Electronics Guide

The Corrosion Cell

Electrochemical corrosion requires four essential elements: an anode where metal oxidation occurs, a cathode where reduction reactions take place, an electrolyte providing ionic conductivity between sites, and an electrical path connecting anode and cathode. Removing any element stops corrosion, forming the basis for protection strategies.

At the anode, metal atoms release electrons and become positive ions that dissolve into the electrolyte. Common anodic reactions include iron oxidizing to ferrous ions, copper becoming cupric ions, and aluminum forming aluminum ions. These reactions consume metal, creating the material loss characteristic of corrosion. The electrons released flow through the metallic path to the cathode, where they participate in reduction reactions.

Cathodic reactions consume electrons without dissolving the cathode material. In neutral or alkaline environments, oxygen reduction predominates, with dissolved oxygen combining with water and electrons to form hydroxide ions. In acidic environments, hydrogen evolution occurs as hydrogen ions accept electrons to form hydrogen gas. These cathodic reactions are essential drivers of the corrosion process.

Electrode Potential and the Galvanic Series

Different metals exhibit different tendencies toward oxidation, quantified by their electrode potential. The galvanic series ranks metals by their potential in a specific environment, typically seawater. Metals with more negative potentials (active metals) serve as anodes when coupled with more positive (noble) metals, corroding preferentially to protect the noble metal.

In electronics, commonly encountered metals span a significant range of the galvanic series:

• Most active (anodic): Magnesium, zinc, aluminum, steel
• Intermediate: Tin, lead, nickel, brass, copper
• Most noble (cathodic): Silver, palladium, platinum, gold

The potential difference between coupled metals influences corrosion rate, with larger differences generally accelerating galvanic attack. However, actual corrosion behavior depends on additional factors including polarization characteristics, area ratios, and environmental specifics.

Kinetics and Rate-Controlling Factors

While thermodynamics determines whether corrosion is possible, kinetics determines the rate. Activation polarization reflects the energy barrier for electrochemical reactions at electrode surfaces. Concentration polarization results from depletion of reactants or accumulation of products near electrode surfaces. Resistance polarization accounts for ohmic losses in the electrolyte.

In practical electronics applications, corrosion rate is often controlled by oxygen diffusion to cathodic sites or by resistivity of thin electrolyte films. Understanding rate-limiting mechanisms guides selection of appropriate protection strategies.

Galvanic Corrosion

Galvanic corrosion occurs when dissimilar metals contact electrically in the presence of an electrolyte. The more active metal corrodes preferentially, protecting the more noble metal. This mechanism is ubiquitous in electronics where diverse metals must connect functionally.

Critical galvanic couples in electronics include aluminum wire bonded to gold or copper pads, where aluminum oxidizes rapidly; tin-plated leads soldered to copper traces, where the potential difference drives tin dissolution; and steel hardware contacting copper or brass, accelerating iron corrosion. The severity depends on potential difference, relative areas (small anodes corrode rapidly), and environmental moisture.

Prevention strategies address the requirements of galvanic cells: selecting metals closer in the galvanic series, minimizing area ratios to favor large anodes, applying barrier coatings to separate metals from electrolyte, and controlling environment to exclude moisture.

Electrochemical Migration

Electrochemical migration (ECM) poses particular danger to closely spaced conductors under electrical bias. When an electrolyte bridges adjacent conductors at different potentials, metal ions dissolve from the anode conductor, migrate through the electrolyte toward the cathode, and deposit as metallic dendrites that grow until bridging the gap and creating short circuits.

Silver is particularly susceptible to ECM due to high ion mobility and low electrodeposition overpotential. Tin, lead, and copper also migrate readily under appropriate conditions. Migration rates increase with voltage, temperature, contamination, and humidity. Modern fine-pitch electronics with conductor spacings below 100 micrometers face elevated ECM risk.

Prevention requires controlling all factors: eliminating ionic contamination through thorough cleaning, applying conformal coatings as moisture barriers, using migration-resistant surface finishes, and designing adequate spacing for the operating environment. Testing methods including surface insulation resistance (SIR) measurement characterize ECM susceptibility.

Crevice Corrosion

Crevice corrosion concentrates attack within narrow gaps where electrolyte becomes stagnant and depleted in oxygen. Differential aeration creates a concentration cell with the oxygen-depleted crevice interior becoming anodic relative to exterior surfaces with normal oxygen access. The crevice acidifies as metal dissolution produces hydrolysis reactions, further accelerating attack.

Electronics present numerous crevice sites: beneath surface-mount components, at connector contacts, under fastener heads, and within hermetic package seal regions. Once established, crevice corrosion can progress rapidly to functional failure.

Design practices minimize crevice formation through continuous sealing, avoiding tight fits where stagnant electrolyte accumulates, providing drainage from potential crevice sites, and applying corrosion inhibitors to susceptible regions.

Pitting Corrosion

Pitting produces highly localized attack that penetrates deeply into metal while surrounding surfaces remain relatively unaffected. Pits initiate at surface defects, inclusions, or damage to protective oxide films where local chemistry differs from bulk conditions. Once initiated, pits become autocatalytic as accumulating corrosion products acidify the pit interior and exclude oxygen.

Pitting is particularly dangerous because small surface indications can mask extensive subsurface damage. A conductor that appears sound may have sufficient cross-section reduction from hidden pitting to fail under current loading. Stainless steels, aluminum, and copper alloys are susceptible to pitting in chloride-containing environments.

Prevention relies on selecting pitting-resistant alloys, excluding chlorides and other aggressive species, maintaining passive films through proper surface treatment, and applying protective coatings to vulnerable surfaces.

Atmospheric Corrosion

Atmospheric corrosion occurs through thin electrolyte films formed by humidity adsorption, condensation, or contamination deposition. Even in indoor environments, airborne pollutants including sulfur dioxide, hydrogen sulfide, nitrogen oxides, and chlorides concentrate in moisture films to create aggressive conditions.

The time of wetness, the duration that surfaces support liquid water films, strongly influences atmospheric corrosion rates. Temperature cycling that produces condensation, high ambient humidity, and hygroscopic contamination all extend wetness time. Critical relative humidity thresholds exist above which corrosion accelerates markedly, typically 40-60 percent depending on contamination levels.

Control measures include maintaining low relative humidity, filtering air to remove pollutants, cleaning to remove deposited contamination, and applying protective finishes or coatings. Hermetic sealing eliminates atmospheric corrosion by excluding the external atmosphere entirely.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) results from the combined action of tensile stress and corrosive environment, causing brittle fracture of normally ductile materials. Cracks propagate through the material at stress levels well below yield strength, often with minimal apparent corrosion product formation.

Electronics materials susceptible to SCC include brass (particularly in ammonia environments), stainless steels (in chloride environments), and aluminum alloys. Residual stresses from manufacturing processes including forming, welding, and soldering provide the mechanical driving force. The failure mode is sudden and catastrophic, with little warning from visual inspection.

Prevention addresses both stress and environment: stress relief through annealing, using SCC-resistant alloys, excluding aggressive species, and applying compressive surface treatments that counter tensile stresses.

Humidity and Moisture

Moisture provides the electrolyte essential for electrochemical corrosion. Relative humidity above critical thresholds allows sufficient moisture adsorption to form conductive surface films. This threshold varies with surface contamination level, with heavily contaminated surfaces supporting corrosion at lower humidity.

Condensation presents more severe conditions than humidity alone, providing substantial liquid water volumes. Temperature cycling through the dew point, operation in humid environments without adequate ventilation, and proximity to moisture sources all promote condensation. Design for drainage, ventilation, and temperature stability reduces condensation risks.

Temperature Effects

Temperature influences corrosion through multiple mechanisms. Reaction kinetics accelerate with temperature following Arrhenius-type relationships, roughly doubling for each 10 degrees Celsius increase. However, higher temperatures also reduce oxygen solubility and can promote protective oxide formation, sometimes reducing net corrosion rate.

Temperature cycling produces particularly damaging conditions through repeated condensation, differential thermal expansion stressing protective layers, and fatigue of corrosion protection systems. Electronics experiencing wide temperature excursions require robust protection against cyclic environmental stresses.

Atmospheric Contaminants

Airborne pollutants dramatically accelerate corrosion by providing aggressive species and increasing electrolyte conductivity:

• Sulfur compounds: Hydrogen sulfide and sulfur dioxide attack copper and silver particularly aggressively, forming tarnish films that increase contact resistance
• Chlorides: Sea salt and industrial chlorides promote pitting, crevice corrosion, and stress corrosion cracking in susceptible materials
• Nitrogen oxides: NOx compounds form nitric acid in moisture, attacking multiple metals
• Ammonia: Causes stress corrosion cracking in brass and accelerates copper corrosion
• Particulate matter: Hygroscopic particles retain moisture and concentrate corrosive species on surfaces

Industrial environments, coastal locations, and areas with high traffic or combustion sources present elevated atmospheric corrosion challenges requiring enhanced protection.

Operational Electrical Effects

Applied electrical potentials influence corrosion behavior beyond simple galvanic effects. Impressed currents can drive anodic dissolution or provide cathodic protection depending on polarity. Alternating currents induce localized heating and may break down protective films through capacitive charging effects.

Leakage currents through contamination paths accelerate electrochemical migration. Higher voltages increase migration rates and the driving force for dendrite formation. Design practices that minimize leakage paths and limit voltages across susceptible gaps reduce electrically enhanced corrosion.

Polymer Degradation

Polymeric materials in electronics including encapsulants, conformal coatings, cable insulation, and circuit board laminates degrade through thermal oxidation, photochemical attack, hydrolysis, and chemical reaction with environmental species.

Thermal degradation causes chain scission and crosslinking, reducing flexibility and strength while potentially releasing volatile species that contaminate sensitive components. UV exposure initiates photochemical reactions causing discoloration, embrittlement, and surface chalking. Moisture absorption leads to hydrolysis in susceptible polymers and plasticizer leaching that reduces flexibility.

Polymer selection for long-term reliability considers operating temperature relative to polymer capability, UV exposure and need for stabilizers, moisture exposure and hydrolysis susceptibility, and chemical compatibility with surrounding materials and environments.

Intermetallic Formation

Intermetallic compounds form at interfaces between different metals through solid-state diffusion, growing over time and temperature exposure. While some intermetallic formation is necessary for bonding (as in soldering), excessive growth creates brittle layers prone to fracture.

Critical intermetallic systems include copper-tin intermetallics at solder joints, gold-aluminum intermetallics at wire bonds (Kirkendall voiding and purple plague), and nickel-tin intermetallics in lead-free soldering. Growth rates follow Arrhenius temperature dependence, with elevated temperatures dramatically accelerating formation.

Design considerations include selecting compatible metal systems, minimizing thermal exposure during assembly and operation, using barrier layers to slow diffusion, and accounting for intermetallic growth in reliability predictions.

Tin Whisker Growth

Tin whiskers are hair-like crystalline growths that spontaneously emerge from tin and tin alloy surfaces, potentially bridging conductors and causing short circuits. Whiskers grow through compressive stress relief, with stress sources including plating residual stress, substrate CTE mismatch, and intermetallic growth at the tin-substrate interface.

The transition to lead-free electronics increased tin whisker concerns, as lead additions to tin suppress whisker formation. Mitigation strategies include using minimum 3 percent lead content (where regulations permit), applying conformal coatings to contain whiskers, using matte tin finishes with reduced stress, annealing after plating to relieve residual stress, and specifying nickel underlayers as diffusion barriers.

Creep and Stress Relaxation

Metals under sustained loading experience creep (progressive deformation) and stress relaxation (decreasing stress at constant strain). These mechanisms affect connectors maintaining contact pressure, solder joints under thermal cycling stress, and springs providing mechanical bias.

Soft metals including tin-based solders and pure copper creep readily at room temperature. Stress relaxation in copper alloy springs can reduce contact force below functional thresholds. Material selection considering creep resistance, operating temperature relative to homologous temperature, and stress levels ensures long-term mechanical stability.

Material Selection

Selecting inherently corrosion-resistant materials provides fundamental protection. Noble metals (gold, platinum group) resist most corrosive environments but at significant cost. Stainless steels, nickel alloys, and titanium offer excellent resistance for structural applications. Copper alloys provide good atmospheric corrosion resistance for electrical conductors.

Material selection considers not only individual component corrosion resistance but also galvanic compatibility. Minimizing potential differences between contacting metals and avoiding unfavorable area ratios (small anode, large cathode) reduces galvanic corrosion risk.

Surface Finishes and Coatings

Surface treatments modify the metal-environment interface to reduce corrosion:

• Metallic platings: Gold, tin, nickel, and other platings provide barrier protection and modify surface electrochemistry. Plating selection considers thickness adequacy, porosity, and galvanic compatibility with substrate
• Conversion coatings: Chromate, phosphate, and similar treatments form thin protective films that improve corrosion resistance and coating adhesion
• Anodizing: Electrochemical oxidation of aluminum produces thick, hard oxide films with excellent corrosion and wear resistance
• Passivation: Chemical treatments that enhance natural oxide films on stainless steels and other passive metals

Organic Coatings and Encapsulation

Conformal coatings, potting compounds, and encapsulants separate electronic assemblies from corrosive environments:

• Acrylic coatings: Easy application and rework, good moisture resistance, moderate chemical protection
• Silicone coatings: Excellent temperature range and flexibility, good moisture resistance
• Urethane coatings: Superior moisture and chemical resistance, difficult to remove for repair
• Parylene coatings: Vapor-deposited films with excellent uniformity and dielectric properties
• Epoxy encapsulants: Complete encapsulation providing maximum protection with trade-offs in weight, thermal management, and repairability

Coating effectiveness depends on proper surface preparation, adequate thickness and uniformity, absence of defects providing corrosive pathways, and compatibility with operating conditions.

Environmental Control

Controlling the operating environment removes corrosive species or conditions:

• Humidity control: Maintaining relative humidity below critical thresholds prevents electrolyte film formation
• Air filtration: Removing particulates and gaseous pollutants from air supplies reduces aggressive species deposition
• Inert atmospheres: Nitrogen or argon filling excludes oxygen required for cathodic reactions
• Hermetic sealing: Complete isolation from external environment eliminates atmospheric corrosion
• Desiccants: Moisture-absorbing materials maintain low humidity within sealed enclosures

Design for Corrosion Resistance

Thoughtful design eliminates or minimizes corrosion-prone features:

• Avoid water traps and provide drainage from moisture collection points
• Eliminate crevices where stagnant electrolyte accumulates
• Specify adequate conductor cross-sections with corrosion allowance
• Separate dissimilar metals with insulating barriers
• Position vulnerable components away from moisture sources
• Design for cleanability to enable contamination removal
• Provide access for inspection and maintenance of corrosion-critical areas

Accelerated Corrosion Testing

Standard accelerated tests provide comparative data and qualification evidence:

• Salt spray (fog) testing: Continuous exposure to salt mist per ASTM B117 provides severe marine-type corrosive stress. Useful for comparing coating systems but correlation to field performance requires careful interpretation
• Cyclic corrosion testing: Alternating wet-dry and temperature cycles better represent actual exposure conditions than continuous salt spray
• Mixed flowing gas testing: Controlled exposure to pollutant gas mixtures simulates industrial or urban atmospheres with greater environmental relevance
• Humidity testing: High humidity exposure at elevated temperature accelerates moisture-related degradation. Variants include steady-state exposure and cycling through condensation conditions

Electrochemical Testing

Electrochemical methods quantify corrosion behavior and protection effectiveness:

• Potentiodynamic polarization: Scanning electrode potential while measuring current reveals corrosion rate, passivity behavior, and breakdown potentials
• Electrochemical impedance spectroscopy (EIS): Frequency-dependent impedance measurements characterize coating integrity, corrosion mechanisms, and degradation progression
• Surface insulation resistance (SIR): Measuring resistance between conductors under bias detects electrochemical migration and contamination effects. Critical for qualifying printed circuit assemblies

Failure Analysis

Analyzing corrosion failures provides essential feedback for design improvement:

• Visual examination: Documenting corrosion morphology, location, and extent
• Microscopy: Optical and electron microscopy reveal attack patterns and identify corrosion products
• Chemical analysis: Energy dispersive spectroscopy (EDS) and other techniques identify corrosion products and contaminating species
• Cross-sectioning: Metallographic preparation reveals subsurface damage and corrosion penetration depth
• Environmental sampling: Analyzing operating environment identifies corrosive species and exposure conditions