Objective: To analyze the cause of mildew on aluminum alloy frame used for photovoltaic module. Methods: Substrate composition analysis, oxide film thickness analysis, scanning electron microscope morphology and energy spectrum analysis were used to study the "mildew" part and normal part of aluminum alloy frame. Results: It was found that "mildew spot" was caused by point corrosion of aluminum alloy, which was precipitated after corrosion. Conclusion: Wet environment can lead to spot corrosion of aluminum alloy, halogen ion on sample surface accelerates corrosion effect, cutting off the halogen ion environmental conditions can reduce the occurrence of corrosion, and prevent the fracture of photovoltaic module after installation.

Photovoltaic module (or solar cell module) with aluminum alloy frame as the key support and installation of the photovoltaic module, its quality reliability will be directly related to the actual installation of the photovoltaic module after the normal operation or not. Aluminum alloy frame as a photovoltaic module peripheral support material, need to withstand the test of wet heat, wet cold, salt spray and other harsh environment in 25 years, if there are quality problems, will affect the actual service life of photovoltaic modules.

In this paper, to help enterprises find the cause of aluminum alloy frame "mildew", provide technical support for enterprises to solve disputes, and provide technical basis for the management and protection of the problem.

1. Test sample and method

The alloy brand and supply state of the aluminum alloy frame for the photovoltaic module are: 6063-T5. The problem part and the normal part were removed from the aluminum alloy frame, and a total of two samples were analyzed for comparison.

1.1 Chemical composition analysis

The experimental materials were the aluminum alloy substrates of the "mildew" part and the normal part. X-ray fluorescence spectrometer (QUANT 'X, manufactured by ThermoFisher, USA) was used to analyze the chemical composition of the substrates of the two parts.

1.2 Analysis of oxide film thickness

The test materials were the aluminum alloy substrates of the "mildew" part and the normal part. The thickness of the anodic oxide film of the two parts was analyzed and measured using an eddy current thickness meter (ElektroPhysik 7400F1.5). Each sample was tested twice separately (two parallel test results were taken).

1.3 Micromorphology and energy spectrum analysis

The micromorphology and EDS spectra of the aluminum alloy frame inside and in the normal part of the "mold" were observed by OMEGA field emission scanning electron microscope of Zeiss ZESS.

2. Results and discussion

2.1 Analysis results of chemical composition and average film thickness

The results of chemical composition and average film thickness are shown in Table 1. As can be seen from the chemical composition analysis in Table 1, the material of normal frame and "mildew spot" frame both meet the requirements of brand 6063, and there is no difference in film thickness.

2.2 Pore morphology and energy spectrum analysis results

The scanning electron microscope analysis and EDS analysis of the holes are shown in Table 2.

2.3 Analysis and discussion

According to the differences of elements in the holes of aluminum frame and the surface of aluminum frame in Table 2, except for the same elements, anionic elements such as S and Cl are mainly present in the holes, while cationic elements such as Ca are present around the holes. According to the literature. A microcell is formed here, resulting in electrochemical corrosion, resulting in rapid dissolution of the pure aluminum as the matrix of the aluminum frame, the formation of corrosive holes, and eventually the appearance of point corrosion. Anionic elements such as S and Cl were found in each hole, and Cl ions would be adsorbed on the matrix at the hole defect in a humid environment, forming a micro-zone battery structure and forming a positive corrosion feedback, which further aggravated the local micro-zone corrosion. In the pure aluminum area, the potential is more negative and it is the anode. Under the action of corrosive media such as S and Cl ions, the dissolution of Al matrix results in the formation of corrosion pits with residues. In the humid environment, the corrosion continues to deteriorate and the corrosion pits develop in depth, so that the surface appears as corrosion pits with residues and eventually "mildew" precipitates appear on the surface of aluminum frame.

conclusion

After analysis, the "mildew" on the surface of the aluminum frame is caused by point corrosion. The reason for the point corrosion damage is that the photovoltaic module is stored in a humid environment, especially the existence of active anionic chlorine, a corrosive element. The presence of halogen ions on the surface of the sample accelerates the corrosion effect. It is recommended to conduct accelerated salt spray test on the aluminum alloy frame to confirm the corrosion protection effect, so as to prevent the fracture of the aluminum frame after the installation of photovoltaic modules.