Plumbing 101 - Part 2 brass part failure: the return of an old problem
By Frédéric Bourgeois Eng., M.A.Sc., Metallurgist, Isabelle Murray P. Eng., M.A.Sc., Metallurgist, Assistant Manager, Montreal,
We recently noticed an important rise of the failure rate of brass made parts installed in modern plumbing networks. Brass has been utilized in such networks without any problems for many years. However, we have documented failures of brass parts which had been in service in plumbing networks for a surprisingly short period of time. Some parts failed after less than a year of service. Certain parts even failed after only a few weeks of service. Failures related to brass made plumbing parts are sometimes causing important material damage as well as loss of water supply. Failure of brass made plumbing parts is a complex problem since many variables and phenomenon can play a role in such failures. A metallurgist (material specialist) has the knowledge to identify the various mechanisms that lead to the failure of parts such as brass made plumbing constituents. A metallurgist has, in fact, an excellent knowledge of all the macroscopic and microscopic characteristics associated to various types of metal parts fracture mechanism. Such a material specialist also knows which tools and procedures will permit the identification of the failure mechanism that led to a part failure.
This technical paper is about metallurgical aspects related to a common material: brass. More precisely, all metallurgical aspects related to brass made parts behavior when exposed to aqueous media are detailed in this paper. Accelerated/premature corrosion phenomenon of brass parts, occurring even if these parts are utilized under normal operating conditions, is presented in this paper. The main goal of this article is to provide answers as to why some brass made parts are subjected to premature failure in potable water plumbing networks.
Brass Parts for the Dumb
Brass is actually not a pure metal. It is, in fact, an alloy which is prepared by mixing liquid copper and liquid zinc. Other elements, such as lead or tin, are also added to brass, in smaller proportion. In fact, brass parts are mainly composed of copper to which a certain quantity of zinc is added. There are many kinds or types of copper and zinc alloys that can be considered as "brass". The zinc content of such a material typically varies from 15% to about 40%.
Mechanical and corrosion resistance of brass parts are significantly influenced by their zinc content. More precisely, corrosion resistance of a brass part is greatly affected by zinc since this element is less noble (or more reactive) than copper. In fact, a brass part susceptibility to a selective and premature corrosion phenomenon is directly linked to its zinc content. The higher the zinc content in a part, the more susceptible this part is to selective corrosion mechanism. Such degradation mechanism can even be provoked by a liquid normally considered as non-corrosive: potable water.
Brass Corrosion Phenomenon
Brass parts can be affected by an accelerated corrosion phenomenon when their zinc content is higher than 15%. Moreover, parts with high zinc content are particularly susceptible to degradation related to this corrosion mechanism. As such, parts with zinc content higher than 35% are particularly sensitive to such degradation phenomenon. We recently realized that many brass made parts, installed in water plumbing networks, were made of alloys which contained about 40% zinc. Such parts are highly susceptible to premature degradation phenomenon when exposed to potable water. In fact, the resistance of such parts is more attributable to appropriate physical/chemical characteristics of the potable water itself rather than the alloy corrosion resistance.
The corrosion resistance of brass parts can be mostly related to the parts themselves or, more precisely, their metallurgical characteristics. First, zinc is a more reactive metal than copper. The more zinc a part contains and the more this part becomes susceptible to corrosion. As a result, parts containing as few as about 15% zinc can be subjected to selective corrosion phenomenon.
Even if a part corrosion resistance drops with increased zinc content, its inner microscopic structure (microstructure) remains the same up to a certain level of added zinc. More precisely, there is only one phase present in brass parts when the zinc content is below about 30%. This microscopic phase is commonly named "Alpha" (?).
An important microstructural change is found in brass inner microscopic structure when more than about 35% zinc is added to a part. More precisely, a second phase, highly susceptible to premature and selective degradation phenomenon, appears in the microstructure. This phase is more commonly called "Beta" (?).
The above-mentioned microstructural change in brass parts can be compared to a glass of water to which salt is added. In that case, water would represent copper and the salt would be the zinc added to generate brass alloy. First, adding salt to a glass of water generates a single phase product, namely salted water. Up to a certain amount (or quantity) of added salt, all the salt is completely dissolved in the water. Salted water is the only phase in the glass since nothing else but that specific phase can be found in that glass. Brass behaves in a similar manner. More precisely, zinc can be dissolved inside copper up to a certain level to generate a single phase "Alpha" alloy.
When too much salt is added to water, salt precipitates and accumulates at the bottom of the glass. At that point, the glass of water contains two phases: salted water as well as precipitated salt. Again, brass behaves in a similar manner. More precisely, when more than about 35% zinc is added to copper, a second phase, named "Beta", appears in addition to the "Alpha" phase. It is precisely when such "Beta" phase appears in brass that the alloy corrosion resistance to potable water drops significantly.
Zinc rich brass alloys and "Beta" phase are particularly sensitive to a well known and documented corrosion phenomenon when exposed to potable water, namely dezincification. This corrosion mechanism was first documented in the 1920's. It had been exhaustively studied by well known organisms such as ASM (American Society of Materials) and AWWA (American Water Works Association).
Dezincification is a corrosion phenomenon characterized by a selective removal of the zinc from brass made parts when they are exposed to an electrolyte (water, potable water). A porous and weak copper rich structure with poor mechanical properties remains locally in the microstructure (inner microscopic structure) due to selective zinc removal. As a result, the areas of the part affected by dezincification are containing a copper rich microstructure where the copper content is significantly higher than the level found in the rest of the brass part. These areas are also characterized by gray and/or red/pink colored products, typical of copper rich compound. The rest of the brass part (unaffected areas) stays rather yellow or gold colored. In certain cases, when a part is heavily corroded, the red corrosion products can be observed during a simple "naked eye" visual examination.
Since they are porous, the dezincified areas act as weak zones and contribute to diminish the mechanical properties of a part. The more a part gets corroded (dezincified), the more its mechanical properties drop until the part gets too corroded to resist to its normal operating stresses. At that moment, the part breaks due to dezincification corrosion phenomenon.
Materials engineers can, by applying specialized techniques and using specific metallurgic instruments, characterize broken brass parts in order to identify if dezincification played a role in their failure.
For example, a sample of a broken brass part, commonly referred to as metallography, can be prepared using a well known technique in the field of metallurgy. Such metallography, when observed under a microscope, will reveal dezincified areas or corroded "Beta" phase inside a brass part. In other words, metallurgical failure analysis can determine if dezincification played a role in part failure.