Table 1 The main environmental risk indicators and the screening reasons (degradation mechanism)

Temperature,

Relative humidity

Temperature and humidity are the primary factors influencing the corrosion of all metals

Changes in temperature affect the chemical reaction rate occurring on the metal surface directly. As the temperature increases, the corrosion rate rises accordingly

Similarly, humidity plays a significant role in the formation and thickness of the water film on the metal surface. As the relative humidity increases, the metal surface is more likely to absorb the water film, causing other atmospheric pollutants to become more soluble on the surface of the bronze cultural relics, thus accelerating corrosion

Furthermore, these two factors interact with each other. When the absolute humidity remains constant, decreasing temperatures will lead to an increase in relative humidity. The combined effect of temperature and humidity can accelerate the formation of oxides on the surface of metal, resulting in the development of a rust layer and ultimately leading to damage to cultural relics

Lighting

Lighting plays an important role in the museum environment. Light is a kind of external energy source that can be converted into heat energy. The lighting used in showcases can affect the ambient temperature, thereby accelerating the corrosion rate of metal cultural relics

At the same time, light will destroy the protective oxide layer on the surface of metal relics, causing damage to the internal matrix. Additionally, UV light causes the breakdown of ozone, which is another significant pollutant. This process leads to increased corrosion

Pollutants

In the International Cultural Heritage Conservation Science and Technology Conference, acid pollutants, specifically volatile organic acids like formic acid (HCOOH) and acetic acid (CH3COOH), were emphasized as significant pollutants in museum environments. In recent years, numerous scholars from both domestic and international backgrounds have conducted their research on the corrosion of metal cultural relics

In comparison to inorganic acids, formic acid and acetic acid have lower acidity levels. However, when they combine with the water film absorbed on the metal surface, the ionized H + will reduce the pH of the liquid film, thereby accelerating corrosion. At the same time, organic acids will also accelerate the existing corrosion on the surface of relics, potentially leading to aggravated matrix corrosion

Particulate matter serves as a carrier of pollutants in the atmosphere. The smaller the particle size, the greater the penetration ability. Organic acids and other substances can utilize these particles to reach the metal surface, resulting in the damage to metal culture relics

Particularly, some fine particles (PM2.5) can even pass through the loose rust layer on metal surface, carrying pollutants and causing damage to the matrix. Water-soluble ions such as Cl⁻ and NH⁴⁺ can reach the surface of metal cultural relics through particles as a medium. These ions dissolve in the water film on the metal surface, forming a salt solution that accelerates the electrochemical corrosion

Biological factors

Biological factors can also play a significant role in promoting the corrosion of metal culture relics. The influence of microbial corrosion depends on several factors, including the properties of relics, the types of microorganisms, and the environmental conditions

The main corrosion bacteria includes iron bacteria, thiobacillus ferrooxidans and sulfate reducing bacteria. These microorganisms utilize metal as a source of nutrition, leading to the production of organic acids and harmful gases. In addition, it involves the biological promotion in the corrosion process