FACTORS INFLUENCING CORROSIVITY OF THE ATMOSPHERE

9.4 FACTORS INFLUENCING CORROSIVITY OF THE ATMOSPHERE

In all except the most corrosive atmospheres, the average corrosion rates of metals are generally lower when exposed to air than when exposed to natural waters or to soils. This fact is illustrated by the data of Table 9.3 for steel, zinc, and copper in three atmospheres compared to average rates in seawater and a

T A B L E 9.3. Comparison of Atmospheric Corrosion Rates with Average Rates in Seawater and in Soils a

Environment Corrosion Rate (gmd)

Copper Rural atmosphere

Steel

Zinc

0.014 Marine atmosphere

0.032 Industrial atmosphere

a Atmospheric tests on 0.3% copper steel, 7 1 2 - year exposure, from C. Larrabee, Corrosion 9 , 259 (1953).

Atmospheric rates for zinc and copper, 10 - year exposure, from Symposium on Atmospheric Exposure Tests on Non - Ferrous Metals , ASTM, 1946. Seawater data from Corrosion Handbook , H. H. Uhlig, editor, Wiley, New York, 1948. Soil data for steel are averaged for 44 soils, 12 - year exposure; for zinc,

12 soils, 11 - year exposure; for copper, 29 soils, 8 - year exposure – from Underground Corrosion , M. Romanoff, Circ. 579, National Bureau of Standards, Washington, D.C. 1957.

196 ATMOSPHERIC CORROSION

variety of soils. In addition, atmospheric corrosion of passive metals (e.g., alumi- num and stainless steels) tends to be more uniform and with less marked pitting than corrosion in waters or soils.

Specifi c factors infl uencing the corrosivity of atmospheres are particulate matter, gases in the atmosphere, and moisture (critical humidity). In the United States, the Environmental Protection Agency (EPA) compiles data on ambient outdoor - air quality from information obtained at air - monitoring stations, where concentrations of the more common gases found in trace amounts in the atmo- sphere are measured continuously. Because of the importance of trace constitu- ents on atmospheric corrosion behavior, the engineer or architect would be well advised to review air - quality data for a corrosion assessment at the design stage, to avoid disappointment at the operational stage. Microclimatic conditions (e.g., east versus west exposure, sunlight, wind direction, proximity to a highway where deicing salts are used) can be different from macroclimates and should be considered.

9.4.1 Particulate Matter

Particulate matter (PM) is the term for a mixture of solid particles and liquid droplets in air. PM consists of several components, including acids, organic chemi- cals, metals, and soil or dust particles. The importance of atmospheric dust was established in the early experiments of Vernon [11] , who exposed specimens of iron to an indoor atmosphere, some specimens being entirely enclosed by a cage of single - thickness muslin measuring several inches larger in size than the speci- men. After several months, the unscreened specimens showed rust and appre- ciable gain in weight, whereas the muslin - screened specimens showed no rust whatsoever and had gained weight only slightly.

Particulate matter is found near roads and some industries, in smoke and haze; it can be directly emitted from sources such as forest fi res, and it can be formed when gases emitted from power plants and automobiles react in the air.

The NAAQSs for particulate matter, revised in 2006, are set at 150 μ g/m 3 as the

24 - h average for coarse particulates (2.5 - to 10 - μ m diameter) and are set at

35 μ g/m 3 as the 24 - h average for fi ne particulates ( ≤ 2.5 - μ m diameter) [12] . Particle pollution is controlled by reducing directly emitted particles and by reducing emissions of pollutants that are gases when emitted, but form particles in the atmosphere. In the United States in 2006, the average concentration of fi ne par-

ticulates amounted to about 13 μ g/m 3 [13] .

Particulate matter can be a primary contaminant of many atmospheres. In

the 1950s, it was estimated that the average city air contained about 2 mg/m 3 , with higher values for an industrial atmosphere, reaching 1000 mg/m 3 or more [14] . It was estimated that more than 35,000 kg of dust per km 2 (100 tons per square

mile) settled every month over an industrial city [15] . In contact with metallic surfaces, particulate matter infl uences the corrosion rate in an important way. Industrial atmospheres carry suspended particles of

carbon and carbon compounds, metal oxides, H 2 SO 4 , (NH 4 2 ) SO 4 , NaCl, and other

FAC TORS INFLUENCING CORROSIVIT Y OF THE ATMOSPHERE

salts. Marine atmospheres contain salt particles that may be carried many miles inland, depending on magnitude and direction of the prevailing winds. These substances, combined with moisture, initiate corrosion by forming galvanic or differential aeration cells; or, because of their hygroscopic nature, they form an electrolyte on the metal surface. Dust - free air, therefore, is less apt to cause cor- rosion than is air heavily laden with dust, particularly if the dust consists of

water - soluble particles or of particles on which H 2 SO 4 is adsorbed.

9.4.2 Gases in the Atmosphere

The carbon dioxide normally present in air neither initiates nor accelerates cor- rosion. Steel specimens rust in a carbon - dioxide - free atmosphere as readily as in the normal atmosphere. Early experiments by Vernon showed that the normal carbon dioxide content of air actually decreases corrosion [16] , probably by favoring a more protective rust fi lm.

A trace amount of hydrogen sulfi de in contaminated atmospheres causes the observed tarnish of silver and may also cause tarnish of copper. The tarnish fi lms are composed of Ag 2 S and a mixture of Cu 2 S + CuS + Cu 2 O, respectively. Important atmospheric pollutants are sulfur dioxide (SO 2 ), nitrogen oxides (NO x ), ozone (O 3 ), and chloride ions (Cl − ). Sulfur dioxide and nitrogen oxides form corrosive acids, and ozone is a powerful oxidizing agent. Sulfur dioxide originates predominantly from the burning of coal, oil, and gasoline. In New York City in the 1950s, it was estimated that about 1.5 million tons of sulfur dioxide were produced every year from burning coal and oil [17] . This amount was equivalent to burdening the atmosphere with an average of

6300 tons of H 2 SO 4 every day and has been reduced by limiting the allowable sulfur content of fuels burned within city limits. Since fuel consumption is higher in winter, sulfur dioxide contamination is also higher (Fig. 9.2 ). It is also obvious from this cause that the average sulfur dioxide content of the air (and

Figure 9.2. Variation of average sulfur dioxide content of New York City air with time of year [17] .

198 ATMOSPHERIC CORROSION

T A B L E 9.4. Variation of SO 2 Content of Air with Distance from Center of City (H. Meller, J.

Alley, and J. Sherrick, quoted in Ref. 17) City

Parts per Million

Miles Kilometers

Philadelphia – Camden

St. Louis

Washington, D.C.

corresponding corrosivity) falls off with distance from the center of an industrial city, and it is clear that this effect is not as pronounced in the case of a residential city, such as Washington, D.C. (Table 9.4 ).

Although the SO 2 levels in the air of many major centers of population

around the world may be similar to the data in Fig. 9.2 , the engineering and industrial responses to the U.S. Clean Air Act have resulted in a major decline

in the levels of air pollutants. About 10% of SO 2 emissions result from industrial

processes, and transportation sources contribute most of the remainder. The EPA

estimates that ambient SO 2 levels have decreased by more than 50% since 1983.

In comparison with the data in Fig. 9.2 , the annual average concentration of SO 2

in New York City air in 2006 was about 0.01 ppm, and the NAAQS for SO 2 is

0.03 ppm (annual arithmetic mean) [18] .

A high sulfuric acid content of industrial and urban atmospheres shortens the life of metal structures (see Tables 9.2 and 9.3 ). The effect is most pronounced for metals that are not particularly resistant to sulfuric acid, such as zinc, cadmium, nickel, and iron. It is less pronounced for metals that are more resistant to dilute sulfuric acid, such as lead, aluminum, and stainless steels. Copper, forming a pro- tective basic copper sulfate fi lm, is more resistant than nickel or 70% Ni – Cu alloy, on which the corresponding fi lms are less protective. In the industrial atmosphere

of Altoona, Pennsylvania, galvanized steel sheets [0.381 kg zinc per m 2 , 0.028 mm thick (1.25 oz zinc per ft 2 , 1.1 mil thick)] began to rust after 2.4 years, whereas in

the rural atmosphere of State College, Pennsylvania, rust appeared only after

14.6 years [19] . Copper exposed to industrial atmospheres forms a protective green - colored

corrosion product called a patina , composed mostly of basic copper sulfate, CuSO 4 · 3Cu(OH) 2 . A copper - covered church steeple on the outskirts of a town may develop such a green patina on the side facing prevailing winds from the city, but remain reddish - brown on the opposite side, where sulfuric acid is less readily available. Near the seacoast, a similar patina forms, composed in part of basic copper chloride.

FAC TORS INFLUENCING CORROSIVIT Y OF THE ATMOSPHERE

Industrial atmospheres can cause S.C.C of copper - base alloys, mostly accounted for by presence of nitrogen oxides (see Section 20.2.3 ). Copper did not fail, but copper alloys containing > 20% Zn failed on exposures for up to 8

years [20] . Nickel is quite resistant to marine atmospheres, but is sensitive to sulfuric acid of industrial atmospheres (Table 9.2 ), forming a surface tarnish composed of basic nickel sulfate. Corrosion in the industrial atmosphere of New York City is about 30 times higher than in the marine atmosphere of La Jolla, California, and about 20 times higher than in the rural atmosphere of State College, Pennsylvania (Table 9.2 ).

9.4.3 Moisture (Critical Humidity)

From previous discussions, it is apparent that, in an uncontaminated atmo- sphere at constant temperature, appreciable corrosion of a pure metal surface would not be expected at any value of relative humidity below 100%. Practi- cally, however, because of normal temperature fl uctuations (relative humidity increases on decrease of temperature) and because of hygroscopic impurities in the atmosphere or in the metal itself, the relative humidity must be reduced to values much lower than 100% in order to ensure that no water condenses on the surface. In very early studies, Vernon discovered that a critical relative humidity exists below which corrosion is negligible [21] . Experimental values for the critical relative humidity are found to fall, in general, between 50% and 70% for steel, copper, nickel, and zinc. Typical corrosion behavior of iron as a function of relative humidity of the atmosphere is shown in Fig. 9.3 . In a complex or severely polluted atmosphere, a critical humidity may not exist [22] .

An important factor determining susceptibility to atmospheric corrosion of

a metal in a particular environment is the percentage of time that the critical humidity is exceeded [23] . This period of time is called the “ time of wetness. ” It is determined by measuring the potential between a corroding metal specimen and a platinum electrode [23, 24] . Surface moisture from either precipitation or condensation is the cell electrolyte. To estimate atmospheric corrosion rates, Kucera et al. designed the device shown in Fig. 9.4 [25, 26] . The cell B is placed about 1 m above ground level with the surface inclined at 45 ° . An electronic integrator automatically integrates cell currents over extended periods of time. Calibration using data from weight - loss experiments at the same site shows that the electrochemical technique is suitable for estimating short - term variations in corrosion rates [26] .

Atmospheric corrosion testing is important to the suppliers of metals and to the engineers and architects who use metals under atmospheric conditions. Reviews have been prepared summarizing the atmospheric corrosion standards and testing procedures of the American Society for Testing and Materials

200 ATMOSPHERIC CORROSION

Figure 9.3. Corrosion of iron in air containing 0.01% SO 2 , 55 days ’ exposure, showing critical humidity [21] .

Figure 9.4. General arrangement of electrochemical device for measurement of atmospheric corrosion: A is a zero resistance ammeter, the circuit of which is shown on the right; B is an electrochemical cell with (a) electrodes and (b) insulators; C is an external emf. [25, 26] . ( Copy- right ASTM INTERNATIONAL. Reprinted with permission. )

REMEDIAL MEASURES