Atmospheric Corrosion

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2 Corrosion of mild steel Corrosion of aluminium Exfoliation corrosion of aluminium aerospace alloy

3 Given the large surface areas of materials exposed to the atmosphere, annual losses in the UK are of the order of several hundred million pounds Compared with, for example, the effect of immersion sea water on the corrosion of metals, atmospheric corrosion is not as degrading. Additionally, not continuous in action

4 Consider here corrosion processes in the natural environment of the atmosphere. Introduce following topics: The Atmospheric Environment Deposition Processes

5 The Atmospheric Environment; Composition The atmosphere, also the sea, is one of the most common natural environments to which materials are exposed. Unlike most corrosion environments, the atmosphere does not provide a constant exposure condition.

6 For example, to determine the corrosion rate of a steel specimen, may undertake following test: Laboratory measurement in 3.5% sodium chloride solution at 20ºC Use defined mass transfer (i.e. constant stirring rate) Under such conditions, expect the corrosion rate to be generally reproducible (± 50%)

7 Similar specimen exposed in the atmosphere does not experience a constant environment; hence, measured corrosion rates are much less reproducible. Main components of the atmosphere have an effectively constant composition; however, minor components that affect corrosion may vary significantly.

8 AVERAGE COMPOSITION OF THE NATURAL ATMOSPHERE Substance Nitrogen (N 2 ) Oxygen (O 2 ) Water vapour (H 2 O) Argon (Ar) Carbon dioxide (CO 2 ) Other rare gases (Ne, He, etc.) Hydrocarbons (mainly CH 4 ) Hydrogen (H 2 ) Nitrogen oxide (N 2 O) Ammonia (NH 3 ) Sulphur dioxide (SO 2 ) Oxides of nitrogen (NO x ) Hydrogen sulphide (H 2 S) Concentration (vol% or ppm by vol) 78.1% 20.9% 0-5% 0.93% 380 ppm 30 ppm 2 ppm 0.5 ppm 0.3 ppm < 0.1 ppm 10-3 ppm 10-3 ppm 10-3 ppm

9 Concentrations of water vapour vary naturally over large range. For example, on a hot, humid day there may be several percent H 2 O vapour in the air. Conversely, concentration drops to close to zero on a cold, dry day.

10 Natural air pollution 3 main primary sources of natural air gas pollution: Volcanic action Vegetation Animal wastes. Additionally, 2 primary sources of particulate pollution: Sea-spray Dust from the earth Secondary sources of pollution arise from transformations in the atmosphere, e.g. ozone and other photochemical action.

11 Pollution is generally classified into primary species that remain unchanged in the atmosphere after emission. Secondary species result from transformation (change) from some other primary species. Pollutants also classified as either gaseous, particulate ( i.e. dust from ground), or aqueous (i.e. dissolved in rain).

12 SOURCES OF NATURAL POLLUTANTS Primary Volcanic action Biological respiration Animal waste Vegetation Soil microbes Sea spray Soil (earth) from the ground Natural (forest) fires Secondary Oxidation of SO 2 Reaction of NH 3 + SO 4 2- Reaction of sea-spray with SO 3 Oxidation of N 2 during lightning Atmospheric photochemistry Plant hydrocarbons + O 3 Primary SO 2, H 2 S, HCl, Cl 2 gaseous; Volcanic ash - particulate CO 2 gaseous NH 3, organic sulphides gaseous Hydrocarbons gaseous N 2 O gaseous Cl -, SO 2-4 particulate aerosol Inorganic dusts - particulate CO 2 gaseous Ash and smoke - particulates Secondary SO 3 gaseous SO 2-4 particulate aerosol (NH 4 ) 2 SO 4 particulate aerosol HCl gaseous NO/NO 2 (NO x ) gaseous O 3 (ozone) gaseous Hydrocarbon peroxides and radicals ( Blue mountains of Virginia ) gaseous

13 Man-made atmospheric pollution Main sources of man-made pollution from burning fossil fuels, and other combustible materials in processes such as waste incineration. Additional atmospheric pollution arises from chemical processing, sewage treatment and farming.

14 SOURCES OF MAN-MADE POLLUTION Primary General combustion (all fuels) Fossil fuel combustion (oil, coal, gas) Incineration (waste, plastics, coal) Transport (vehicle exhaust) Chemical processing Sewage treatment Farming (domesticated animals) Farming (growing crops) Secondary Oxidation of SO 2 by O 3 Reaction of NH 3 + SO 4 2- Oxidation of NO x by O 3 Hydrocarbons + O 3 Primary CO, CO 2 gaseous Fly-ash, carbon (smoke) particulate SO 2, NO x gaseous HCl, Cl 2 gaseous NO x, CO, hydrocarbons gaseous carbon (smoke) particulate Hydrocarbons, sulphides, NH 3 gaseous Sulphides, NH 3 gaseous CH 4, NH 3, sulphides gaseous Hydrocarbons (CH 4 ) gaseous Secondary SO 3 gaseous SO 2-4 particulate aerosol (NH 4 ) 2 SO 4 particulate aerosol H 2 SO 4 acid rain NO 3- - particulate aerosol HNO 3 acid rain Hydrocarbon peroxides and radicals ( Photochemical smog ) gaseous

15 Climate effects; weather at a specific location. Weather changes from place-to-place, from country-tocountry and from season-to-season. General weather pattern for a region of a country usually termed macroclimate. Significant local variations (microclimate effects) with height or distance from the sea; through human activity (e.g. cities usually slightly warmer and more humid than the countryside); very locally (e.g. depending on arrangement of buildings).

16 High smokestacks may transport pollutant emissions over many hundreds of kilometres Medium height sources may transport to 50 kilometres Low emitter, i.e. domestic chimney, transports up to 250 metres Exhaust system, about metres

17 Pollutant deposition Pollutants in the atmosphere consist of: dry species: gases, dry salt particles, dusts and smoke, wet species; gases, ionic salts dissolved in water droplets. While suspended in the atmosphere, contribute to general atmospheric pollution- reduction in atmospheric quality. No influence on atmospheric corrosion or ground quality until deposit onto a surface or the ground.

18 Dry deposition Gases or particulates (particles) transport to a surface by 3 main mechanisms: advection due to the airflow (wind) convection due to thermal air currents diffusion due to concentration gradient of the species between the surface and atmosphere Dry deposition generally accounts for most pollution transport to a surface: ~ 70% of deposition on land.

19 Wet deposition Although most atmospheric pollution transport to surfaces in the dry state, significant quantity dissolves in water droplets in clouds and eventually falls as precipitation (i.e. rain, snow, or fog). Depending on the local climate, amounts to about 30% pollutant deposition. Note: gaseous pollutants dissolving in moisture film on surface is counted as dry deposition.

20 CONCENTRATION RANGES FOR ATMOSPHERIC POLLUTANTS Pollutant SO 2 NO x HCl Smoke and ash Sea-salt aerosol Other dusts Industrial (μg m -3 ) Urban (μg m -3 ) Rural (μg m -3 ) < < < 20 Inland Coastal (air) 30 μg m μg m -3 (rain) 5 mg dm mg dm μg m -3 Note that 1 μg m -3 = 1 ppb (part per billion 10 9 )

21 Acid rain Describes all forms of deposition, including wet and dry, that eventually result in acidification of surface water. Important to recall that, in the absence of any acidic pollutants such as SO 2, NO x and HCl, natural rainwater is slightly acidic due to the presence of atmospheric CO 2.

22 The natural ph of rainwater readily calculated from following equilibrium: CO 2 + H 2 O H + + HCO 3 - K = [ H ].[ HCO [ CO ] 2 3 ] K = 1 x [H + ] = [HCO 3- ] [H + ] 2 = 1 x x pco 2 Volume concentration of CO 2 in the atmosphere in 2018 was ~400 ppm: hence, [H + ] 2 ~ => [H + ] = = ph 5.5

23 Atmospheric classifications ISO9223, international standard: provides a classification scheme for ranking pollution in a particular climatic location. Scheme based on deposition rates of SO 2 and Cl -.

24 SULPHUR DIOXIDE CHLORIDE SO 2 class Deposition rate mg/m 2 /day Concentration mg/m 3 Chloride class Deposition rate mg/m 2 /day P 0 <10 <12 S 0 <3 P S P S P S 3 >300

25 Humidity and time of wetness Atmospheric corrosion, like all metallic corrosion, only proceeds when sufficient water present to solvate ions generated during anodic and cathodic reactions. Atmospheric humidity clearly important in determining when atmospheric corrosion proceeds. Atmospheric humidity is a measure of amount of water vapour in air.

26 Humidity defined in two ways: Absolute humidity = Concentration (pressure) of water vapour in the air Relative humidity = Actual water vapour pressure/ Saturated (maximum) water vapour pressure

27 Relative humidity Defines the thermodynamic water vapour activity at any temperature. Important when considering condensation of water on a surface. From the definition, at RH = 100%, liquid water is a stable phase and condensation occurs. For RH < 100%, water vapour is the stable phase and, normally, condensation does not occur.

28 The saturated or maximum concentration of water vapour in air varies as a function of temperature: At 100ºC water boils, so the maximum vapour pressure of water is 1 atmosphere. Below 100ºC, the water vapour pressure is less than 1 atmosphere For a given humidity, temperature below which water vapour condenses termed the dewpoint. Metal corrosion can not occur unless sufficient liquid water on surface. Time during which the RH exceeds a critical value is defined as the Time-of-Wetness of the surface.

29 From 0 to 40 o C water content of saturated air roughly doubles for every 11 o C rise in temperature: If air at 50% humidity is cooled by 11 o C, it becomes saturated with water vapour, increasing the atmospheric corrosion rate of metals If air saturated with water vapour at given temperature heated by 11 o C, relative humidity falls to 50% Fundamental to atmospheric corrosion

30 ISO9223: classification scheme for ranking a particular climate in terms of its time-of-wetness. Time-of-wetness (TOW) calculated as the total time when the RH is greater than 80% at temperatures above 0ºC. Note, when raining, RH by definition is equal to100%.

31 WETNESS CLASS TIME OF WETNESS (hours/year) (%) EXAMPLES OF OCCURRENCE T 1 <10 <0.1 Indoor T Indoor, unheated T Outdoor, dry/cold climate, ventilated buildings. T Outdoor temperate climate. T 5 >5200 >60 Tropical outdoor or surf.

32 Corrosion rates of steel with time of exposure at different locations worldwide

33 Surface condition In atmospheric corrosion, electrochemical (anodic and cathodic) reactions occur in very thin layer of water condensed on the corroding surface. Physical and chemical nature of the surface (e.g. roughness and contamination), as well as amount of water condensed on the surface, control rate of corrosion. Typical thicknesses of water layers on a clean (freshly polished) metal surface are given in following Table:

34 CONDITIONS WATER LAYER THICKNESS Wet during rainfall Covered by dew or condensation 100% relative humidity < 100% RH > 500 μm μm 1 5 μm < 1 μm

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36 General atmospheric corrosion of metals ISO9223 also provides a classification scheme for the corrosivity of an environment based on the expected corrosion rates of materials exposed outdoors for 1 year. Thus, from a knowledge of the pollution classifications and the TOW (time of wetness) classification, the corrosivity classification can be derived.

37 CORROSIVITY CATEGORY CORROSION RATES OF METALS Units Carbon steel Zinc Copper Aluminium C 1 (very low) g m -2 y -1 µm y negligible C 2 (low) g m -2 y µm y C 3 (medium) g m -2 y µm y C 4 (high) g m -2 y µm y C 5 (very high) g m -2 y µm y

38 Factors affecting critical relative humidity Water condensation can occur on metals at relative humidity less than 100% from two mechanisms: Firstly, small diameter cracks and pores on the surface, i.e. in corrosion product layer, can significantly reduce the equilibrium humidity for condensation. A long thin crack can stabilise water at ibase even though external humidity relatively low. Process termed capillary condensation and is the reason why a polished metal surface corrodes initially more slowly than rusty surfaces.

39 Second mechanism for reduction in critical RH is important and relates to chemical contamination (e.g. by pollutants) on the surface. Dried salt on a surface will have a tendency to dissolve in condensed water to give, initially, a saturated solution of the salt on the metal surface. The dissolved salt reduces thermodynamic activity of water and, hence, equilibrium RH in air immediately above surface. Following Table shows the RH at which water condenses on a surface, contaminated with the salt.

40 RH OF CONDENSATION ON A POLLUTED SURFACE AT 20ºC Polluting salt Na 2 SO 4 (NH 4 ) 2 SO 4 NaCl MgCl 2 ; CaCl 2 Calcium and magnesium chloride are components of sea salt. RH of condensation 93% 81% 78% 35% Surface polluted with sea salt is wet at all RH > 35%

41 Influence of humidity on the corrosion rate of zinc

42 Atmospheric corrosion of selected metals

43 Measurements show that the corrosion rate during drying is many times higher than when a surface is just wet. Hence, wetting and drying are critical components of the mechanism of corrosion. Thus an artificial (accelerated) test should use wet-dry cycles in order to be realistic.

44 Corrosion rate of iron during wetting and drying

45 Brown rust layer formed in the atmosphere generally consists of a crystalline iron oxide-hydroxide, lepidocrocite, FeO(OH); magnetite, Fe 3 O 4, may also form. The anodic reactions in atmospheric corrosion of iron are: Formation of ferrous hydroxide: Fe + 2H 2 O Fe(OH) 2 + 2H + + 2e - Oxidation of Fe(OH) 2 to lepidocrocite: Fe(OH) 2 FeO(OH) + H + + e- Oxidation of Fe(OH) 2 to magnetite: 3Fe(OH) 2 Fe 3 O 4 + 2H 2 O + 2H + + 2e- The last two reactions are reversible, hence both magnetite and lepidocrocite can be reduced to ferrous hydroxide.

46 Zinc is considerably more active than iron, but corrodes in the atmosphere at a rate that is about times less. The corrosion product layer that forms is highly protective. Like iron, the corrosion rate of zinc follows a power law relationship, but the value of the exponent is close to one, i.e. zinc tends to have linear corrosion kinetics. Linear kinetics make it easy to predict the lifetime of a galvanized coating on steel.

47 During atmospheric corrosion of zinc, a thin, non-porous zinc hydroxide layer initially forms. Subsequently transforms, at its interface with the atmosphere, to a basic zinc salt that is slightly soluble. Corrosion product layer thickness stays approximately constant, being determined by the solubility of the basic salt. In turn, this is determined by the concentration of pollutants in the atmosphere

48 Formation of zinc hydroxide: Zn + 2H 2 O Zn(OH) 2 + 2H + + 2e - Reaction with CO 2 : 4Zn(OH) 2 + CO 2 3Zn(OH) 2.ZnCO 3 + H 2 O basic zinc carbonate Reaction with SO 3 : 5Zn(OH) 2 + SO 3 4Zn(OH) 2.ZnSO 4 + H 2 O basic zinc sulphate Reaction with HCl: 5Zn(OH) 2 + 2HCl 4Zn(OH) 2.ZnCl 2 + 2H 2 O basic zinc chloride

49 Few metals are exposed in the atmosphere without some kind of additional protection usually painting. Zinc and steel are the most common but copper (and copper alloys) and lead are also used uncoated. Both materials have similar corrosion mechanisms to zinc, but considerably lower corrosion rates.

50 Problems can arise with buildings clad with passive metals, such as aluminium or stainless steel, which are then presumed to be maintenance free. However, although the general corrosion rates may be low, airborne particulate deposition can initiate local pitting corrosion. Is is important to keep such surfaces clean by regular washing in order to remove the build-up of salts and other deposits

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