By the end of this topic, you should be able to...
explain corrosion resistance, reactivity (food safe), hygroscopy and flammability.
Guiding Question
How do material properties and classifications aid material selection for a specified manufacturing process?
💡 Did You Know? The Iron Pillar of Delhi has stood in the open air for over 1,600 years — and it has not rusted. Contemporary iron structures exposed to the same environment would be severely corroded within decades. The pillar's corrosion resistance was not accidental — its unusually high phosphorus content promotes the formation of a stable, adherent iron hydrogen phosphate surface layer that suppresses further oxidation. Without any understanding of chemical properties, this phenomenon is inexplicable and unreproducible. Chemical properties — corrosion resistance, oxidation behaviour, chemical compatibility, and degradation mechanisms — determine whether a material survives its service environment over the intended product lifespan.
What Are Chemical Properties?
Chemical properties describe how a material responds to its chemical environment — interactions with water, oxygen, acids, alkalis, biological organisms, light (photochemical), and other chemical species present in the service context.
Unlike physical properties (which describe responses to physical stimuli without material change) and mechanical properties (which describe responses to applied forces), chemical properties describe interactions that alter the material itself at the molecular or atomic level — changing its composition, structure, or mass.
Chemical properties are critically important in material selection because they determine:
Service life — How long does the material retain its properties before degrading?
Safety — Does the material release toxic compounds into food, water, or the environment?
Maintenance requirements — Does degradation require protective coatings, regular replacement, or controlled storage conditions?
End-of-life behaviour — Does the material degrade safely or persist environmentally?
Fire safety — Does the material contribute to fire spread or toxic combustion products?
As established in A3.1.3 (Material Properties Overview) and A3.1.4 (Physical Properties), a complete material profile requires evaluation across all three property groups.
Corrosion Resistance
Corrosion is the degradation of a material through chemical or electrochemical reaction with its environment, resulting in progressive deterioration of the material's structure, properties, and dimensional integrity.
Corrosion resistance is therefore the ability of a material to withstand this degradation — to retain its chemical composition, surface integrity, and functional properties when exposed to a corrosive environment over time.
Corrosion is not limited to metals — polymers degrade through chemical attack (chemical resistance), ceramics dissolve under acid attack, and timber degrades through biological and chemical processes. However, electrochemical corrosion of metals — particularly ferrous metals (iron and steel) — is the most economically significant corrosion mechanism in engineering design.
The global economic cost of corrosion is estimated at approximately 3.4% of global GDP — approximately USD 2.5 trillion annually (NACE International, 2016 study). Approximately 25–30% of this cost is considered preventable through better design decisions, including appropriate material selection.
Design rule:Â
When two materials are in electrical contact with an electrolyte present, the material with the more negative electrode potential (higher in the table) will corrode. The greater the potential difference, the more severe the galvanic corrosion of the anodic material.
Corrosion Protection Strategies — Systematic Summary
Strategy | Mechanism | Examples | Design Considerations |
Passivation | Allow natural passive oxide layer to form and maintain itself | Aluminium, titanium, stainless steel | Must maintain intact passive film — damage or specific chemistries can depassivate |
Anodising | Electrochemically thicken the natural oxide layer on aluminium | Type II anodising: 5–25 µm​; Type III (hardcoat): up to 150 µm | Excellent corrosion and wear resistance; can be dyed for colour; brittle if thick |
Galvanising | Coat steel with zinc — zinc is anodic to steel and sacrificially protects it (sacrificial anode protection) | Hot-dip galvanised steel (50–100 µm Zn coating); electrogalvanised sheet | Zinc coating provides both barrier AND sacrificial protection — even when coating is scratched, zinc around the scratch corrodes preferentially, protecting the steel |
Electroplating | Deposit thin metallic layer of noble metal on substrate | Chrome plating (Cr on steel); tin plating (food cans); nickel plating | Barrier protection only — once coating scratched through, galvanic corrosion accelerates (coating acts as cathode; small scratch area = intense local corrosion of substrate) |
Painting / organic coating | Apply barrier layer of polymer-based coating | Epoxy primer + polyurethane topcoat; powder coating | Requires surface preparation; mechanical damage breaks barrier; some primers contain corrosion-inhibiting pigments (zinc phosphate, strontium chromate) |
Sacrificial anodes | Attach a more reactive metal that corrodes preferentially, protecting the structure | Magnesium or zinc sacrificial anodes on ship hulls, offshore platforms, buried pipelines | Anode must be periodically replaced as it is consumed |
Impressed current cathodic protection | Apply external electrical current to make the protected structure cathodic | Offshore platforms, underground pipelines, ship hulls | Requires continuous power supply and monitoring; electrical engineering system required |
Material substitution | Replace corrosion-susceptible material with inherently resistant alternative | Replace steel with aluminium, titanium, CFRP, or polymer for corrosive environments | Most fundamental design-level solution — eliminates corrosion rather than managing it |
Alloying | Add alloying elements that improve corrosion resistance | Add Cr (>10.5% → stainless steel); add Mo (improved pitting resistance in stainless grades) | Standard metallurgical approach; 316 stainless (Mo\text{Mo}Mo addition) resists chloride pitting better than 304 |
Reactivity (Food Safety)
In the context of design technology and material selection, reactivity (particularly as it relates to food safety) refers to the propensity of a material to chemically interact with food, beverages, food preparation environments, or the human body.Â
Food safety in materials science is thus the evaluation of whether a material is chemically inert (non-reactive) in contact with:
Foodstuffs (including acidic foods — vinegar, citrus, tomatoes; alkaline foods — egg whites; fatty foods — oils; salty foods — brines)
Beverages (including acidic drinks — coffee, carbonated beverages; alcoholic drinks — ethanol solutions)
Food processing chemicals (cleaning agents — sodium hydroxide, chlorine-based sanitisers; acids — citric acid, phosphoric acid)
The human body (skin contact, implantable devices, food packaging with diffusion into food)
Core regulatory principle:Â
Food contact materials must be chemically inert under actual use conditions — the combination of time, temperature, food type, and contact geometry must all be considered. A material safe for cold water contact may not be safe for acidic hot beverage contact.
The BPA Case Study — A Material Safety Design History
Bisphenol A (BPA) is a monomer used in the synthesis of polycarbonate (PC) and epoxy resins. It was identified in the 1990s–2000s as an endocrine-disrupting chemical (EDC) — capable of binding to oestrogen receptors in the human body at extremely low concentrations and interfering with hormonal signalling systems, with potential developmental and reproductive health effects.
BPA is present in polycarbonate as residual unreacted monomer and can migrate into food/beverage in contact with PC containers — particularly when heated (e.g. warming baby bottle in microwave) or when exposed to acidic or alkaline solutions.
Regulatory response timeline:
Year | Action |
2008 | Canada: First country to classify BPA as a toxic substance; banned from baby bottles |
2011 | EU: Ban on BPA in baby bottles (polycarbonate) |
2012 | USA (FDA): Banned BPA in baby bottles and sippy cups |
2020 | EU EFSA: Revised Tolerable Daily Intake (TDI) for BPA reduced by factor of 100,000 from previous level |
2023 | EU: Further restrictions on BPA in food contact materials |
Ongoing | Replacement materials (Tritan, PPSU, glass, stainless steel) have largely replaced PC in consumer food contact applications |
Design implication:Â
This case illustrates that food safety is not static — a material approved for food contact at one time may subsequently be restricted or banned as toxicological evidence develops. Design decisions must account for the precautionary principle in food safety material selection, and designers must maintain current regulatory awareness. The replacement of PC water bottles with Tritan copolyester bottles is a design technology example of material substitution driven entirely by chemical property concerns — food safety reactivity.
Hygroscopy
Hygroscopy is the tendency of a material to absorb moisture (water vapour or liquid water) from its surrounding environment and retain it within its structure. A hygroscopic material is one that absorbs water readily from the atmosphere.
The related term hydrophilic describes materials that have a thermodynamic affinity for water (water spreads readily on their surface — contact angle < 90°). Hydrophobic materials repel water (contact angle > 90°, water beads up on surface).
Consequences of Hygroscopy in Design
Moisture absorption has significant consequences across multiple dimensions of material performance:
1. Dimensional change (swelling):
When a hygroscopic material absorbs water, water molecules insert between polymer chains or cellulose fibres, increasing inter-chain spacing and causing dimensional swelling. This is analogous to thermal expansion but driven by moisture rather than temperature.
2. Mechanical property degradation:
Water acts as a plasticiser for many polymers — it inserts between polymer chains, increasing chain mobility, reducing intermolecular forces, and lowering the effective glass transition temperature
3. Processing implications — Injection moulding:
Hygroscopic polymers that have absorbed atmospheric moisture during storage will, when heated in the injection moulding barrel to processing temperature, cause moisture to flash to steam, producing:
Splay marks (streaks) on the moulded part surface
Voids and bubbles within the moulded part
Hydrolytic chain scission (chain length reduction → reduced mechanical properties)
Surface roughness and cosmetic defects
FDM 3D printing application:Â Hygroscopic FDM filaments (particularly Nylon, PLA, PETG, PC filaments) absorb atmospheric moisture during storage. Printing with moisture-laden filament produces rough, stringy prints with reduced mechanical properties. Proper storage in sealed containers with desiccant (silica gel), and pre-drying in a filament dryer at appropriate temperature before printing, are essential process controls.
4. Electrical property degradation:
Water is a polar, ionically-conducting medium. When an insulating polymer absorbs moisture, the absorbed water reduces its electrical resistivity and increases dielectric loss, compromising electrical performance.
5. Biological degradation:
Hygroscopic materials that absorb and retain moisture create conditions for biological degradation — fungal growth (mould, mildew, rot) and bacterial proliferation.
Flammability
Flammability is the ease with which a material ignites and sustains combustion when exposed to a heat source or ignition stimulus.
It encompasses:
Ignitability — The ease with which a material begins to burn (related to ignition temperature and flash point)
Flame spread rate — How quickly combustion propagates across the material surface
Heat release rate (HRR) — The rate at which thermal energy is released during combustion — the most critical fire safety metric
Smoke production — Quantity and toxicity of combustion products
Extinction behaviour — Whether a material self-extinguishes when the ignition source is removed (self-extinguishing) or continues to burn independently
Chemical Properties Interaction Matrix
Chemical properties do not operate in isolation — they interact, and in many design situations, multiple chemical property constraints must be satisfied simultaneously:
Design Scenario | Corrosion Resistance | Reactivity (Food Safe) | Hygroscopy | Flammability | Optimal Material |
Outdoor kitchen structure | Must resist rain, atmospheric moisture, sea air | Must not leach into food preparation area | Must not absorb moisture and swell/degrade | Must not contribute fuel to barbecue fire | Stainless 316 structure; aluminium fittings; HDPE cutting surfaces |
Children's drink bottle | Good — resists daily dishwasher cycles | Must not leach BPA, lead, or plasticisers | Low — maintains dimensional integrity | Not primary concern (ambient temperature product) | Food-grade stainless 316 or Tritan copolyester; PE lid |
Electric vehicle battery housing | Resists road salt and moisture | Not food contact — vehicle application | Must not absorb moisture (electrical insulation) | Must be self-extinguishing adjacent to Li-ion energy store | Aluminium alloy housing; UL94 V-0 rated polymer inner components; LSOH cable insulation |
Medical device housing (autoclavable) | Resists high-temperature steam sterilisation cycles (aggressive to many plastics) | Must not leach into patient contact surfaces | Low — dimensional stability required under humid sterilisation cycle | Not primary concern in clinical setting, but materials must not emit toxic volatiles | PPSU (polyphenylsulfone), PEEK, or stainless 316 |
Public playground equipment | Resists atmospheric weathering (rain, UV, freeze-thaw) for 15+ year service life | Food-safe if children mouth components | Low — dimensional stability, no rot, no swelling | Must not contribute fuel load in public area | Hot-dip galvanised steel; aluminium alloy; HDPE; powder-coated mild steel (with regular maintenance) |
FDM 3D-printed outdoor sign | Resists UV, rain, and thermal cycling | Not food contact | Must not warp or degrade with moisture absorption | Ambient — low fire risk; relevant if near electrical system |
Key Vocabulary
Precise definitions are the foundation of explanation. Every term below should be used in your answers — with its definition embedded naturally in your response.
Term | Precise Definition |
Corrosion | Electrochemical degradation of a material — typically a metal — through reaction with its environment, most commonly involving oxygen and moisture |
Oxidation | A chemical reaction in which a material loses electrons to oxygen — the fundamental mechanism of most metallic corrosion |
Passive layer | A thin, dense, chemically stable oxide film that forms spontaneously on certain metals (aluminium, stainless steel, titanium) and prevents further corrosion by acting as a physical barrier |
Galvanic corrosion | Accelerated corrosion of the more reactive metal (anode) when two dissimilar metals are in electrical contact in the presence of an electrolyte |
Sacrificial anode | A block of reactive metal (zinc or magnesium) deliberately attached to a structure so that it corrodes preferentially, protecting the structural material |
Chemical migration | The transfer of chemical molecules from a material into substances it contacts — the primary food safety concern for food-contact materials |
Food-safe / food-grade | A material classification indicating that the material will not transfer harmful chemicals to food in contact with it under specified conditions of use |
BPA (Bisphenol A) | A monomer used in polycarbonate synthesis that can leach into food and beverages — an endocrine disruptor linked to hormonal disruption, now banned from baby products in many jurisdictions |
Hygroscopy | The ability of a material to absorb and retain water molecules from the surrounding atmosphere, causing dimensional changes and property alterations |
Hydrophobic | A material that repels water — water beads on its surface and is not absorbed into its structure (e.g. PTFE, wax, polyethylene) |
Moisture content | The mass of absorbed water as a percentage of the dry mass of the material |
Flammability | The ease with which a material ignites and sustains combustion — characterised by flash point, ignition temperature, LOI, flame spread rate, and heat release rate |
Limiting Oxygen Index (LOI) | The minimum concentration of oxygen (% by volume) in an atmosphere required to sustain combustion of a material. Materials with LOI > 21% are self-extinguishing in normal air |
Intumescent | A fire protection material that swells and chars when heated, forming an insulating barrier that protects the substrate from fire |
Flame retardant | A chemical additive incorporated into a material to increase its resistance to ignition and reduce its rate of flame spread |
Endocrine disruptor | A chemical that interferes with the hormonal (endocrine) system of living organisms — a critical concern in food-contact and medical material selection |
Practice Questions
The command term for this learning objective is EXPLAIN — "give a detailed account including reasons or causes." Describing what happens is worth half marks at best. Full marks require you to explain why it happens at a chemical or molecular level and what the consequence is for design decisions. Every question below is structured to reward that depth.
Question 1Â (4 marks)
A packaging designer is selecting a polymer for a microwaveable ready meal tray that will be in direct contact with food during heating to 120°C. Explain why chemical reactivity (food safety) is a critical property in this selection and evaluate the suitability of polypropylene (PP) versus polycarbonate (PC) for this application.
Examiner's hint: Define chemical migration as the transfer of molecules from material into food. Explain that temperature dramatically increases migration rate — this is especially critical for a microwaveable product. PP: high melting point ~160°C, very low migration, food-approved, no hazardous monomers — suitable. PC: contains BPA monomer which has demonstrated leaching, particularly at elevated temperatures, classified as endocrine disruptor, banned from food contact in many jurisdictions — NOT suitable for hot food contact despite good thermal and mechanical properties. Conclude: PP is the appropriate selection. Physical and mechanical properties of PC are irrelevant if the chemical safety requirement fails.
Question 2Â (4 marks)
Explain how hygroscopy affects the performance of nylon (polyamide) components used in a precision mechanical assembly — such as gears inside a medical device used in a hospital environment.
Examiner's hint: Define hygroscopy as moisture absorption from the atmosphere into the material structure. Explain mechanism — polar amide groups in nylon attract water molecules which diffuse between polymer chains, acting as plasticiser. Three consequences to explain: (1) dimensional change — nylon absorbs up to 8.5% moisture at saturation, causing gears to swell and potentially bind on shafts or increase backlash; (2) reduced stiffness and strength — water molecules disrupt interchain hydrogen bonding, lowering Young's modulus and tensile strength by up to 20%; (3) hospital environment has controlled humidity ~50% RH, so equilibrium moisture content must be established — design solution: moisture-condition nylon components before assembly so they are dimensionally stable in service.
Question 3Â (6 marks)
A designer is developing a children's wooden toy for the 0–3 years age group. The toy will be painted, handled daily, potentially put in the mouth, and washed regularly in warm soapy water. Explain how three chemical properties should influence the material and finishing selection decisions for this toy. For each property, identify what is required and justify your reasoning in terms of the specific use context.
Examiner's hint: This is a full 6-mark explanation — two marks per property for definition + design application. Suggested three: (1) Chemical reactivity / food safety — paint and surface finishes must not leach toxic compounds when mouthed by infants — specify water-based, food-safe paints; EU Toy Safety Directive (EN 71-3) limits migration of hazardous elements including lead, cadmium, chromium from surface coatings. (2) Hygroscopy — wood is hygroscopic; repeated washing and drying causes swelling/shrinkage, surface finish cracking, and bacterial harbouring in surface cracks — specify sealed, lacquered, or varnished surfaces to act as moisture barrier. (3) Flammability — children's products must meet ignition resistance requirements; specify timber species and finishes with appropriate fire classification; water-based finishes are less flammable than solvent-based alternatives. For each: define the property, state the requirement, explain WHY in this specific context of infant use.
Sources
Ashby, M.F. (2011). Materials Selection in Mechanical Design. 4th edn. Butterworth-Heinemann, Oxford.
Babrauskas, V. and Peacock, R.D. (1992). 'Heat release rate: the single most important variable in fire hazard.' Fire Safety Journal, 18(3), pp. 255–272.
Callister, W.D. and Rethwisch, D.G. (2018). Materials Science and Engineering: An Introduction. 10th edn. Wiley, New York.
European Commission (2004). Regulation (EC) No 1935/2004 of the European Parliament and of the Council on Materials and Articles Intended to Come into Contact with Food. Official Journal of the European Union.
European Food Safety Authority (2023). Food Contact Materials. EFSA, Parma. Available at: www.efsa.europa.eu
International Baccalaureate Organization (2024). Design Technology Guide: Diploma Programme. First Assessment 2027. IBO, Geneva.
NACE International (2016). International Measures of Prevention, Application and Economics of Corrosion Technology (IMPACT) Study. NACE International, Houston.
Ulrich, K.T. and Eppinger, S.D. (2015). Product Design and Development. 6th edn. McGraw-Hill Education, New York.
Vandenberg, L.N. et al. (2012). 'Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses.' Endocrine Reviews, 33(3), pp. 378–455.
Linking Questions
Why is a good understanding of material properties important when designing structural systems? (A3.2)
When do the physical properties of materials restrict the ability to use certain prototyping techniques? (A2.2)
How do the properties of a material influence the choice of manufacturing techniques for a product? (A4.1)
How can the characteristics of a material limit the effectiveness of modelling and prototyping as designs are developed? (B2.2)
How important is an understanding of the mechanical properties of a material when considering structural and mechanical systems, and their applications? (A3.2, A3.3, B3.2, B3.3)
Which classifications of properties are important when developing electronic systems and their applications? (A3.4, B3.4)
How could the continued development of biodegradable materials influence designers’ ability to address aspects of design for sustainability and design for a circular economy? (C2.1, C2.2)
Why is a thorough understanding of materials key for effective product analysis and evaluation of products? (C3.1)
How do design decisions related to the properties of materials and components impact a product’s life-cycle analysis? (C3.2)