CSI Division
Division 5 - Metals
Section
Metal Materials
Last Modified
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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
The following guidelines provide general information on the characteristics and common uses of bronze and identify typical problems associated with the material. See also "Checklist for Inspecting Bronze Failures".
Gayle, M., Look, D. and Waite, J. Metals in America's Historic Buildings: Uses and Preservation Treatments. Washington, DC: Department of the Interior, National Park Service, 1992.
Preservation Science. "Preventing Galvanic Corrosion. By Choosing the Right Materials". Web. 2008.
Weaver, M. Conserving Buildings: Guide to Techniques and Materials (1st Edition). New York: Wiley, 1997.
Zahner, L. W. Architectural Metal Surfaces. New York: Wiley, 2004.
Bronze is an alloy of copper which can vary widely in its composition. It is often used where a material harder than copper is required, where strength and corrosion resistance is required and for ornamental purposes. The variations in bronze (both in proportion and elemental composition) can significantly affect its weathering characteristics. "True" bronze is a combination of approximately 90% copper (Cu) and 10% tin (Sn), however there are three major classes or types of "bronzes" used in sculpture and construction. They are:
Statuary Bronze - approximately 97% copper (Cu), 2% tin (Sn) and 1% zinc (Zn); this composition is the closest to "true" bronze.
Architectural Bronze - actually more of a "leaded brass", this composition is commonly composed of approximately 57% copper (Cu), 40% zinc (Zn) and 3% lead (Pb).
Commercial Bronze - composed of approximately 90% copper (Cu) and 10% zinc (Zn).
Traditionally, a copper alloy which contains zinc is a "brass"; a copper alloy which contains tin (not exceeding 11%) is a "bronze". Bronze composition may vary significantly however, and contemporary bronzes are typically copper alloys which may contain silicon (Si), manganese (Mn), aluminum (Al), zinc (Zn) and other elements, with or without tin (Sn).
In its "raw" state, bronze is a semi-pink or salmon-colored metal; however it is rarely seen in its pure state. Bronze usually exhibits some patination or corrosion so that its color normally ranges from lime green to dark brown. Exposed bronze undergoes continuous change and progresses through several predictable "stages" of oxidation and corrosion. The stages of bronze corrosion vary in duration and time of onset, based on many factors, including:
Composition of the bronze
Patination or other protective treatments applied at the foundry
Weather
Location and exposure to rain, sun, and other climatic conditions
Atmospheric pollutants
Scheduled maintenance/cleaning
Adjacent materials including residual core materials
Statuary bronze is typically used in outdoor sculpture. Its forms are almost limitless since it may be cast in any shape for which a mold can be devised. The most common types of forms include the human figure, landscapes, battle scenes, animals, weapons, decorative elements such as stars, rosettes, etc., and plaques.
Architectural bronze is typically used for:
Door and window frames
Door and window hardware
Mail boxes and chutes
Trim or rails
Furniture hardware
As a general rule, architectural applications seek to preserve the natural, highly polished "pinkish" finish of raw bronze, in contrast to the patination of outdoor sculpture/ornament. This is achieved by the frequent polishing and oiling of bronze/brass decorative and structural elements, or the application of clear lacquers which must be renewed on a periodic basis.
Bronze has good resistance to:
Industrial, rural and marine atmospheres
Weak acids if suitably shielded with appropriate protective coatings.
Bronze has poor resistance to:
Ammonia
Ferric and ammonia compounds
Cyanides
Urban pollution
Acid rains
Bird droppings
Problems may be classified into two broad categories: 1) Natural or inherent problems based on the characteristics of the material and the conditions of the exposure, and 2) Vandalism and human- induced problems.
Although there is some overlap between the two categories, the inherent material deterioration problems generally occur gradually over long periods of time, at predictable rates and require appropriate routine or preventive maintenance to control. Conversely, many human induced problems, (especially vandalism), are random in occurrence; can produce catastrophic results; are difficult to prevent, and require emergency action to mitigate. Some human induced problems, however, are predictable and occur routinely.
Bronze, like cast iron, is a manufactured product. Copper is extracted from natural ores and alloyed with tin to create a metal which does not exist in nature. Many of the inherent problems relate to the normal physical process of the bronze "returning to nature", i.e. to the most stable states of its components.
Additionally, most outdoor bronze is erected with a foundry applied patina of some type. The actual surface patina could be one of dozens of different composites as a result of the foundry applied finishes. Each of these finishes may react differently with the environment and result in different corrosion types and rates.
Regardless of which finish exists, the bronze will begin the deterioration process described below, where the surface will be subjected to the alteration of the patina through oxidation and sulfurization. Patinated and protected surfaces will resist the effects of exposure more than bare metal; therefore, such pieces will maintain their original appearance longer and exhibit changes more slowly.
Corrosion of one form or another is the chief cause of the deterioration of metals, including statuary and architectural bronze. The degree of corrosion which occurs, and the corrosion by-products which result, are affected by several factors including bronze composition or formulation, environmental conditions and adjacent materials.
While the composition of bronze does affect the rate of corrosion, it has been generally recognized that composition is one of the least significant factors in bronze deterioration. The existence of chemicals in the atmosphere, such as chlorine, sulfur, and nitrogen oxides, in the presence of moisture, is the most significant cause of bronze deterioration.
There are numerous causes and symptoms of corrosion, including:
Uniform O xidation or Corrosion: Corrosion attacks the metal surface evenly.
Pitting: Attacks the metal surface in localized areas.
Selective Attack: When a metal is not homogenous throughout, certain areas may be attacked in preference to others.
Erosion: When a corrosion-resistant oxide layer is removed and the bare metal beneath corrodes.
Oxygen Cell Corrosion (or Atmospheric Corrosion): The most common form of corrosion; Moisture containing environmental gases (carbon dioxide, oxygen, sulfur compounds, soot, fly ash, etc.) produces chemical corrosion on the metal.
Galvanic Corrosion: The increased corrosion of a metal due to its contact with another metal, or in some cases, the same metal.
Galvanic corrosion causes extensive deterioration to the attacked metal(s), and in turn the corrosion products stain and streak the adjacent surfaces.
It is an electrolytic reaction. For this to occur, there must be an anode (negatively charged area), a cathode (positively charged area), and an electrolyte (conducting medium). The electrolyte can be rainwater, condensation, acid, alkali, or a salt. The formation of an anode and a cathode may occur due to the presence of impurities, difference in work hardening, or local differences of oxygen concentration on the surface.
Stress Corrosion Cracking: Attacks areas in a metal which were stressed during metal working.
Humidity, temperature and condensation: Affect the rate of corrosion; in a marine environment, aerosols can deposit chloride and other salts which will accelerate the rate of atmospheric corrosion.
The bronze corrosion process goes through five predictable stages. The specific results of each stage can differ due to combinations of atmospheric elements, bronze composition, patination, and other protective treatments such as waxing, oiling or lacquering.The five stages are:
Induction is when normal oxidation takes place, normally producing the dark brown copper oxide film which can be a protective barrier against future pollutants. The actual film composition is dependent upon the type and concentration of pollutants in the atmosphere, upon the duration of exposure, and upon the relative degree and duration of wetness on the surface. High concentrations of sulfides in the atmosphere can dramatically alter the result of stage 1, producing less protective, even potentially damaging films. The rate of oxidation can also have an effect on long term durability of the surface finish; oxides formed over longer time periods seem much more resistant to deterioration.
The conversion of the topmost metallic surface to copper sulfate normally begins to occur on surfaces with the most severe exposure, such as horizontal surfaces. Oxygen deprivation and deposition of particulates and moisture create a catalytic situation where electrolytic reactions occur. (This is the same principle as a battery, where the charged ions move from a positive to a negative pole.) The visual symptom of this phase is the formation of thin, light green patches on the more exposed areas.
Run-off streaking and scab formation occurs at a slower rate than the two previous stages but the consequences are significant. Copper sulfates and sulfides may have been formed during the earlier stages, yet the degree of solubility of these compounds may vary widely. It is during Stage 3 that the familiar streaking and uneven discoloration may occur due to differential weathering of the corrosion by-products. This erosion can continue until uneven blackish areas or island- like scabs are present on the surface.
Pitting may spread around the black scab formation; the pitting can also continue to spread below what appears to be a stable surface. Pitting is generally caused and accelerated by microscopic particles of chlorides deposited from the air, and if chlorides are present below a crust or a barrier coating, the corrosion can continue unchecked and invisible to casual observation.
Complete conversion of all exposed surfaces to the bright blue-green copper sulfate is the final stage of corrosion. The result is the familiar solid green bronze with the lime- green color and a matte texture. This condition is sometimes misperceived as the desirable end condition, but it is actually a phase of active corrosion.
Unprotected areas of raw bronze will oxidize, or combine with oxygen present in the air, resulting in a thin film of copper oxide along the surface of the exposed bronze. The resulting appearance is a flat, dark brown surface. The most common example to which most users can relate is the process of oxidation of a copper penny. The specular (shiny) finish of a new penny is familiar, as is the shift to the dark, red-brown finish as the surfaces oxidize over time.
This normal process of oxidation is a form of corrosion. The resultant oxide film is less reactive than raw bronze and forms a stable, protective barrier with a greatly reduced rate of oxidation.
Bronze also reacts with many atmospheric pollutants, especially sulfur compounds, which are normally found in the atmosphere as sulfur dioxide and hydrogen sulfide. Both are produced in industrial manufacturing processes. Concentrations of these gasses are generally greater in or near urban and industrial areas; therefore higher rates of corrosion can normally be expected in such areas. The initial symptom of sulfurization is the appearance of patches of light green primarily on exposed surfaces. This usually begins on horizontal surfaces which receive the greatest exposure to rains and water run-off.
A general layer of surface corrosion can eventually spread over the entire metallic surface, resulting in an overall bright green surface. The uniform green surface is often accepted by the general public, and others, as protective and the normal state of bronze. This is a misconception, and one which has probably resulted in the public acceptance of appearances which are actually symptoms of corrosion and deterioration. The sulfides and sulfates will continue to form in the presence of moisture and atmospheric sulfur compounds. The presence of green corrosion products on the bronze is always an indication of active corrosion. The pattern and result of this process will vary based upon several environmental factors such as wind, rain, pollutants, patina, and the nature of previous corrosion.
Differential weathering due to winds, rain and surface orientation can result in uneven corrosion with patterns of green streaking on a dark blackish surface.
The process of sulfurization is complicated by two factors, both of which result in aesthetically unacceptable appearances; appearances which are generally perceived as neglect and deterioration. Uneven black and green streaking of bronzes is one of the most disfiguring problems which can occur with bronze. Random dark (black) and light (green) streaks follow the contours downward, resulting in distracting visual patterns with no relationship to the form or texture of the surface of the work. The artistic details which give form and definition to the bronze become extremely obscured by streaking which results from two phenomena:
Differential solubility of the corrosion products, and
Electrochemical processes between the dark (black) and light (green) areas.
The streaking of bronze indicates a differential corrosion of the bronze which will be permanently disfiguring. Two different surface corrosion products are dissolving at significantly different rates. The geological analogy is the formation of canyons by the erosion of the land surface. Where such corrosion has already occurred, conservation techniques are likely to be required. Early indications of streaking should be given serious attention in the inspection process, and called to the attention of the Regional Historic Preservation Officer (RHPO) at the earliest possible time.
Bronze disease is the result of exposure to chlorine compounds which can come from any saline source, such as contact with saline soils, atmospheric pollutants or airborne salt spray near bodies of salt water. The chlorine reacts with the copper in bronze to form copper chloride. The primary symptom is pitting, and the process can proceed unchecked below apparently sound patinas, or protective coatings.
The copper chloride is relatively unstable and the only way to arrest the continuing corrosion is the complete removal of the chlorides using electrochemical methods. All such methods of chloride removal are advanced conservation techniques requiring the employment of a skilled professional.
Bronze is cast in a foundry process which consists of the pouring of molten bronze into a mould containing a central core. Frequently this core material is gypsum or plaster of Paris, and occasionally portions of the core are left inside the casting. It is possible for the core material to migrate through the casting wall over time and appear on the exterior surface of the bronze.
The removal and repair of core migration problems is not a maintenance procedure and will require an "existing conditions analysis" supporting a proposed conservation treatment. The RHPO should be notified of the problem following its identification. The most common symptom is the appearance of whitish spots, which gradually enlarge, in the bronze surface.
Corrosion of bronze, unlike that of natural stones, is in part an electro-chemical phenomenon. Points of negative electrical potential called cathodes and points of positive potential called anodes form on the bronze. In the presence of moisture, the corrosion process is driven by an electrical differential between the two points. This process can occur at a highly accelerated rate.
An electric potential can develop between both large and small areas. Atmospheric pollutants, especially chlorides, can be deposited on the surface of bronze. Tiny "islands" of corrosion can form, rapidly eroding/converting away the bronze metal and resulting in tiny voids or pits in the surface of the bronze. Pits may begin small and increase in size due to the continued electrochemical action and deposition within the pits. This may continue as long as moisture is present.
Pitting may be pinpoint or broad, as in patterns of deep etching created by differential erosion. (Also see: Bronze Disease)
Bird, or other animal, droppings may collect on the surface of bronze and (because of the acidic nature) may accelerate localized corrosion and deterioration. Droppings can also build up in sheltered areas, providing concentrations of damaging chemical agents of deterioration.
Galvanic corrosion, also known as dissimilar metal corrosion, occurs when two dissimilar metals are brought into contact with one another. One of the metals will corrode, and the other will remain intact. As an example, if bronze is brought into contact with iron, the iron will frequently begin to corrode. Galvanic corrosion is caused by an electric potential between two dissimilar metals in the presence of water or moisture, where the water's electrolytes allow the flow of metallic ions from the more active metal, or the anode, to the more noble metal, or the cathode. The movement of these metallic ions represents a physical loss of metal from the metal being corroded. It can continue until the source metal is completely gone.
Below, thirteen construction metals are ranked according to their susceptibility to corrosion, from most to least susceptible, or from active to noble. This type of ordered list is called a Galvanic Series chart.
The rate of the transfer of iron from the passive to the active metal is determined by the difference in electrode potential between the two metals. Therefore, the farther apart two metals are in the list below, the more likely the active metal (higher on the list) is to corrode.
Zinc
Aluminum
Galvanized streel
Cast iron, mild steel
Lead
Tin
Brass, bronze
Copper
Silver solder
Stainless steel
Silver
Graphite
Gold
Galvanic corrosion typically occurs where dissimilar metals are used as connectors or parts of a building's armature. It can be stopped by replacing the more active metal with a more noble metal such as stainless steel. When two dissimilar metals must be in contact with one another, the risk of corrosion can be substantially reduced by applying a coating to both of the materials but especially to the noble metal, or applying a sacrificial metallic coating that is more active than both of the metals.
The relative mass or sizes of the two metals in contact will also determine the rate at which galvanic corrosion occurs. As an example, in a bronze plaque with iron bolts, the bolts would corrode rapidly, but an iron plaque with bronze or copper bolts would exhibit a much lower, almost negligible, amount of galvanic corrosion as a result of its contact with the bolts. Therefore, bolts and other fasteners should be made of more noble metals where possible.
Erosion or "wearing away" of metal from the surface may be due to natural or environmental factors, or due to man-induced factors such as excessive handling or rubbing. Erosion due to human contact is by far the most serious problem, but erosion can occur due to the abrasive action of wind-driven pollutants.
Natural erosion will be a slow process and one which is, therefore, difficult to detect. It will be most obvious on outdoor bronze or in exposed locations. Industrial settings and areas where there are higher concentrations of airborne particulates, which can act as abrasives, also offer the possibility for higher rates of erosion. Natural, wind-driven abrasion will be generally so slow that it will be most apparent when comparing different exposures/orientations of bronze which has been in service for long periods. The differential loss of detail between protected and exposed surfaces will begin to be apparent over many years. Examination for this differential weathering should be part of any inspection.
Abrasion: Causes removal of the protective metal surface. Some metals such as zinc are relatively soft and therefore vulnerable to abrasion damage, especially in areas similar to roof valleys where the metal can be worn paper-thin.
Fatigue: Failure of metal that has been repeatedly stressed beyond its elastic limit, due to failure to provide necessary allowances for thermal expansion and contraction caused by temperature differences.
Creep: The permanent distortion of a soft metal which has been stretched due to its own weight. Thin areas of the metal will be among the first to fail. Can be found in lead sculptures which have inadequate or corroded internal armature.
Heat: Usually in the form of fire, will cause many metals to become plastic, distort, and fail.
Distortion: Permanent deformation or failure may occur when a metal is overloaded beyond its yield point because of increased live or dead loads, thermal stresses, or structural modifications altering a stress regime.
Chemical and mechanical processes can cause the breakdown or reduced effectiveness of structural metal fixings such as bolts, rivets, and pins. Stress failure is often a contributor to breakdown situations. Iron connections which are water traps are particularly susceptible.
Most bronze corrosion can be characterized as "general" or "uniform" and "pitting", with occasional signs of selective attack. Galvanic corrosion appears mostly in connection with pins, bolts, and replacement parts in different metal. Erosion is apparent most often in bronzes in fountains. Stress corrosion is less apparent in bronze than in brass, but could be a factor in some cases in bronze sculptures.
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Q. I am facing problem in process of plating on brass. I want to finish yellow bronze over it. Can any body suggest me the actual process. Please help.
I am a chemical Engineer running my own industry at Delhi, India. This time I am doing job work for watch and Sanitary industry giving gold, silk and matte finish. I have develop various color. For one of my project I need bronze/yellow finish over brass. I have done similar color but it is not accurate that much I want. Can you help me in this matter.
MANISH TYAGI
- DELHI, India
Ed. note: Robert Probert explains some coloration issues and approaches below.
A. I am having some difficulty in understanding the question. Is it the antique finish you are looking for? For colors over brass you can also try electrophoretic lacquer with dye.
Gurvin Singh
Mohali, Punjab, India
Q. Would anyone have information on bronze electroplating systems?
Where they can be bought, how do they work, etc.
Thanks, Joe.
Joe Prevatt
metal fabrication - Tallahassee, Florida
"Electroplating Engineering Handbook"
by Larry Durney
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A. Hi Joe. Bronze is one of a few dozen metals and alloys that can be electrodeposited. It can be plated for both functional and decorative reasons. There would be a number of preliminary cleaning and acid activation stages on the substrate, then the plating step proper, then possibly some tarnish preventers, or lacquers, etc.
You can buy an amateur level plating system for a few hundred dollars, or a professional level small brush plating system for a few thousand dollars. But you can also spend a million dollars or even more for a fair size automated bronze plating installation. Please tell us about your needs, background, and company, and we'll be happy yo continue the discussion.
Good luck & Regards,
Ted Mooney
, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. "Bronze" means different things to different people.
To the metallurgist bronze is an alloy of copper and tin.
To the architect or plating job shop, bronze is a "color". To get the particular shade of color you need, you may experiment by mixing copper plating solutions with zinc plating solutions sometimes having to add a bit of an alkaline tin. You might also get the color bronze you need by merely increasing the copper content in a standard brass plating solution.
In the 1950's some architect in Atlanta specified a particularly difficult shade for the wall light switch covers in the restoration of the state capitol. As a co-op student from Georgia Tech working at a dirty job shop, I was able to get his color by blending my standard brass solution with our standard alkaline stannate tin solution.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
⇦ (tip: readers rarely show interest in abstract questions, but people's actual situations usually prompt responses)
Q. I would appreciate being led to bronze electroplating procedure.
Majid Ghahari
- Tehran, Iran
A. Hi, cousin Majid. We have a number of threads on line here that specifically address bronze electroplating, including this one and:
topic 1114, "Bronze Plating Solution Make-Up, Parameters, Analysis"
Are you looking for a heavy functional bronze for engineering purposes or a decorative layer on top of nickel plating ... a handful of parts or a major installation? There are whole libraries devoted to electroplating, so it's difficult to distill it down to a few paragraphs until you tell us your situation. Are you working for an established plating shop and experienced in many different plating procedures like brass, but not bronze . . . or have you not electroplated at all yet and need an intro to electroplating in general first? Thanks.
Best of luck and Regards,
Ted Mooney
, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. We are trying to plate out bronze (8-15% Sn) 0.022 thick on small disk shaped parts for wear. We use a potassium/cyanide electrolyte solution and copper anodes. We have a problem with layering of tin-rich zones that typically begins after the parts have been plating for some hours, that become so pronounced as to ruin the parts. However, this problem comes and goes, and often the parts are just fine. We use constant current density. Does anyone have an idea as to what causes this layering?
William Dirkin
Manufacturer - Kalamazoo, Michigan
"Modern Electroplating"
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A. Hi William.
It's important to remember that being able to electrodeposit an alloy at all is quite a feat :-)
Copper, being more noble than tin, would be expected to plate out exclusively in a bath where ions of both copper and tin are in solution. The way it becomes possible is by restricting free ions to near zero by complexing/sequestering the metals with the right amount of caustic, the right amount of cyanide, the right amount of tin salts, etc.
Lots of things can affect the copper-tin ratio, but Lowenheim notes in Modern Electroplating ⇨
that higher temperatures increase the ratio of tin deposited. Assuming you don't have cooling coils in the tank, is it possible that the temperature is not being monitored but is rising over the course of those "some hours"?
Luck & Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Looking for advice on plating techniques for outdoor metal finishes. I'm an architect and furniture designer, and I'm working on a bench design that uses metal structurally. I want it to be dark in color, like bronze or gun-blued steel. But I also want it to be thin, so structurally I need to use steel. (We looked into bronze, but it would need to be 3/4" or 1" thick plate - heavy and expensive!) Is it possible to have mild steel plated in bronze? Will that be a durable finish when exposed to weather? (Obviously we don't want the steel to rust.) Or any other ideas for dark patinated finishes (not paints or powdercoats) for exterior metalwork? Thanks!
Peter Larsen
architect - San Francisco, California, USA
A. Copper plating with oxidised or patina finish may be the only viable option for architectural use.
Khozem Vahaanwala
Saify Ind
Bengaluru, Karnataka, India
Q. I want to give gold effect on artificial jewellery.Fine gold is very costly. So, I want to use Yellow Bronze Plating. If someone can help me giving workable formulation. I don't want to use any proprietary chemicals
Thanks,
Zafar Mahmud
Electroplating shop - Lahore, Pakistan
A. Zafar,
yellow bronze does not give gold effects. Try gold dyed lacquering.
Regards
T.K. Mohan
plating process supplier - Mumbai, India
A. Hi Zafar. In topic 37526 Geoff Smith describes Pinchbeck Alloy, Dutch Metal, and Nordic Gold -- brasses designed to look a lot like gold. That should be at least a start.
Luck & Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Hi My Name is KISHORE. Actually I got a problem with my plating unit -- it is bronze plating. When I am starting plating arranging desired current, but if it would be down 0 current and also plating surface getting straight lines after plating, so kindly solve my problem. Mostly I like to know the testing procedure to get the Bronze salt-a, salt-b, Potassium cyanide and Potassium hydroxide in the chemical.
thank you..
Kishore Singu
shop floor manager - Hyderabad.Ap&India
A. Hi Kishore. We probably have either language problems, or keyboard conversion issues because I can't understand your question at all -- apologies. But it is usually much better to buy a proprietary bonze plating process from a plating process supplier so you can get local, hands-on help.
Readers will probably be happy to help, but we would need to quite a bit of data like concentrations, amperages, temperature, etc., from you to try to guess what is wrong. And if you could e-mail a pic of "getting straight lines after plating", that would probably help a lot!
Luck & Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. My company manufactures and repairs aircraft hydraulic parts. Over the last several months, we have been noticing that our bronze plated parts are being attacked by some sort of chemical reaction that can best be described as black "smudging" and pitting on the bronze plated face. The problem presents itself after the parts have been installed and run on a test stand. The operational environment of the parts is characterized by high rotational speeds and high oil temperatures (225 °F-240 °F). We have manufactured these parts for 10 years or so and have just now experienced this issue. We have tried three different plating vendors and have had the same results. This leads me to believe that there may be an issue with the base material (O6 tool Steel). Could the graphite content of the tool steel be drawn to the surface of the part during the etching phase of pretreatment and become included in the bronze plating itself? If so, would this cause an adverse reaction similar to what we have seen? Thanks.
Warren Musselman
- Agoura Hills, California
A. Hi Warren.
I don't have an answer but have a suggestion for a trial. Some alkaline solutions, like zinc, have difficulty plating on high carbon steel. I wouldn't expect it with tin or copper, but alloy plating can be tricky. Since three shops have failed to plate the material correctly I'd suggest having one of them try a nickel strike before the bronze plating.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. I'd like to know differences between bronze plating and brass plating.
Currently I'm using brass plating to do antique brass finish, but mostly I'm not able to match the antique finish as per the sample I receive from clients. How can I adjust the colour in brass plating, like more yellow or antique brass. Many times I fail to match antique finish colour. Please advise.
Shahid Ahmad
- Dubai, U.A.E.
A. Hi, Shahid.
We appended your inquiry to a thread where Robert Probert suggested that if you're good at it and patient it may even be possible to simply mix an alkaline brass and an alkaline tin bath for some different colors. The previously mentioned topic 1114 notes that you can buy the solutions from Macdermid or other vendors, and also includes some formulation information.
Despite these options though, you might consider trying to get the desired color via electrophoretic lacquering, or blackening & clearcoating on your existing plating if color is your chief concern.
Regards and good luck,
Ted Mooney
, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
⇦ (tip: readers rarely show interest in abstract questions, but people's actual situations usually prompt responses)
Q. Can a yellow bronze plated object be successfully plated with gold plating?
Mr. Bhupendra Upadhyay
- Mumbai, India
A. Hi Bhupendra.
I don't see why this should present a problem, but an intermediate layer of nickel is probably desirable if you are looking for a bright gold plate; otherwise, an intermediate layer of copper and probably a gold strike might be necessary. But answers to questions with little context are often misleading because there are so many possible "ifs, ands & buts" that can't all be covered in every forum answer. So please tell us as much about your actual situation as you can for the best answers. Thanks.
Regards,
Ted Mooney
, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q.
Hi folks,
I have been sent some 'gold' coloured buttons (from some sort of garment I believe), there is a customer complaint regarding tarnishing and having looked at them they do have brown patchy tarnishing in certain areas, maybe approx. 10% of surface area in seemingly random patches, looks like brass tarnishing.
I have analysed them microscopically and using XRF and they are a 60/40 brass with approx. 2 microns copper underplate and what appears to be a yellow tin bronze (quite gold coloured) which is approx. 1 micron. there is no lacquer coating.
I was wondering what the general tarnish resistance of yellow tin bronze is? I think it is pretty good but not as high as the white bronze?
I assume copper will migrate into the thin bronze plating (and tin from the bronze will migrate into the copper)? This will lower the tarnish resistance of the bronze plating I assume? Any idea how long does this take at room temperature, are we talking just months or years? And could copper migration be a cause of the tarnishing, would it very significantly lower the tarnish resistance?
I think a typical yellow bronze is something like 80% copper, 10 -15% tin and 2.5 - 5% zinc, that correct?
I was wondering what to tell the customer as an explanation for the tarnishing, and how big an effect this copper migration may have. The bronze plate is only about 1 micron but that should be thick enough to be reasonably coherent and not porous I assume. I don't suppose there is a significant galvanic effect between bronze and copper.
The actual environment/chemicals, etc. the buttons have been exposed to and how long the buttons have been in use is relevant, but unfortunately that info wasn't provided.
Well, any help/comments would be greatly appreciated.
Steve
Steve Jones
Technologist - Sheffield, UK
Right, well that's all a lot clearer now. Thanks for the help.
Steve Jones
[returning]
Technologist - Sheffield, UK
Hi Steve. Sorry that 4 days passed rather than you getting a quick answer; if your need is urgent, there are metal finishing consultants eager to be retained.
My guess is that it may be cyanide bleed-out, but I do not have sufficient bronze plating experience to answer your questions, and verbal descriptions of defects are not always easily understood. People help each other out here when they can in this public forum, but a reader highly experienced in that sort of bronze plating and with available time to help with your problem might only drop by every few weeks. Please consider this a permanent resource which allows you to search tens of thousands of previous Q&A's about metal finishing, as well as a place to enjoy the camaraderie of your peers -- unfortunately it's not a place that can always deliver personal help on every possible metal finishing question, let alone immediately. Apologies.
Good luck, and please try to be the change you'd like to see ... we always have hundreds of open questions, some of which would surely benefit from your input :-)
Regards,
Ted Mooney
, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. I am a chem engineer running a bronze plating bath. We have had a dramatic ramp up on scrap peels, but only on specific substrates (we run multiple steel alloys through this bath).
One of the things we're seeing is an immersion tin layer that is depositing before the bronze and inhibiting bonding. What may be causing this condition and how do I stop this? I'm not even sure where to begin looking.
About the same time as our scrap spike, we had a couple of changes in our process.
1) We added a water softener/RO system to our already-existing DI water filter system. The water we are using is still DI, just now - instead of direct from well, the water is "pre-treated through the softener/RO before the DI filters.
2) We moved our rectifiers on our electroclean tanks (after being stationary for 40+ years). All the leads & connections were replaced new.
Is there anything specific that I should be looking for?
Any help is appreciated!!!
Thanks,
Dawn
Dawn Baeckeroot
- Bellaire, Michigan
A. Hi Dawn,
The one thing that immediately comes to mind when you mention problems only with specific steel substrates is that leaded steel will surely give you a problem if it's not specially activated with a fluoride-bearing acid. Might any of the steel be leaded?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Hi. I use the "makeup solution" in the plating workshop to make the bronze bath. Does anyone know the nature of this solution? Thank you.
Fateh heydari
- Iran
Practical Electroplating Handbook
by N.V. Parthasaradhy
on
AbeBooks
or
Amazon
(affil links)
A. Hi Fateh. Bronze is an alloy of copper and tin, so those two metals are probably in the makeup solution, and most bronze plating baths are cyanide-based. Parthasaradhy ⇨
suggests copper cyanide, potassium stannate, potassium cyanide, potassium hydroxide and Rochelle salt. Copper is complexed by cyanide, and tin by hydroxyl, making it possible to control the ratio of copper and tin that are plated.
Lowenheim's books, both "Electroplating" [adv: this book on eBay , Amazon, AbeBooks] and "Modern Electroplating" [adv: this book on AbeBooks, eBay, Amazon] offer good coverage of bronze plating including formulas. Do you know whether your anodes are bronze, copper, or what? Please tell us about your situation and why you want to know the "nature" of the replenishment solution -- your question might be simple and generic, but it might involve trying to reverse-engineer proprietary information and we can't participate in crowd-sourcing stuff like that. Thanks!
Regards,
Ted Mooney
, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Plating for watch components: I have several watch cases that I want to use to build better watches. I'm starting with a large case that is "bronzed" but I want a good bronze plating on it. The dial is apparently unique (size and shape) and I have to have a new one custom made and want it bronzed also; but then lacquered to prevent oxidization while I want the case to age.
(After this will come a large chronometer and a stupid green case that I love the texture of and I think it would look great in rose gold. These are 1 at a time projects, but there are several and I know there will be more as I break down what I have into good and garbage).
Patrick Hudson
- Lehigh Acres, Florida
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
Q. I have a Cincinnati Victor Luminaire funeral fan from the 1920's that has a bronze or orangey copper color plating. I'm looking to re-plate and restore the finish because the cast iron has lost a lot of finish.
Does anyone in the USA do Bronze plating or can match a color?
Can someone share some advice how I can use my rectifier and plate it myself to get a close color. Also I was thinking copper plating and chemically changing the color but not sure what chemical can change copper to a bronze color.
To see color, look up Luminaire funeral fan under images on the web.
James Thomas
- Folsom, California
Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)
A. Hi James. Earlier on this page Robert Probert describes how to adjust the shade of the plating but, unfortunately, cyanide bronze plating is not for home or amateur use because cyanide is such a powerful poison that it can't be used without training and proper facilities. There are many plating shops which offer bronze plating, but identifying those which are set up to handle consumer items like this rather than high production runs may take a lot of googling and phoning.
Luck & Regards,
Ted Mooney
, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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