Material Deterioration by Heat, Air, and Pressure

What Is Material Deterioration?

Material deterioration is any progressive change in a material’s physical, chemical, or mechanical properties that reduces its ability to perform its intended function. Heat, air (oxygen), and pressure are three of the most common deterioration agents in engineering applications – and they frequently act together rather than independently. A polymer seal exposed to hot pressurised air experiences all three simultaneously: thermal degradation, oxidative attack, and mechanical compression set. Understanding each mechanism individually, and their interactions, is essential for predicting service life and selecting materials for elevated temperature or pressure applications.

Deterioration by Heat

Thermal Degradation of Polymers

Polymers degrade thermally through chain scission (breaking of the polymer backbone) and crosslinking (formation of additional bonds between chains). Chain scission reduces molecular weight, causing loss of tensile strength, elongation, and toughness – the material becomes brittle and weak. Crosslinking stiffens the material initially but eventually embrittles it. The rate of thermal degradation follows Arrhenius kinetics – a 10 degree C increase in temperature approximately doubles the degradation rate for many materials, the basis for ASTM accelerated thermal ageing tests. TGA (thermogravimetric analysis) characterises the onset of thermal degradation.

Thermal Fatigue and Cycling

Repeated thermal cycling induces fatigue through differential thermal expansion between bonded materials or through the material itself cycling through phase transitions. Solder joints in electronics, thermal barrier coatings in gas turbines, and brazed joints in heat exchangers all fail by thermal fatigue. Each cycle generates plastic strain at the interface; fatigue life depends on the strain per cycle and the material’s ductility. Testing is conducted per ASTM E2368 (thermomechanical fatigue) or mission-profile cycling relevant to the application. The failure mode is typically cracking at the interface or in the coating.

Creep at Elevated Temperature

Creep is time-dependent plastic deformation under constant stress at elevated temperature. For metals, creep becomes significant above approximately 0.4 times the absolute melting temperature. Creep rupture – failure after extended time at elevated stress and temperature – is the design-limiting failure mode for power plant turbine blades, steam pipes, and high-temperature fasteners. ASTM E139 covers creep testing of metallic materials. For polymers, creep is significant even at room temperature for stressed parts – this is why plastic pipe pressure ratings are time-dependent (the 1,000-hour and 100,000-hour ratings differ).

Deterioration by Air and Oxygen

Oxidation of Metals

Metal oxidation forms surface oxide layers that can be protective (alumina on aluminium, chromia on stainless steel) or non-protective (rust on carbon steel, which flakes off and exposes fresh metal). The rate of protective oxide growth follows parabolic kinetics – the oxide layer slows its own growth by acting as a diffusion barrier. Non-protective oxides grow linearly. High-temperature oxidation of turbine alloys, furnace equipment, and exhaust components is characterised by thermogravimetric analysis (ASTM E1131) and cyclic oxidation tests. Aluminium alloying in steel and chromium in superalloys are the primary strategies for oxidation resistance.

Oxidative Degradation of Polymers

Oxygen attacks polymer chains through a free radical chain reaction – autoxidation – that is accelerated by heat, UV radiation, and metal ion catalysts. The reaction produces hydroperoxides that decompose to produce more radicals, propagating the degradation. Antioxidant additives (hindered phenols, phosphites) interrupt the chain reaction by scavenging radicals or decomposing hydroperoxides. Oxidative induction time (OIT) testing by DSC (ASTM E1858 or ISO 11357-6) measures the resistance of a polymer to oxidative degradation by measuring the time to exothermic oxidation onset under oxygen at elevated temperature.

Corrosion and Atmospheric Oxidation

Atmospheric corrosion of metals combines oxygen with moisture, pollutants (SO2, chlorides), and electrochemical driving forces. Carbon steel in humid air rusts at rates of 25-100 micrometres per year; galvanised steel corrodes at 1-5 micrometres per year; stainless steel at 0.001-0.1 micrometres per year depending on chloride exposure. Salt spray testing (ASTM B117) and cyclic corrosion testing (SAE J2334) accelerate atmospheric corrosion for coating evaluation. Corrosion rate measurement by mass loss or electrochemical methods (ASTM G102) characterises material performance in aggressive environments.

Deterioration by Pressure

Compression Set in Elastomers

When an elastomeric seal is compressed and held at temperature, it undergoes permanent deformation – compression set – that reduces its sealing force over time. Compression set results from stress relaxation through bond breaking and reformation at elevated temperature and through physical rearrangement of polymer chains. ASTM D395 Method B measures compression set of elastomers at defined temperature and time conditions. A compression set of 25% means the seal has permanently deformed to retain 25% of the original deflection as permanent strain. Sealing pressure decreases as compression set increases – failure occurs when the sealing force drops below the threshold to maintain a seal.

Pressure-Induced Stress Corrosion Cracking

Stress corrosion cracking (SCC) occurs when a susceptible material under tensile stress is exposed to a specific corrosive environment. Pressure vessels and pipelines are inherently under tensile stress from internal pressure; if the material-environment combination is susceptible, SCC can cause catastrophic failure at stresses far below the yield strength. Austenitic stainless steel in chloride solutions, high-strength steel in hydrogen sulfide (sulfide stress cracking), and aluminium alloys in chloride are classic SCC systems. ASTM G36 (SCC in boiling magnesium chloride), ASTM G44 (alternate immersion in salt water), and NACE TM0177 (sulfide stress cracking) are standard SCC test methods.

Fatigue Under Cyclic Pressure

Pressure vessels and components subject to repeated pressurisation – autoclaves, hydraulic cylinders, compressed gas cylinders, fuel injectors – accumulate fatigue damage from cyclic stress at the pressure boundary wall. Fatigue life is governed by the hoop stress range, the pressure cycle frequency, and stress concentrations at nozzles, welds, and geometric discontinuities. ASME BPVC Section VIII Div. 2 requires fatigue analysis for pressure vessels subject to cyclic service. Pressure cycle testing to failure establishes the actual fatigue life of the component design.

Industry Specifications

• Thermal Ageing of Polymers: ASTM E2368 (thermomechanical fatigue), ASTM E1858 (OIT), IEC 60216 (thermal endurance)
• Creep Testing: ASTM E139 (metallic materials), ASTM D2990 (plastics creep and relaxation)
• Oxidation of Metals: ASTM E1131 (TGA), ASTM B117 (salt spray), SAE J2334 (cyclic corrosion)
• Elastomer Compression Set: ASTM D395 (compression set), ASTM D573 (rubber deterioration in air oven)
• Stress Corrosion Cracking: ASTM G36, ASTM G44, NACE TM0177, ASTM G64 (SCC ratings for aluminum)
• Pressure Vessels: ASME BPVC Section VIII (design and fatigue analysis), ASTM E466 (fatigue testing)

Conclusion

Heat, air, and pressure each deteriorate materials through distinct mechanisms – thermal degradation, oxidation, and creep/compression set/SCC, respectively. In most real applications, two or three of these agents act simultaneously, and their combined effect is more severe than any single agent alone. Testing must replicate the combined exposure to generate relevant data. Elevated temperature with oxygen is more severe than elevated temperature in nitrogen; pressure cycling with a corrosive medium is more severe than either alone. Material selection, protective coatings, and antioxidant additives are the primary engineering controls against deterioration.

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