General corrosion characteristics of brass
Brass, a Cu-Zn alloy with zinc as the main alloying element, gets its name from its yellow color.
Depending on the type and content of added alloying elements, brass can be categorized into three main types: single-phase brass, duplex brass, and specialty brass.
When the zinc content is below 36%, it forms a single-phase α solid solution, therefore known as α brass. When the zinc content varies between 36% and 45%, it becomes α+β duplex brass.
Brass with zinc content above 45% is impractical due to excessive brittleness due to excess β phase. Special brasses are formulated by adding elements such as Sn, Mn, Al, Fe, Ni, Si, Pb, etc., to the Cu-Zn base.
Brass corrodes slowly in the atmosphere and also has a low corrosion rate in pure fresh water (0.0025-0.025 mm/year). However, it corrodes a little faster in seawater (0.0075-0.1 mm/year).
Fluorides have a minimal impact on brass corrosion, chlorides have a more significant effect, while iodides cause severe corrosion. In water containing gases such as O2, CO2, H2S, SO2, NH3, etc., the corrosion rate of brass increases sharply.
Easily corrodes in mineral water, especially water containing Fe2(SO4)3. Brass corrodes severely in nitric and hydrochloric acids, corrodes more slowly in sulfuric acid and is resistant to NaOH solutions. Brass has better resistance to impact corrosion than pure copper.
Special brasses have better corrosion resistance than regular brass. Adding about 1% Sn to brass significantly reduces dezincification corrosion and improves its resistance to seawater. Incorporating about 2% Pb into brass increases its wear resistance, significantly reducing its corrosion rate in seawater flow.
To avoid dezincification, small amounts of As, Sb or P (0.02%-0.05%) can be added. Naval brass containing 0.5%-1.0% Mn has increased strength and excellent corrosion resistance. In brass containing 65% Cu and 55% Cu, replacing part of the Zn with 12%-18% Ni changes the color to silvery white, which is why it is called nickel silver or German silver.
This alloy presents excellent resistance to corrosion in saline, alkaline and non-oxidizing acids. Extensive replacement of Ni by Zn prevents dezincification. In addition to these corrosion characteristics, brass also undergoes two significant forms of corrosion: dezincification and stress corrosion cracking.
Stress corrosion cracking of brass
Factors that influence stress corrosion cracking in brass include the corrosive medium, stress, alloy composition, and microstructure. A specific alloy only undergoes corrosion cracking under certain media and specific stress conditions.
(1) Corrosive Medium
Brass under tensile stress can suffer from stress corrosion in all environments containing ammonia (or NH4+), as well as in the atmosphere, sea water, fresh water, high temperature and high pressure water, and steam. For example, the breakage of brass bullet casings during the summer rainy season (also known as “season breakage”) is a typical example of stress corrosion cracking in brass.
Furthermore, the morphology of brass stress corrosion cracking can be intergranular or transgranular. In film-forming solutions, intergranular fractures mainly occur, while in non-film-forming solutions, transgranular fractures are more common.
It is generally believed that the mechanism of stress corrosion cracking of brass involves the formation of a brittle film of cuprous oxide on the surface of the brass in film-forming solutions. This film fractures under stress and strain, leading to propagation of the crack into the base metal, which then stops due to sliding, exposing the crack tip to the corrosive solution.
The process of intergranular penetration, film formation, brittle fracture, and crack propagation repeats itself, resulting in a stepped fracture surface. In non-film-forming solutions, stress causes preferential dissolution of brass surface dislocations, leading to the propagation of cracks along the path of highest dislocation density, causing fracture.
In brass with lower zinc content, dislocations are mainly cellular and grain boundaries have the highest dislocation density, leading to intergranular fractures.
In high-zinc brass, dislocations are mainly planar and stacking faults are the areas of highest dislocation density, leading to transgranular fractures.
Furthermore, the congregation of zinc atoms at dislocations under tension increases the reactivity at these locations, thus increasing the rate of crack propagation with higher zinc content.
Experimental studies show that, under atmospheric conditions, industrial atmospheres most easily cause stress corrosion cracking in brass, with the shortest fracture life, followed by rural atmospheres; marine atmospheres have the least effect.
These differences in atmospheric environments are due to variations in SO2 content (higher in industrial atmospheres, lower in rural atmospheres and almost non-existent in marine atmospheres).
In summary, the substances that primarily cause stress corrosion cracking in brass are ammonia and its derivatives, or sulfides. The effect of ammonia is well recognized, while the role of sulfides is less clear. Furthermore, steam, oxygen, SO2, CO2, CN- have an accelerating effect on stress corrosion cracking.
(2) Stress
Tensile stress is a necessary condition for the occurrence of stress corrosion cracking in brass. The higher the tensile stress, the greater the sensitivity to stress corrosion cracking.
Eliminating residual tensile stress through low-temperature tempering can prevent stress corrosion cracking in brass.
(3) Composition and Microstructure of the Alloy
The higher the zinc content in brass, the greater its sensitivity to stress corrosion cracking. The specific zinc content below which stress corrosion cracking does not occur depends on the nature of the medium.
For example, brass with less than 20% zinc content generally does not suffer from stress corrosion cracking in natural environments, but brass with low zinc content may suffer from stress corrosion cracking in ammonia water.
The effects of other alloying elements on stress corrosion cracking are as follows:
Silicon effectively prevents stress corrosion cracking in α brass. Si and Mn improve the resistance of α+β and β brass to stress corrosion cracking. Under ammonia atmospheres, elements such as Si, As, Ce, Mg improve the stress corrosion resistance of α brass.
Under atmospheric conditions, Si, Ce, Mg, etc., increase the resistance to stress corrosion cracking. Exposure tests in industrial atmospheres indicate that the addition of Al, Ni and Sn to Cu-Zn alloys reduces their tendency to undergo stress corrosion cracking.
