Copper is commonly used in a wide range of industries. However, its extreme ductility makes it prone to deformation and scratches. On this page, you can learn how to prepare scratch-free copper samples for metallographic examination quickly and effectively.
Download the full application note
Although metallic copper occurs naturally, it is mainly extracted from sulfide ores in a metallurgical smelting process. There are four main steps.
1. An initial smelting process extracts copper concentrates, resulting in copper matte (75 % Cu).
2. In a converter, air is blown into the liquid matte to oxidize the sulfides, resulting in blister copper (96-98 % CU).
3. Blister copper is refined in an anode furnace, resulting in anode copper (99 % Cu).
4. Impurities (including Ni, Pb, Ag, Pd and Au) are removed through electrolytic refining, resulting in copper cathodes (99.99 % Cu).
A small amount of copper is also produced by hydro-metallurgical process.
Fig. 1: Copper with red copper oxides, dark field, 500x
Fig. 2: Oxygen-free copper, etched with ammonium peroxydisulfate, 100x
Fig. 3: Copper cathode, etched according to Klemm, 100x
By adding aluminum, manganese, iron, nickel, tin or even small amounts of lead, special types of brass with specific chemical or mechanical properties can be produced.
Fig. 4: α- brass, color etched, 200x
Fig. 5: α-β brass cast (CuZn40Pb2) with grey-blue lead inclusions, unetched, 500x
Fig. 6: α-β brass cast, etched according to Klemm, light α- solid solution in dark matrix of β- solid solution, 100x
Bronze (copper-tin alloys)
Bronzes are divided into three types: wrought alloys, cast alloys, and bell casts. Depending on the required properties, small amounts of zinc, phosphorus, lead, nickel or iron can be added.
| Some specific brass alloys and their properties | |
| Gun metal (Cu-Sn-Zn) | Good corrosion resistance; a low friction coefficient |
| Aluminum bronzes (up to 11 % aluminum) | High strength at elevated temperatures; very good corrosion resistance |
| Beryllium bronze | High strength; high hardness; does not produce sparks when striking or impinging other metals |
| Copper-nickel alloys | Excellent corrosion resistance |
| Copper-nickel-zinc alloys (German silver) | High strength; good corrosion resistance; easy to form |
When working with copper and its alloys, metallography is generally used for grain size measurement, as well as to perform purity checks by qualifying and quantifying the copper oxide content.
In certain brasses, it may be necessary to determine the distribution of lead, as this can influence the machining process.
In cast alloys, metallography is generally used for general structure evaluation, to assess the distribution of eutectic or lead, and to evaluate the presence of shrinkage cavities or porosity.
Fig. 8: Bronze cast (CuSn10), etched with iron (III) chloride, dendritic structure α-δ eutectoid, 200x
Pure copper is soft and ductile, which makes it easy to deform and prone to scratches. Even bronzes and some of the harder brasses may be susceptible to severe scratching. This creates a problem for the metallographer. However, there are some simple solutions:
- Avoid coarse grinding abrasives
- Give a thorough diamond polishing with soft cloths
- Use chemical-mechanical fine polishing
Read on for a detailed description of how to prepare copper and its alloys for metallographic analysis quickly and accurately, without scratches or deformation.
Fig. 9: Pure copper wire, with final polish of OP-S, DIC, 200x
Fig. 10: Same sample as Fig. 9 with final polish of OP-S-ammonia/water/hydrogen peroxide mixture, DIC, 200x
Fig. 11: α-β brass cast, mechanically polished, unetched, 200x
Fig. 12: Same specimen as Fig.11 electrolytically polished, unetched, 200x. The lead inclusions are pulled out and therefore appear larger and more numerous
There are numerous etchants for copper and its alloys that are relatively easy to apply. Most cast alloys are not difficult to etch. However, it can be difficult to find the right etching solution for some wrought alloys, especially if they have been severely cold worked. In these cases, a color etch can be useful.
It is important to note that lead is attacked by the etchants and only black voids will remain after etching. Therefore, micrographs to document the quantity and distribution of lead must be taken before etching. The color of pure lead is grey-blue.
Fig. 13: Bronze cast (CuSn8Pb), unetched, showing large and small blue-grey lead inclusions, pale blue α-δ eutectoid discernable, 500x
Fig. 14: Same specimen as in Fig.13, color etched according to Klemm. The dendritic structure with light blue eutectoid and blue lead inclusions can be seen, but small lead inclusions cannot be clearly differentiated, 500x
Fig. 15: Bronze cast (CuSn10), etched with iron (III) chloride, dendritic structure α-δ eutectoid, 200x
| Application | Etchant |
| Grain area etch for copper, brass and bronzes | 100 ml water. 10 g ammonium peroxydisulfate. Use fresh! |
| All types of copper | 100-120 ml water or ethanol. 20-50 ml hydrochloric acid. 5-10 g iron (III) chloride. (concentration variable) |
Grain boundaries Grain areas |
25 ml distilled water. 25 ml ammonia water. 5-25 ml hydrogen peroxide, 3% Less hydrogen peroxide More hydrogen peroxide |
| α-β brass | 120 ml water. 10 g copper (II) ammonium chloride. Add ammonia water until precipitate dissolves |
| Fast and good polish for pure copper | 100 ml water. 100 ml ethanol. 19 g iron (III) nitrate |
| Colour etch according to Klemm | 100 ml cold saturated sodium thiosulfate. 40 g potassium metabisulfit |