CUPRONICKEL

Cupronickel is defined as a copper (Cu)-based alloy in which nickel (Ni) constitutes at least 10% by mass or acts as the primary alloying element. This category of alloys does not respond to a single fixed composition but encompasses a range of proportions that allow for the adjustment of its mechanical, thermal, and chemical properties according to the desired application.

It is important to distinguish cupronickel from other alloys where nickel is present along with significant concentrations of additional elements such as zinc (Zn), tin (Sn), or aluminum (Al). In these cases, the resulting alloy is not classified as cupronickel but as nickel bronze, a distinct family with its own characteristics. The difference lies in the relative proportion of the elements: while in cupronickel, nickel dominates as the secondary component after copper, in nickel bronze, the presence of other alloying elements substantially modifies the material's structure and behavior, generating differentiated properties in terms of both resistance and chemical reactivity.

Quality cupronickel is characterized by containing exclusively copper (Cu) and nickel (Ni) as primary components, accompanied only by controlled trace amounts of other added elements. Among these, manganese (Mn) is deliberately incorporated to prevent intergranular oxidation during the casting process. Conversely, elements such as sulfur (S), oxygen (O), or phosphorus (P) are considered undesirable and are rigorously avoided, as their presence compromises the structural integrity and chemical stability of the alloy.

There are five recognized grades of cupronickel, none of which are registered as trademarks. In all cases, the composition remains simple: a mixture of copper and nickel, two metals completely miscible with each other in any proportion. This compatibility allows the nickel content in the copper matrix—or vice versa—to vary from values as low as 0.01% to concentrations close to 95%. However, when nickel becomes the base metal, i.e., the most abundant component by mass, the alloy ceases to be classified as cupronickel and becomes part of a distinct category, such as Monel, a special alloy with a predominance of nickel.

Likewise, alloys combining copper, nickel, and zinc (Zn) should not be considered cupronickel grades. This triad gives rise to a different family known as alpaca or German silver, also called hotel silver or imitation silver. Although these alloys may exhibit visual and structural similarities to cupronickel, their properties and applications are distinct, and therefore, they should not be confused or classified under the same metallurgical denomination.

The alloy known today as cupronickel arrived in Europe through trade routes established with China long before the discovery of nickel (Ni) as a chemical element on the European continent, and even before the arrival of the Spanish in America. In China, this alloy was called "Pai-Tong", a term that literally translates to "white copper." In that context, it was considered a new metal, similar to copper (Cu), although its exact composition was unknown. Since it was obtained from the same mines where copper was exploited, and shared many of its physical properties, confusion between the two materials was common.

Cupronickel is a native alloy, meaning it can be found naturally in mineral deposits, just as iron–nickel (Fe–Ni) combinations are found in metallic meteorites. The high chemical affinity between copper and nickel allows both metals to coexist in their natural state, and when melted together, they generate a whitish mixture that is harder and more resistant than pure copper, with a remarkable ability to resist corrosion, especially against sulfur compounds.

The "Pai-Tong" alloy was particularly well-received in countries like Germany, where it was valued for its balance between cost and functional properties. Its price was situated between that of copper and silver (Ag), making it an attractive option for the manufacture of cutlery, ornaments, and costume jewelry. Over time, nickel was isolated as a pure element, which allowed its deliberate incorporation into alloys with copper to produce the first European "white copper."

In Germany and the Nordic countries, nickel was originally known as "Kupfernickel", a compound word where Kupfer means copper, and Nickel refers to a figure from local folklore—a goblin or mischievous entity—who, according to popular belief, replaced high-quality copper with a harder-to-work white metal. The Spanish term "cuproníquel" derives from this Germanic denomination, retaining the original semantic structure.

Although in some regions of Germany, elemental nickel is still called "Kupfernickel", the denomination has been simplified to "Nickel" to avoid confusion between the pure metal and the copper–nickel alloy. This terminological distinction has also been adopted in English-speaking countries.

Over time, cupronickel ceased to be a rarity imported from the East to become an alloy mass-produced in Europe, used as an economical alternative to silver. Subsequently, the inclusion of zinc (Zn) in the standard formula of copper and nickel gave rise to what is known as "German silver" or alpaca. It is worth noting that "Alpacca" is, in effect, a registered trademark, although the term has been generalized to refer to this family of ternary alloys.

Cupronickel alloys come in different grades according to the relative proportion of copper (Cu) and nickel (Ni), which determines their physical, chemical, and aesthetic properties. Each grade responds to specific needs for strength, durability, and cost, and is used in industrial, monetary, and marine contexts.

A composition of 90 parts copper and 10 parts nickel produces an alloy of an opaque yellowish hue, with a "dirty" appearance. This grade, the most economical due to its low nickel content, is used to a limited extent, although it exhibits acceptable resistance in marine environments.

The variant with 85 parts copper and 15 parts nickel offers a white color with a slight yellowish tint. It is even less common than the previous one, and its use is restricted to very specific applications.

When the proportion reaches 80 parts copper and 20 parts nickel, the result is a white alloy with a subtle bluish tone. This grade is frequently used in the manufacture of coins, such as the outer discs of two-euro coins. Its corrosion resistance is high, and it maintains its luster under normal conditions. Interestingly, this composition presents a whiter appearance than pure nickel, which makes it particularly attractive for aesthetic and functional applications.

The grade with 75 parts copper and 25 parts nickel constitutes cupronickel by definition. It is the toughest, most durable, and most corrosion-resistant, with excellent stability in seawater even after decades of continuous exposure. This grade is used in the manufacture of coins such as US "nickels"—five-cent coins—whose formula has remained practically unchanged since the late 19th century. During World War II, nickel was temporarily replaced by a mixture of manganese (Mn) and silver (Ag), both easily soluble in copper, due to the high strategic demand for the metal. In wartime contexts, nickel is considered indispensable, to the point that silver is preferred as a temporary substitute in non-critical applications. This need has led the United States Federal Government to explore more economical alternatives for coin minting, although cupronickel remains the preferred option for its performance and longevity.

Cupronickel is an alloy with a typically white, opaque, and non-shiny appearance, notable for its excellent technical performance in multiple applications. Its austenitic (γ) structure gives it a combination of toughness, ductility, and malleability that allows it to be easily forged, extruded, machined, coiled, and welded. Furthermore, it is completely recyclable and does not exhibit magnetic properties, which expands its versatility in industrial and technological environments.

The corrosion resistance of cupronickel surpasses that of any other copper-based alloy, especially in marine environments. In seawater, it behaves as virtually inert, suffering no significant damage even after prolonged exposure, both intermittent and continuous. This resistance is due to the combined action of copper and nickel: the chloride ion (Cl⁻), present in saline environments, cannot attack the alloy due to its high copper content, while alkaline salts—such as those of sodium (Na⁺) or magnesium (Mg²⁺)—are repelled by the nickel. Consequently, its behavior in freshwater is even more favorable.

A notable property of cupronickel is its ability to inhibit bacterial proliferation on its surface. Copper, recognized for its bactericidal action, prevents the development of metal-eating microorganisms, a phenomenon known as biofouling. This characteristic makes it an ideal material for applications in humid or sanitary environments where surface hygiene is critical. It is worth noting that, although pure nickel can permit the growth of certain bacteria, the dominant presence of copper in the alloy neutralizes this possibility.

The most widely used composition is 75 parts copper and 25 parts nickel, considered the standard grade. This balance offers excellent mechanical strength, high toughness, and a lustre similar to silver, which has favored its use as a monetary metal. Its ease of manufacture—through stamping, pressing, or other processes—makes it an efficient and durable alternative to more costly metals.

Regarding its chemical behavior, cupronickel shows high resistance to reducing acids, including diluted hydrofluoric acid. However, its performance in concentrated oxidizing acids, such as nitric (HNO₃) or sulfuric (H₂SO₄), is limited, so its use in these environments is not recommended.

When surface corrosion occurs, cupronickel acquires a greenish or blue-green color, resulting from the formation of compounds like sulfates or urates on the surface. These residues can be easily removed by cleaning with soap and water or a neutral detergent.

In economic terms, cupronickel presents a higher cost than common alloys like AISI 304 or AISI 316L stainless steels, but lower than specialized high-end alloys such as Hastelloy or Incoloy. The final price of the alloy depends mainly on the nickel content, a metal more expensive than copper, which in turn exceeds the value of iron (Fe), aluminum (Al), or zinc (Zn). This price relationship reflects the technical quality and durability that cupronickel offers in demanding applications.

Cupronickel is widely used in coin manufacturing, where its wear resistance, ease of stamping, and visual similarity to silver make it an efficient and durable alternative. Although cutlery has traditionally been a secondary application in the domestic sphere—now largely replaced by stainless steel—in the industrial environment, cupronickel retains a fundamental role in components exposed to severe corrosive conditions.

Its superior resistance to seawater attack makes it ideal for manufacturing mechanical parts such as screws, valves, nuts, gaskets, heat exchangers, pipes, and ball or roller bearings. These applications are especially relevant in conduction systems and marine machinery, where continuous exposure to saline environments demands materials with high chemical and mechanical stability.

In desalination plants, cupronickel is used in equipment for the production of table salt (NaCl), taking advantage of its immunity to the action of the chloride ion (Cl⁻). In the chemical industry, it is employed for processing corrosive substances that cannot be handled with conventional stainless steels, due to their vulnerability to elemental chlorine, diluted halogens, and hot alkalis. In certain processes, cupronickel also acts as a catalyst, thanks to its structural resistance and stability against reactive agents.

A standout technical advantage is its behavior at sub-zero temperatures. Unlike non-austenitic steels, which tend to become brittle in cryogenic conditions, cupronickel maintains its toughness and ductility, making it a suitable material for applications in extreme climates, industrial refrigeration systems, and polar environments. This property, along with its chemical and mechanical resistance, solidifies cupronickel as a versatile, high-performance alloy.

The corrosion resistance of cupronickel is one of the main reasons why this alloy has become a material of choice in aggressive environments, especially in marine and industrial applications. Its behavior against corrosion is exceptional, even when continuously exposed to seawater, brine, humid chloride-laden atmospheres, or chemical solutions that would rapidly deteriorate other metals. This resistance is due to the spontaneous formation of a passive film composed of copper and nickel oxides, which acts as a protective barrier against chemical attack. This layer is stable, adherent, and self-repairing, meaning that if it is mechanically damaged, it tends to regenerate in the presence of oxygen, maintaining the protection of the base metal.

Unlike other alloys, cupronickel does not exhibit problems of localized corrosion, such as pitting or stress cracking, even in turbulent flow conditions or in the presence of marine microorganisms. In fact, its smooth surface and low susceptibility to biofouling make it particularly useful in piping systems, heat exchangers, and components submerged in seawater, where the accumulation of organisms can compromise operational efficiency. Furthermore, cupronickel shows remarkable resistance to weak organic acids, hot alkalis, and corrosive gases like sulfur dioxide, expanding its field of application in the chemical and petrochemical industries.

Another important aspect is that this resistance is not compromised by temperature variations. Cupronickel maintains its chemical and mechanical stability in both warm environments and cryogenic conditions, without becoming brittle or losing its structural properties. This combination of corrosion resistance, durability, and reliable behavior across a wide range of temperatures makes cupronickel a highly valued alloy in naval engineering, desalination plants, thermal installations, and industrial processes where exposure to corrosive agents is constant and severe.

Despite their corrosion resistance, they should not come into contact with the body (costume jewelry or alternative jewelry) due to their high nickel content, which can cause allergies (nickel scabies), and because copper readily forms sulfates. It is not used in medicine. They should not be worn on the body.

Due to the high price of nickel, it is common to reduce its content in alloys in favor of adding zinc or, more rarely, manganese. Although these elements do not offer the same corrosion resistance or toughness as nickel, they are more economical and accessible. Alpacca, for example, is a more ductile and malleable alloy than typical cupronickel, and it has gained popularity in the world of costume jewelry. With it, pieces of great finish and beauty can be achieved. I myself am a witness to this: once, at a medieval fair in A Coruña, we visited some Andalusian artisans who showed me the art of filigree, a cultural heritage from the Arab period in the south of the Peninsula. Their works were of extraordinary delicacy. Alpacca is widely used in these types of creations because it is easier to work than cupronickel and also cheaper. At first glance, it looks like silver, and with it, tiaras, pendants, bracelets, and ornamental pieces are made that enjoy great acceptance.

In Spain and other European countries, cupronickel was not always the chosen material for minting coins. On many occasions, the chemical composition included large proportions of zinc instead of nickel, due to cost and scarcity. The color of these alloys, commonly known as alpacca or German silver, is very similar to cupronickel and they show remarkable resistance to seawater.

Zinc, in addition to being more economical, improves the ductility and malleability of the alloy, although it does not possess the whitening power of nickel. However, it alloys easily. A typical combination might be: 70 parts copper, 20 parts zinc, and 10 parts nickel. As can be seen, this formula is much cheaper than traditional cupronickel, composed of 75 parts copper and 25 parts nickel, but it is quite similar in appearance.

It is also possible to use manganese in alloys. This element has the advantage of whitening much more than zinc, although its corrosion resistance is lower, and the resulting alloys are more difficult to stamp. There are more complex compositions that include small quantities of aluminum, tin, and other metals, although they are not usually employed for monetary purposes.

During World War II, the United States government withdrew five-cent coins —the famous "nickels"— which contained 25% nickel, to melt them down and allocate the metal to the armaments industry. Nickel was essential for the manufacture of special steels, high-performance stainless steels, superalloys, magnets, and other strategic components. In its place, an alloy of copper, silver, and manganese was created that mimicked the color of the original cupronickel and was used until the end of the conflict in 1945. These coins are highly valued by collectors today, and although they contain little silver, their historical value gives them a high price in the numismatic market.

Once the war ended, nickel was reincorporated into the composition of U.S. coins, and has remained so until the present day (January 2016). As I have already mentioned, the U.S. government has sought more economical alternatives, and I personally believe they will soon find a viable solution, if they haven't already. It is curious that nickel has not been removed from the monetary repertoire, considering its high cost and increasing industrial demand.

Alpacca, also known as Alpaca, Argentan, Albata, German silver, Hotel silver, Imitation silver, or Nickel silver, among other names, is an alloy of copper, nickel, and zinc patented in Germany in 1824 or 1825 (the date is subject to debate). It was registered by the manufacturer Berndorf as the result of a competition held a year earlier in that country, whose objective was to create an alloy as similar as possible to pure silver. The Henninger brothers were the winners of this contest.

At the time, Alpacca caused a real sensation, as at first glance it was —and still is— practically indistinguishable from pure silver, especially to untrained eyes. Although many variants exist, the standard composition is based on 60 parts copper, 20 parts nickel, and 20 parts zinc.

The success of Berndorf's Alpacca quickly spread throughout Europe. To understand its impact, one must place it in its historical context: the Industrial Revolution was just beginning, the bourgeoisie was emerging as a new dominant class, and the world was transforming rapidly. Capitalism opened the doors of big cities to peasants who for centuries had lived tied to hard labor and without alternatives. Research and development were encouraged in all corners of the continent. In England, families who could not afford silver cutlery resorted to pewter, but the Germans went further.

Even before the colonization of America, copper-like metals, resistant to corrosion and staining, known by the Chinese as pai-tong ("white copper"), were imported from India and China. This alloy remained a mystery for three centuries until Constredt isolated nickel in 1721, at a time when chemistry and alchemy were still intertwined. It was in Germany, after the discovery and exploitation of nickel, that the copper-nickel combination gained relevance: cupronickel was born, still in use today. A century later, after multiple attempts—deliberate or accidental—the addition of zinc to the copper-nickel base increased the ductility and malleability of the alloy, allowing it to be cold-worked. It was soon appreciated by jewelers and costume jewelry artisans who could not afford the luxury of working with pure silver in an era marked by profound inequalities, where old elites still hoarded much of the wealth of the old and new worlds.

The introduction of Alpacca as an alternative to silver propelled German metallurgists to the forefront of an innovation process that continues to this day. The so-called "new silver" expanded throughout Europe, including Spain, and displaced original silver in many applications. Of course, there was also a dark side: who wouldn't have tried to take advantage of its resemblance to sell "silver" jewelry that was actually Alpacca? Such was its similarity that it was almost impossible to distinguish them with the naked eye, which explains the success of the alloy.

Although Alpacca was once a prestigious registered trademark, copper, nickel, and zinc alloys were already known long before. Their manufacture and composition varied according to national tradition and even from artisan to artisan. There is evidence of similar alloys —what we would today call "imitation silvers"— being used in regions as diverse as Persia, India, or China, although in many cases their creation was accidental, a result of technical ignorance.

Today, Alpacca is still used, though to a lesser extent, probably due to the rising price of nickel and copper. In cutlery, it has been practically displaced since the mid-20th century by stainless steel, which is more economical and less allergenic. It is important to remember that Alpacca contains high doses of nickel, which can cause allergic reactions in some people with prolonged use. Therefore, its use in jewelry is not recommended unless the alloy is of high quality and the wearer has no known nickel allergies.

Alpacca, properly speaking, refers to a standard composition that remained a secret for decades. It is assumed that this composition was close to the one previously described: Copper, 60 parts; Nickel and Zinc, 20 parts each. However, there are multiple variables. For example, an alloy of Nickel (30 parts), Copper (50 parts), and Zinc (20 parts) can also be considered "Alpacca," although it is often preferred to simply call it "German Silver"; it is essentially the same thing.

The confusion stems from "Alpacca" being (or at least having been) a registered trademark, meaning its formula was defined by the manufacturer. All combinations of these three metals look similar, but only the ideal formula truly resembles genuine Silver in color, tone, and luster. Some modifications do not include Nickel but rather Manganese (either for economic reasons or due to the aforementioned nickel allergy) and are still considered "German Silver." Other white copper alloys, such as Cupronickel (Copper + Nickel) or Bullion (Copper + Silver), are not German Silver but belong to their own families.

In a word: they have it all. They are easy to manufacture, resist corrosion (even by seawater), can be easily soldered, are tough, ductile, and malleable, 100% recyclable, non-volatile, non-toxic, and exhibit a great luster. They do not stand out for their strength; these are alloys that need to resist corrosion and maintain their "youthful" appearance for the longest possible period. They can be formed into wires (coils) and thin sheets that are then hammered into sculptures, artworks, etc.

They are germicidal: the high copper content prevents the formation and reproduction of microorganisms, making them suitable for components in ships that spend a long time underwater or in places where such capacity is required, for example, for coating hospital door handlaes.

The formation of a protective oxide layer of complex chemical formula, fostered by the high presence of Zinc and Nickel, prevents the alloy from the aggressive effects of Sulfur and its combinations, which are very reactive with Copper and its traditional alloys (Bronzes and Brasses). German Silver alloys are very versatile. The price is slightly high, likely due to the high demand for Copper and Nickel, which has increased the cost of these types of alloys.

As with most copper alloys, Alpaca exhibits excellent corrosion resistance in a wide variety of environments. The presence of nickel in its composition significantly enhances this property, granting it greater stability against external agents. The alloy behaves very stably in contact with fresh and salt water, dry and humid air, as well as organic substances like food and beverages. It also shows good resistance to sulfur and its natural compounds, especially common in urban and industrialized atmospheres. However, contact with sulfuric acid in any concentration must be avoided, as it aggressively attacks the alloy.

Alpaca also tolerates contact with human sweat, detergents, soaps, and everyday cleaning products well, making it suitable for use in cutlery and personal items. Its resistance to alkalis and their combinations is remarkable, which reinforces its utility in domestic and industrial settings.

However, the alloy is not immune to all chemical agents. Strongly oxidizing solutions can rapidly deteriorate it, and although reducing acids react more slowly, they can also cause corrosion over time. Therefore, it is recommended not to expose Alpaca to aggressive acids or extreme chemical environments if its integrity and appearance are to be preserved.

Alpacca has been used for over a century as a substitute for precious Silver in all applications where the price of silver is beyond the buyer's reach. It is used to manufacture cutlery (except for knife blades – which have always been Steel, and then Stainless Steel since its introduction), jewelry: rings, chains, earrings, pendants, bracelets, etc.; sculptures (usually small due to the price of nickel); mechanical parts not subjected to great stress; and for manufacturing imitation coins, medals, and trophies in all types of sporting events. For example, in Formula 1, Alpacca is used to make the trophy given to each podium driver, usually with a superficial plating of genuine Silver. This is because trophies are awarded at each Grand Prix and are not returned, instead becoming part of the driver's personal collection, at least theoretically.

In major sporting events where all attention is focused on a particular trophy, real Silver is usually used: the "Orejona", the highest prize in the Champions League, is made of sterling silver. However, in some cases, unofficial copies can be found either at the request of a particular client or for security reasons (e.g., for exhibition during the team's parade through the city). Nevertheless, such copies are penalized and are not usually manufactured without the consent of the corresponding organization.

I have decided to help you a bit more by providing important details because I am aware of the danger of deceptions and traps that people are often subjected to by unscrupulous and disreputable artisans. Additionally, I have included some tips for its maintenance, health safety, etc. 

The most direct and reliable way to determine if a piece is authentic silver or Alpacca is through a chemical test performed by a jeweler or metallurgist. This procedure involves lightly scraping the surface of the piece with an abrasive and applying a specific acid that reacts differently depending on the metal. While highly effective, this method can be aggressive to the object, so it is only recommended if you are certain you want to proceed with it. 

Another option is to evaluate hardness using the Mohs scale. However, this test is not easy to apply accurately to finished objects and is not considered entirely reliable for distinguishing between silver and Alpacca. The same applies to density: although silver is denser than Alpacca, the differences can be difficult to detect without appropriate instruments and precise measurements. 

A more accessible and revealing test is to observe the metal's behavior over time. Authentic silver tends to form a black patina as it oxidizes, while Alpacca develops greenish tones, either light or dark, due to the presence of copper in its composition. This difference in corrosion is one of the most useful clues for identifying the material. 

In reality, any experienced silversmith can distinguish between silver and Alpacca almost at first glance. Although they may look identical initially, it is not that difficult to differentiate them with a little practice and knowledge. 

It is also important to note that some counterfeit pieces are coated with a thin layer of authentic silver. These surface platings can deceive the eye, making the object appear to be real silver. Some coatings are so thin that they wear off easily, while others are thicker and more difficult to detect without specialized tools. Therefore, it is always advisable to consult a professional if you have any doubts about the authenticity of a piece.