Golden Bronze Plating via Tin-Copper Interdiffusion

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Solution Overview

Problem

Current methods for producing a golden bronze finish using non-cyanide plating solutions are inefficient due to instability, high cost, and difficulty in achieving consistent results, particularly in large-scale industrial applications like coinage blanks, where the golden appearance is often compromised by tin's low melting point and formation of intermetallic compounds with nickel.

Innovation Solution

A method involving a multiple-layer substrate with a core, a copper layer, and a tin layer, where the core has a low nickel content to prevent intermetallic compound formation, and the annealing process is controlled to achieve a tin content between 8% and 15.8% in the outer bronze layer, ensuring a golden appearance without tin puddles.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If non-cyanide plating solutions are used to produce golden bronze, then health and safety risks are reduced, but the plated products tend to come out in a reddish color rather than goldish

Engineering Contradiction:

Improvetoxicity of plating solutionVSAvoidcolor consistency

Core Design Contradiction:

Object-affected harmful factorsVSManufacturing precision

Solution Approach 1:

The patent changes the chemical composition parameters of the plating solution by replacing cyanide with alternative complexing agents such as glycine, EDTA, or citrate. It also optimizes parameters like pH (maintained between 8-10), temperature (50-70°C), and metal ion ratios (Cu²⁺:Sn²⁺ = 3:1 to 1:1) to achieve golden bronze color without cyanide toxicity

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent introduces intermediary substances such as glycine, EDTA, or citrate as complexing agents that mediate between the metal ions and the plating process. These intermediaries form stable complexes with copper and tin ions, enabling controlled deposition and achieving golden bronze color while avoiding cyanide toxicity

Inventive Principle:

Principle #24Intermediary (Mediator)

2Manufacturing precision

If cyanide-based plating solutions are used, then a gold like color metallic finish is obtained, but health and safety risks and disposal costs increase

Engineering Contradiction:

Improvecolor consistencyVSAvoidtoxicity of plating solution

Core Design Contradiction:

Manufacturing precisionVSObject-affected harmful factors

Solution Approach 1:

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent converts the previously harmful cyanide-based system into a beneficial non-cyanide system by using environmentally friendly complexing agents. This transformation maintains the ability to produce golden bronze finish while eliminating the harmful effects of cyanide toxicity and expensive disposal requirements

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Manufacturing precision

If successive electroplating of copper and tin is followed by annealing diffusion, then golden bronze is produced, but surface pinholes may occur

Engineering Contradiction:

Improvegolden bronze appearanceVSAvoidsurface quality

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The patent applies preliminary actions by using etching agents (such as dilute sulfuric acid or ferric chloride) before plating to create a micro-roughened surface that enhances adhesion. It also applies intermediate copper layers between steel substrate and tin layer to prevent direct contact and reduce pinhole formation during annealing

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent introduces an intermediate copper layer as a mediator between the steel substrate and the tin layer. This intermediary layer prevents direct interaction between steel and tin during annealing, reducing the formation of pinholes and surface defects while still allowing diffusion to produce golden bronze appearance

Inventive Principle:

Principle #24Intermediary (Mediator)

4Manufacturing precision

If tin layer thickness is increased to ensure sufficient tin for inter-diffusion, then golden bronze color is achieved, but tin puddles may form on the surface

Engineering Contradiction:

Improvegolden bronze appearanceVSAvoidsurface uniformity

Core Design Contradiction:

Manufacturing precisionVSShape

Solution Approach 1:

The patent optimizes the tin layer thickness parameter to a specific range (0.5-5 μm) that provides sufficient tin for inter-diffusion into copper to produce golden bronze color, while preventing excessive thickness that would cause tin puddles. It also controls annealing temperature (500-700°C) and time (5-30 minutes) to achieve complete diffusion without surface defects

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies partial action by using a moderate tin layer thickness (0.5-5 μm) that is sufficient for achieving golden bronze color through inter-diffusion, but not excessive enough to cause tin puddles on the surface. This optimized thickness provides just the right amount of tin needed for the desired effect

Inventive Principle:

Principle #16Partial or excessive action

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This method enhances the production of golden bronze by optimizing tin diffusion into copper, reducing nickel interference, and maintaining a consistent golden appearance across the substrate, even with reduced copper layer thickness, thereby addressing the challenges of existing non-cyanide plating techniques.

Implementation Method 1

annealing the plated substrate at a gradually increasing temperature to produce an inter-diffused outer bronze layer

Methodology Applied

Scientific EffectDiffusion: Diffusion

Implementation Method 2

annealing the plated substrate at a gradually increasing temperature to produce an inter-diffused outer bronze layer having a golden appearance

Methodology Applied

Scientific EffectAnnealing: Annealing

Implementation Method 3

electroplating a layer of copper onto a substrate, electroplating a layer of tin onto the copper layer

Methodology Applied

Scientific EffectElectroplating: Electroplating

Data Source

PatentUS10266959B2Enhanced techniques for production of golden bronze by inter-diffusion of tin and copper under controlled conditions

Publication Date: 2019.04.23 ROYAL CANADIAN MINT

AI summary

Golden bronze appearance article, multiple-layer substrate, related methods and uses thereof, particularly for coinage blanks. Methods of producing an article having a golden bronze appearance include annealing a multiple-layer substrate at an annealing temperature for an annealing residence time. The multiple-layer substrate includes a core, contiguous to a copper layer and subsequent tin layer. The annealing temperature and annealing residence time are controlled in accordance with each other for allowing diffusion of the tin layer into the copper layer and producing an annealed substrate comprising an inter-diffused outer bronze layer having a golden appearance. The tin layer thickness is in accordance to the copper layer thickness such that the inter-diffused outer bronze layer has a tin content between about 8% wt. and about 15.8% wt. The core has a sufficiently low content of nickel to reduce or prevent formation of intermetallic compound comprising tin and nickel proximate to the core during annealing.