The Corrosion Analysis of Desiccant on Insulating Glass.

As a glass product with mature materials and production processes, insulating glass is widely used in construction projects due to its long service life, excellent thermal insulation, heat insulation, and sound insulation properties. In practical application, to improve the service life and performance of insulating glass, it is usually necessary to use a desiccant that can continuously control the low-temperature dew point. However, due to the use of desiccant containing calcium chloride, the insulating glass will be corroded and alkalinized internally, which will eventually lead to failure, fogging, condensation, and other phenomena, and even the glass will fall in serious cases. To avoid the internal corrosion of insulating glass, this article systematically studies and analyzes the causes of corrosion, and puts forward suggestions and solutions for the corrosion and protection of insulating glass.

A piece of severely corroded insulating glass

The insulating glass sample used in this article is a silicone sealant groove aluminum double-sealed building (safety) insulating glass with corrosion failure, which was provided by an Eastern European glass deep processing company served by Jinan LIJIANG Glass. The insulating glass adopts aluminum spacers. , The aluminum spacer is filled with B-type desiccant for insulating glass, and the two pieces of glass are bonded and sealed together by the first butyl hot-melt sealant and the second silicone sealant. The doors and windows using the corrosion-ineffective insulating glass are existing residential doors and windows, and the service time is about 7 years.

First, observe the installation position and environmental conditions of the insulating glass. As shown in Figure 1a, the external silicone sealant of the insulating glass is in good condition, with normal black color and good elasticity, and no obvious damage and corrosion marks are found. However, the corrosion degree of the inner cavity of the insulating glass has been very serious, some areas of the aluminum spacer have been corroded and penetrated, and a large number of white corrosion products have been produced on the surface. As shown in Figure 1b, there is some volatiles on the inner surface of the insulating glass, and the cavity has been fogged due to humidity and the failure is very obvious. The insulating glass was opened, and the aluminum spacer was taken out and disassembled. It was found that the surface of the B-type desiccant inside the aluminum spacer was wet, deliquescence, and powdered, and a large number of white corrosion products were enriched around it, as shown in Figure 1c.

Figure 1 (a) A close-up photo of the corrosion product at the corrosion penetration of the aluminum spacer in the inner cavity; (b) The fogging failure of the hollow glass cavity; (c) The photo after the aluminum spacer is disassembled

The failed insulating glass is supported by aluminum spacers and is bonded and sealed with hot melt butyl sealant so that a dry gas space is formed between the glass layers. Based on the macroscopic corrosion conditions, an intact and a corroded aluminum spacer were selected for comparative analysis of the microstructure. Figure 2a shows the SEM results of the intact aluminum spacer surface. It can be seen that the surface is intact, the substrate is uniform, and there is no corrosion phenomenon. The corroded aluminum spacers have obvious corrosion cracks, vertical and horizontal gullies, and severe local pitting, as shown in Figure 2b, and some flocculent corrosion products exist. EDS was used to analyze the corrosion products. The results are shown in Figure 2c. It can be seen that the corrosion products mainly include four elements: O, Al, Cl, and Ca, and their contents are 48.86, 22.76, 17.30, and 8.99 wt%, respectively. In particular, there is a relatively high content of Cl, which may be the main reason for the severe corrosion of the aluminum spacer, which in turn leads to the failure of the insulating glass.

Figure 2 (a) Surface morphology of intact aluminum spacers; (b) surface morphology of corroded aluminum spacers; (c) EDS analysis of corrosion products

What are corroding aluminum spacers?

To clarify the composition of corrosion products, we performed an XRD analysis on the corroded aluminum spacers, and the results are shown in Figure 3. The surface of the aluminum spacer will be oxidized due to long-term use, so in addition to elemental aluminum, there is also some Al₂O₃  in the phase (pdf#21-0010); at the same time, there will be some desiccant remaining on the surface of the aluminum spacer, which also contains some Al₂O₃  and bulk SiO₂ (pdf#46-1045). It is worth noting that the presence of CaCl₂ (pdf#24-0223) and AlCl₃ (pdf#22-0010) was also detected in the XRD pattern, confirming that the aluminum spacer has been corroded by chlorine element, and the corrosion product is AlCl₃ , this result Consistent with the EDS analysis. However, regarding the source of chlorine elements, further analysis is required.

^{Figure 3 XRD pattern of corroded aluminum spacer surface}

To further observe the effect of chloride ion penetration on the corrosion of aluminum spacer, we carried out SEM characterization of the section of the aluminum spacer, and also compared it with the section of the normal aluminum spacer. From the comparison of Figures 4a and 4b, it can be seen that the material of the normal aluminum spacer section is compact, uniform, with clear lines, and no corrosion is seen, while the section of the aluminum spacer using desiccant has been infiltrated by chloride ions, and the material and structure are complete. The properties are destroyed and the corrosion depth reaches hundreds of microns.

Figure 4 (a) Comparison of cross-sectional morphologies of normal aluminum spacers and (b) corroded aluminum spacers

Through macroscopic observation, the corrosion position of the insulating glass is not the connection between the aluminum spacer and the hot-melt butyl adhesive, that is, the corrosion of the aluminum spacer does not come from the hot-melt butyl adhesive. In addition, a large amount of B-type desiccant is filled in the aluminum spacer, which may be the main cause of corrosion of the aluminum spacer. Therefore, it is necessary to analyze its composition to obtain whether the corrosive element of chlorine originates from the desiccant. Information.

To verify the source of chlorine in the corrosion products, and X-ray fluorescence analysis was carried out on the B-type desiccant. The content of some elements is shown in Table 1, and it was found to contain a large amount of silicon and aluminum elements, which came from the desiccant itself; at the same time, it also There is a certain content of calcium and chlorine elements, is speculated that a certain amount of CaCl₂  may be doped in the desiccant. Therefore, the phase detection of the desiccant was carried out by XRD, and the results are shown in Figure 5. The XRD pattern of the desiccant has typical characteristic peaks of CaCl₂ (pdf#24-0223), which proves that the desiccant is indeed doped with a certain amount of CaCl₂ , which is the source of chlorine and also causes the corrosion of aluminum spacers. one of the fundamental reasons.

 Table 1 Content of some elements in desiccant samples

^{Figure 5 XRD pattern of B-type desiccant}

According to the definition in the relevant international industry-standard "Desiccant for Insulating Glass": Class B desiccant is a water-containing magnesium-rich aluminosilicate mineral with a layered chain structure. 

The ideal molecular formula is (Mg, Al, Fe)5Si8O20(OH) 2(OH2)4·4H2O. 

However, the composition of the samples in this paper has a large deviation compared with that of attapulgite. Therefore, we tested the CaCl₂  content in the desiccant according to the industry standard "Test method for the detection of impurities (calcium chloride, calcium oxide) in the desiccant for insulating glass", as shown in Table 2, the class B desiccant in this paper The content of CaCl₂  is 11.70 wt%.

Table 2 Calcium chloride content in desiccant samples

Corrosion Mechanism of Calcium Chloride and Corrosion Rate of Aluminum Spacer

Based on the above experimental research results, the corrosion on the surface of the aluminum spacer is mainly local pitting corrosion. The calcium chloride in the desiccant absorbs water in the humid environment of the hollow glass cavity and becomes calcium chloride hexahydrate, and deliquesces into a liquid state. Chloride ions migrate to the vicinity of the aluminum spacer along with the liquid water film on the surface of the desiccant and conditionally form enrichment there. Subsequently, a large number of chloride ions are adsorbed on the oxide film of aluminum, replacing the oxygen in the oxide film, and forming soluble AlCl₃ - with Al₃+, so that the metal surface changes from a passivation state to an active state, causing damage to the protective film on the aluminum surface, resulting in Corrosion of aluminum spacers by B-type desiccant occurs.

Corresponding to the actual working conditions of the insulating glass, the external water vapor has always penetrated the inner cavity during the service period, so this type of calcium chloride desiccant will always absorb water and be in a wet state. Because calcium chloride is very easy to absorb water, it will migrate slowly and deliquescence occurs at the aluminum spacer where the moisture changes and fluctuates greatly, especially when the indoor temperature difference changes greatly, deliquescence is more serious. Therefore, it is easy to form a local environment of strong corrosive concentrated salt solution at the position of the aluminum spacer in the hollow glass cavity and the desiccant sealant, causing the aluminum spacer and the hot-melt butyl glue to be seriously corroded, and finally causing the insulation of the insulating glass.

To prevent the corrosion of aluminum spacers and insulating glass by desiccants doped with calcium chloride, it is urgent to understand the corrosion rate of such desiccants on aluminum spacers, and evaluate their corrosion effects on insulating glass, to reduce the corrosion rate of insulating glass. Risk and loss of corrosion failure.

Comprehensive consideration of various factors during the production and service of insulating glass, such as the relative humidity environment during production, the penetration of sealant, the 15-year service life required by the industry standard "Insulating Glass", etc., combined with doping calcium chloride The characteristics of powdering, migration and conditional enrichment of the desiccant, referring to the calcium chloride content of 11.70 wt% in the above desiccant, we prepared a calcium chloride solution with a concentration of 10 wt% to simulate the effect of the above desiccant on the aluminum interval Corrosion rate of strips. As shown in Table 3, the corrosion rate of aluminum spacer bars in 10 wt% calcium chloride solution is 0.0732 mm/a. According to the industry standard for aluminum spacer bars for insulating glass, the wall thickness is not less than 0.3 mm. The use of calcium chloride-doped desiccant will cause the aluminum spacer bars to be penetrated by pitting in 3 to 5 years and then cause external water vapor. Penetration increases, accelerating the corrosion rate. The continuous outward migration of corrosion products will further affect the sealant, resulting in the corrosion failure of the insulating glass.

Table 3 Corrosion rate of aluminum spacer bars in 10 wt% calcium chloride solution

It is recommended to ban calcium chloride and other corrosive desiccants

As a commonly used desiccant, calcium chloride has become the main component of B-type desiccant when mixed with it. Through the comprehensive analysis and testing of the insulating glass samples, it can be seen that the inner cavity of the insulating glass is in a relatively humid state, and the aluminum spacers are directly in contact with the desiccant, and the desiccant is the main water absorption contains a large number of chloride ions. The main source of chloride ions that play an important role in corrosion is the B-type desiccant doped with calcium chloride. 

The specific reasons for the serious corrosion of insulating glass are as follows:

(1) During the production and use of insulating glass, it is in a relatively humid environment. Moisture is easily enriched here, creating a water film (liquid) environment required for the calcium chloride-doped type B desiccant to corrode the aluminum spacer.

(2) There is a high content of calcium chloride in the desiccant, which leads to the slow migration of soluble chloride ions along with the gaps of the desiccant particles and the water film on the surface of the material. The location of the large aluminum spacer leads to a higher level of chloride ion concentration there.

Based on the above research results, to inhibit the corrosion failure of aluminum spacers and insulating glass, the following aspects can be improved and improved:

(1) Use a 3A molecular sieve that meets the requirements of relevant standards as the desiccant for insulating glass. It is forbidden to use calcium chloride and other corrosive desiccants.

(2) Use aluminum spacers that meet the requirements of relevant standards, and choose products with standard aluminum spacer wall thickness, tight back welds, and reasonable air hole permeability to ensure the integrity of the insulating glass structure and reduce water vapor permeability.

(3) In the production process of insulating glass, the humidity and temperature of the environment should be well controlled, and adopts the high-quality hot-melt butyl sealant that meets the automatic production standard should be selected, to ensure that the humidity in the inner cavity of the insulating glass is low and the infiltration of water vapor is less every year.

^{The automatic production standard of insulating glass hot-melt butyl sealant coating.}

(4) It is recommended to revise the industry standard of desiccant for insulating glass as soon as possible, otherwise it will seriously affect the high-quality development of the entire glass industry. It is suggested that clear technical requirements for the stability of the desiccant should be considered in the revision of the industry standard, and the use and doping of materials that will produce corrosive, decomposable, and volatile materials on the insulating glass under actual working conditions are strictly prohibited, to ensure the safety of the insulating glass. quality and longevity.

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