Appearance of the three-point bending test for the electroless Ni-P (high-P type) plated 6061-T6 aluminum alloy specimen.
Figure 5 presents the load-displacement curves for each alloy obtained by the three-point bending test. The displacement required to rupture in both the 2017-T3 and 7075-T651 alloys was not significantly different among the untreated and plated specimens. For the maximum load, the Ni-P plated specimens demonstrated slightly higher values than the untreated and Zn-plated specimens for the 2017-T3 alloy. This improvement is considered to be owing to the hardness and toughness of the Ni-P plating film for both low- and high-P types [19] and based on the law of mixture [20]. No significant differences in the maximum loads were observed among the untreated and plated 7075-T651 alloys, which is owing to the fact that the tensile strength of the 7075-T651 alloy was higher than that of the 2017-T3 alloy, thus reducing the effect of the plating film. Therefore, the bending properties of the 2017-T3 and 7075-T651 alloys were not reduced by either form of plating, and no embrittlement occurred.
Load-displacement curves of the three alloy substrates untreated and plated in the three ways, subjected to the three-point bending test.
For the 6061-T6 alloy, the maximum load for the Ni-P plated specimen was slightly higher than that for the untreated specimen, which can also be explained by the law of mixture, as was the case for the 2017-T3 alloy. However, the displacement required to rupture in the Ni-P plated specimen was less than that of the untreated specimen, clearly confirming embrittlement, which was more pronounced using the high-P Ni-P plating compared to the low-P Ni-P plating, and consistent with the Ni-P plating on the high-strength steel [6]. For Zn plating, no effect of plating on the bending properties was observed, as well as the case for the 2017-T3 and 7075-T651 alloys.
In a similar three-point bending test, when a high-strength steel was used as the base material, Zn plating reduced the rupture stress by more than 50% [21] and Ni-P plating also reduced the rupture stress, and both types of plating induced hydrogen embrittlement [6]. Since it has been shown that Zn and Ni-P plating can absorb hydrogen in high-strength steel substrates [6, 11], it is expected that both Zn and Ni-P plating can absorb hydrogen and induce embrittlement even when the substrate is an aluminum alloy. However, Zn plating on aluminum alloy substrates did not induce embrittlement, whereas only 6061-T6 alloy were embrittled by Ni-P plating. The results indicate that hydrogen absorbed into aluminum alloys by plating causes embrittlement; however, the effect of hydrogen absorption on embrittlement differs depending on the type of plating and alloy. Based on the results of the TDA analysis in the previous section, hydrogen desorption peaks appeared near 150°C for all the alloys owing to Ni-P plating, suggesting that the hydrogen present in the interstitial space of aluminum was responsible for embrittlement. The reason that embrittlement does not occur in the 2017-T3 and 7075-T651 alloys due to Ni-P plating may be due to the passive films that form on each alloy. Zinc and copper in the alloy inhibit passivation, while magnesium promotes passivation. Since the passive film prevents introduction of hydrogen from entering the aluminum alloy [22], the passive film will also affect hydrogen desorption in the aluminum alloy. Therefore, the effect of zincate treatment on inhibiting the formation of passive films may differ for each alloy, which we plan to study in the future.
Besides, the Zn plating on the aluminum alloys did not induce embrittlement. Hydrogen embrittlement of electroless Ni-P-plated 6061-T6 alloy [23] and untreated 7075-T651 alloy [24] in wet environments has been reportedly caused by hydrogen-enhanced localized plasticity deformation (HELP mechanism [25]) owing to hydrogen-dislocation interactions and the resulting microcrack formation, microcrack growth, and transition to intergranular cracks. If embrittlement due to hydrogen introduced by plating follows this mechanism, then Zn plating on aluminum alloys should induce embrittlement in the same manner as Ni-P plating. However, only the Ni-P plating was embrittled, as discussed below.
The authors have previously reported that the hydrogen embrittlement caused by Zn plating on high-strength steel is based on the hydrogen vacancy cluster theory [26–28], in which hydrogen-trapped vacancies migrate and coalesce under external stress to form nanovoids [21]. We speculated that the Zn plating film would prevent the release of the hydrogen-trapped vacancies [21]. If hydrogen embrittlement owing to plating on aluminum alloys is based on the hydrogen vacancy cluster theory, then hydrogen-trapped vacancies are created, and the desorption peak of the hydrogen trapped in the vacancies appears at 200°C to 240°C [14]. Hydrogen desorption from 200°C to 240°C by Ni-P plating is greater than that of Zn plating, which supports the hydrogen vacancy cluster theory. However, the mechanism of hydrogen embrittlement of aluminum alloys by hydrogen based on plating remains unclear and requires a further detailed investigation.
3.4 Fracture surface observationThe SEM images of the fracture surface after the three-point bending test are shown in Fig. 6 for the 6061-T6 alloy, which was embrittled by Ni-P plating, and in Fig. 7 for the 7075-T651 alloy, which was not embrittled by plating. The SEM images present the entire fracture surface and vicinity of the R-part. For the 6061-T6 alloy, the fracture surfaces of the untreated, Ni-P-plated, and Zn-plated specimens exhibited a dimple pattern, suggesting that the fracture was ductile. The 6061-T6 alloy was embrittled by Ni-P plating, but no significant difference was observed on the fracture surface. Kuwada et al. performed low-P Ni-P plating on a 6061-T6 alloy and conducted tensile tests in air and under wet conditions. The results show that the embrittled specimens exhibit intergranular cracking characteristic of brittle fracture [23]. However, no clear intergranular cracking was observed under the laboratory atmospheric conditions in this experiment, suggesting that the local strain rate differed among the three-point bending test with the notched specimens in this experiment and the smooth tensile test without notches, resulting in differences in the fracture surface morphology. Dimple patterns were observed on the fracture surfaces of untreated and all plated 7075-T651 alloy, and similar results were obtained for the 2017-T3 alloy. From the observation of fracture surfaces, it was difficult to estimate the cause of embrittlement of only 6061-T6 alloy by Ni-P plating, for which we plan to conduct experiments by varying the displacement rate.
Secondary electron images of fracture surface of the untreated and plated 6061-T6 alloy specimens after the three-point bending test at low and high magnifications. (a) untreated; (b) electroless-plated with low-P type Ni-P; (c) electroless-plated with high-P type Ni-P; (d) electrolytically zinc-plated. Close-up views correspond to the boxed area “A” in the low-magnification views.
Secondary electron images of fracture surface of the untreated and plated 7075-T651 alloy specimens after the three-point bending test at low and high magnifications. (a) untreated; (b) electroless-plated with low-P type Ni-P; (c) electroless-plated with high-P type Ni-P; (d) electrolytically zinc-plated. Close-up views correspond to the boxed area “A” in the low-magnification views.
In this study, various types of plating were applied to commercial aluminum alloys, the effect of plating on hydrogen embrittlement was investigated by slow-strain-rate three-point bending tests, and the hydrogen embrittlement mechanism was discussed. Ni-P and Zn plating caused the substrates to absorb hydrogen generated during plating, but the trap sites for the absorbed hydrogen differed among the Ni-P and Zn plating. Hydrogen embrittlement could be evaluated by slow-strain-rate three-point bending tests on plated aluminum alloys. Zn plating did not cause embrittlement in any of the alloys. Conversely, Ni-P plating induced embrittlement in the 6061-T6 alloy but not in the 2017-T3 and 7075-T651 alloys. It is not clear why only the Ni-P plating on the 6061-T6 alloy caused embrittlement, and a further investigation is required.
This work was supported by the Light Metal Educational Foundation 2019 integrated advanced research and 2022 Education and Research Foundation.
