Cast copper alloys are widely used in atmospheric and marine environments due to their excellent corrosion resistance. However, their corrosion behavior in these environments is influenced by various factors, leading to different types of corrosion. This article will explore the corrosion characteristics, corrosion types, environmental factors, and phenomena such as stress corrosion and dealloying corrosion of cast copper alloys in atmospheric and marine environments.
Cast copper alloys are widely used in atmospheric and marine environments due to their excellent corrosion resistance. However, their corrosion behavior is influenced by many factors in these environments, resulting in different types of corrosion and corrosion rates. Understanding these characteristics helps optimize the use of cast copper alloys and improve their corrosion resistance.
When cast copper alloys come into contact with substances such as water vapor and salts in atmospheric and marine environments, a passive or semi-passive protective film forms on their surface. This protective film effectively inhibits various types of corrosion, helping the alloys maintain their corrosion resistance for extended periods. Especially in marine environments, the corrosion of cast copper alloys is mainly uniform corrosion rather than severe localized corrosion. This is due to the stable protective film formed on the surface.
Water vapor in the atmosphere and the water film on the metal surface play an important role in the corrosion of cast copper alloys. When the relative humidity reaches the critical level (usually between 50% and 70%), the corrosion rate increases sharply. In addition, atmospheric pollutants such as carbon dioxide (CO₂), sulfur dioxide (SO₂), and nitrogen oxides (NO₂) dissolve in the water film and undergo hydrolysis, leading to the acidification of the water film and destabilization of the protective film. This accelerates the corrosion process. Moreover, ammonia and hydrogen sulfide gases in industrial atmospheres can significantly accelerate the corrosion of cast copper alloys, especially the occurrence of stress corrosion.
The corrosion behavior of cast copper alloys in marine environments is complex. Marine environments can generally be divided into different zones, such as the marine atmospheric zone, the splash zone, the tidal zone, and the fully submerged zone. In these zones, the corrosion rate and type of corrosion of cast copper alloys vary. Especially in the splash zone, the corrosion behavior of cast copper alloys is very similar to that in the marine atmospheric zone, exhibiting good corrosion resistance. Although the high oxygen supply in the splash zone accelerates the corrosion of steel, it helps cast copper alloys maintain their passive state, and the corrosion rate generally does not exceed 5 μm/a.
The corrosion behavior of cast copper alloys in atmospheric and marine environments is not limited to uniform corrosion but may also exhibit various special corrosion phenomena. These corrosion types and phenomena have different formation mechanisms and influencing factors, posing threats to the durability and structural integrity of cast copper alloys. Below are common corrosion types and special corrosion phenomena in cast copper alloys.
Brass is a common representative of stress corrosion in cast copper alloys, with seasonal cracking (also called ammonia cracking) being a common phenomenon. Seasonal cracking typically occurs in tropical regions, especially during the rainy season. Its occurrence is closely related to ammonia and its derivatives, with oxidants and moisture being key factors for stress corrosion in brass. Additionally, environments polluted by sulfur dioxide (SO₂), freshwater, seawater, and other factors can promote stress corrosion cracking in cast copper alloys. Therefore, seasonal cracking is not limited to specific climate conditions but also depends on the presence of ammonia and other corrosive gases in the environment.
Dezincification is one of the most typical dealloying corrosion phenomena in cast copper alloys, especially when combined with stress corrosion. Dezincification corrosion can occur in two forms: a layered dealloying type, which manifests as uniform corrosion and is relatively less harmful, and a deep-penetrating, dendritic dealloying type, which typically leads to pitting corrosion and significantly reduces material strength, posing a greater risk. Additionally, aluminum bronze dealloying and nickel dealloying in white bronze are unique corrosion types in cast copper alloys.
In environments containing ammonia, sulfuric acid, nitric acid, and other solutions, cast copper alloys are prone to stress corrosion cracking. At the same time, galvanic corrosion, pitting corrosion, wear corrosion, and other forms of localized corrosion can also affect cast copper alloys. Although these localized corrosion forms may not affect the overall corrosion resistance of the alloys as much as uniform corrosion, they can cause significant corrosion damage in localized areas, potentially compromising the structural safety of the material.
The corrosion rate of cast copper alloys is closely related to the corrosiveness of the environment. In mild atmospheric environments, the corrosion rate of cast copper alloys is typically between 0.1 and 2.5 μm/a. In heavily polluted industrial atmospheres and marine atmospheres, the corrosion rate may increase by one to two orders of magnitude. Additionally, pollutants in the environment, such as ammonia and hydrogen sulfide, can significantly increase the stress corrosion sensitivity of cast copper alloys like brass, accelerating their corrosion rate.
Cast copper alloys exhibit higher corrosion rates in harsh industrial atmospheres and industrial marine atmospheres. Corrosive gases, such as ammonia, in polluted atmospheres significantly enhance the corrosion rate, especially in high-humidity environments. In contrast, in mild marine atmospheres and rural atmospheres, the corrosion rate is relatively low. This reflects the different impacts of various environments on the corrosion of cast copper alloys.
Cast copper alloys exhibit excellent corrosion resistance in atmospheric and marine environments, especially with the help of protective film formation, which effectively resists various corrosion forms. However, under specific environmental conditions, such as heavily polluted industrial atmospheres, ammonia-containing solutions, or splash zones in seawater, cast copper alloys may still face stress corrosion, dealloying corrosion, and other localized corrosion issues. Therefore, in practical applications, it is important to select appropriate types of cast copper alloys based on environmental characteristics to improve their corrosion resistance and extend their service life.
