Gunmetal - the material
To describe gunmetal in its significance, timelessness and future perspectives, we need to expand our point of view back into pre-Christian times.
The history of humanity was decisively influenced by certain materials that accompany us even into our modern daily life. If one views the various epochs of human history, one can glimpse the deep significance and importance of these materials. They lent important eras in the development of humanity their names: Stone Age, Copper Age, Bronze Age and Iron Age. Whether individual materials from our modern age will gain such fame when viewed by later historians must be answered with a large question mark.
The early knowledge of materials is based solely on their quality and frequency in chemistry. What does that mean? The first metallic materials were only accessible in prehistoric times when they could be found massively in the Earth's crust. They had to be available in the elementary form as the processes to extract metal were unknown. At the same time, that means that they could not participate in the natural corrosion processes. Only a handful of metals could claim this fact for themselves, namely: gold, silver, platinum and copper. Iron is also repeatedly mentioned in this connection. But it must be said that the iron known in prehistoric times was exclusively meteorite iron so its availability in terms of quantity was significantly less than noble metals.
The early awareness of the above-mentioned elements is thus based on their lack or a low amount of affinity to the non-metals we all know about, oxygen, sulphur, chlorine etc. In the end, this fact led to gold, silver and platinum being called noble metals and copper as a seminoble metal. The naming was thus "the first amateurish view of corrosion" of the metallic materials known back then.
That meant copper, widely more available than noble metals as the most important metal for human progress, began its triumphal march already approx. 5000 years BC. Jars, curved plates as water lines and artistic objects were manufactured by beating and forging the materials.
With the dedicated use of energy, i.e. using wind to bring fire to high temperatures, it became possible to create the first parts fabricated through casting.
However, this step was only possible after humankind solved the problem of making a fire resistant vessel to melt the metal. Generally, copper was melted in a stone or in a vessel lined with clay or a soil trough and then conducted to processing.
During this development, copper was also melted in ore-bearing stones or earth without the person doing this actually being aware of this fact.
That was the hour of birth of the first copper-based alloys. If one led tin into the copper, one got „Bronze“; if the element one mixed with the copper was zinc, one got "Brass".
Both materials, due to their commonality and longevity, were quickly propagated throughout human civilisation. But already in this early phase of the development of the use of metals, the users differentiated the metal alloys according to their durability. Brass was worked into decorations and articles of daily use within protected habitats. On the contrary, bronze was more suitable for artistic and daily use articles that might be exposed to weather or even to seawater that could defy these extreme environments for 7000 years and that are available to us today as witnesses of pre-historical developments.
With the introduction of craftsmanship and later through industry, the demand for metallic materials increased so much that the metals native to the Earth's crust could no longer cover this demand. Humanity sought new ways and found the path to iron, aluminium etc. through metal deoxidation, and to large copper deposits.
This step forward for humanity however simultaneously included the modern topic of corrosion resistance and ion leakage in the alloys.
Metal extraction through deoxidation
If one considers extracting metals from the point of view of their compounds, one realises that, e.g., chemically stable oxides can be reduced using carbon and can be raised to a energetically higher, but simultaneously less stable level, see Photo 6. If one equates the terms "stable" and "instable" with a state of equilibrium, one understands that the instable condition can be easily and quickly reverted to the availability of corrosive elements. The electrolyte water provides a nearly inexhaustible potential.
Gunmetal statements in DIN 50930-6
"Gunmetal can be used without restrictions in drinking water"
As illustrated in the table below, the gunmetal alloy is only slightly modified through DIN 50930-6. Only the elements lead and nickel undergo changes. The reduction of lead is merely 1% under the previous permissible minimal value in DIN EN 1982. The amount of nickel with < 0.6% goes only through reduction within the above-mentioned material standard. That means this is not a new but rather a gunmetal modified by reducing its lead content by 1%.
Drinking water applications
|Gunmetal = CuSn5Zn5Pb2||Elements suitable for drinking water according to DIN 50930-6|
|Tin Sn||No information|
|Zinc Zn||No information|
|Lead Pb||max. 3,0 %|
|Nickel Ni||max. 0,6 %|
|Antimony Sb||max. 0,1 %|
|Impurities||Respective max. 0,02 %|
|Release of elements according to DIN 50930-6*|
* DIN 50930-6 merely provides information about the influence on the composition of drinking water by metal ion migration. It does not specify the corrosion restistance of the material.
Examinations of this modified material show that the mechanical values, i.e., stability, expansion and hardness are improved after the material has been modified. This can be thoroughly comprehended. The lead is available elementarily as an element that reduces the cross section in the structure. If its amount is reduced, the occurring tensile stresses from the alpha-base structure can be increasingly accepted and compensated through deformation.
These facts and the yearlong investigations of the alloy based on DIN 50931-1 have resulted in the material after its modification having been certified by DIN 50930-6 for unrestricted use in drinking water installations as a valve and fitting material.
Gunmetal statements in DIN 1988
In extracts, this standard states that the conditions listed below need to be complied with:
- For construction projects, you need to get the current information from the water works about the composition of the drinking water in the respective region (drinking water analysis).
- The material selection is to be based on the water analysis data (parameters according to Table 1, DIN 50930-6) and the proposed application area for materials based on DIN 50930-6.
As in the quoted standard DIN 50930-6 there are no restrictions of any kind for the gunmetal presented here, the material does not underlie any kind of restrictions from this aspect either.