70 Chapter Two Atmospheric Environment Data on atmospheric parameters (humidity, SO 2 etc) Exposure tests Data evaluation Corrosion measurements Corrosivity Classification Corrosion Rate Guidelines Coating Performance Guidelines Materials Selection and Corrosion Control Measures Algorithms (e.g. ISO 9223) Correlation with historical performance data Figure 2.7 Two fundamental approaches to classifying atmospheric corrosivity. TABLE 2.1 List of ISO Standards Related to Atmospheric Corrosion ISO standard Title ISO 9223 Classification of the Corrosivity of Atmospheres ISO 9224 Guiding Values for the Corrosivity Categories of Atmospheres ISO 9225 Aggressivity of Atmospheres—Methods of Measurement of Pollution Data ISO 9226 Corrosivity of Atmospheres—Methods of Determination of Corrosion Rates of Standard Specimens for the Evaluation of Corrosivity 0765162_Ch02_Roberge 9/1/99 4:01 Page 70 Industrial pollution by SO 2 . Two types of units are used: Concentration (␮gиm Ϫ3 ), P C P C Յ 40 P 1 40 Ͻ P C Յ 90 P 2 90 Ͻ P C P 3 Deposition rate (mgиm Ϫ2 иday Ϫ1 ), P D P D Յ 35 P 1 35 Ͻ P D Յ 80 P 2 80 Ͻ P D P 3 Corrosion rate categories. Two types of corrosion rates are predicted: Category Short-term, gиm Ϫ2 иyear Ϫ1 Long-term, ␮mиyear Ϫ1 C 1 CR Յ 10 CR Յ 0.1 C 2 10 Ͻ CR Յ 200 0.1 Ͻ CR Յ 1.5 C 3 200 Ͻ CR Յ 400 1.5 Ͻ CR Յ 6 C 4 400 Ͻ CR Յ 650 6 Ͻ CR Յ 20 C 5 650 Ͻ CR 20 Ͻ CR The TOW categorization is presented in Table 2.2, and the sulfur dioxide and chloride classifications are presented in Table 2.3. TOW values can be measured directly with sensors, or the ISO definition of TOW as the number of hours that the relative humidity exceeds 80 percent and the temperature exceeds 0°C can be used. The methods for determining atmospheric sulfur dioxide and chloride deposition rates are described more fully in the relevant standards (Table 2.1). Following the categorization of the three key variables, the applica- ble ISO corrosivity category can be determined using the appropriate ISO chart (Table 2.4). Different corrosivity categories apply to different types of metal. As the final step in the ISO procedure, the rate of atmo- spheric corrosion can be estimated for the determined corrosivity cate- gory. Table 2.5 shows a listing of 12-month corrosion rates for different Environments 71 TABLE 2.2 ISO 9223 Classification of Time of Wetness Time of wetness, Time of wetness, Examples of Wetness category % hours per year environments T 1 Ͻ0.1 Ͻ10 Indoor with climatic control T 2 0.1–3 10–250 Indoor without climatic control T 3 3–30 250–2500 Outdoor in dry, cold climates T 4 30–60 2500–5500 Outdoor in other climates T 5 Ͼ60 Ͼ 5500 Damp climates 0765162_Ch02_Roberge 9/1/99 4:01 Page 71 TABLE 2.3 ISO 9223 Classification of Sulfur Dioxide and Chloride “Pollution” Levels Sulfur dioxide Sulfur dioxide deposition rate, Chloride Chloride deposition rate, category mg/m 2 иday category mg/m 2 иday P 0 Յ10 S 0 Յ3 P 1 11–35 S 1 4–60 P 2 36–80 S 2 61–300 P 3 81–200 S 3 301–1500 TABLE 2.4 ISO 9223 Corrosivity Categories of Atmosphere TOW Cl Ϫ SO 2 Steel Cu and Zn Al T 1 S 0 or S 1 P 1 111 P 2 111 P 3 1–2 1 1 S 2 P 1 112 P 2 112 P 3 1–2 1–2 2–3 S 3 P 1 1–2 1 2 P 2 1–2 1–2 2–3 P 3 223 T 2 S 0 or S 1 P 1 111 P 2 1–2 1–2 1–2 P 3 2 2 3–4 S 2 P 1 2 1–2 2–3 P 2 2–3 2 3–4 P 3 334 S 3 P 1 3–4 3 4 P 2 3–4 3 4 P 3 4 3–4 4 T 3 S 0 or S 1 P 1 2–3 3 3 P 2 3–4 3 3 P 3 4 3 3–4 S 2 P 1 3–4 3 3–4 P 2 3–4 3–4 4 P 3 4–5 3–4 4–5 S 3 P 1 4 3–4 4 P 2 4–5 4 4–5 P 3 545 T 4 S 0 or S 1 P 1 333 P 2 4 3–4 3–4 P 3 5 4–5 4–5 S 2 P 1 4 4 3–4 P 2 444 P 3 555 S 3 P 1 555 P 2 555 P 3 555 T 5 S 0 or S 1 P 1 3–4 3–4 4 P 2 4–5 4–5 4–5 P 3 555 S 2 P 1 555 P 2 555 P 3 555 S 3 P 1 555 P 2 555 P 3 555 0765162_Ch02_Roberge 9/1/99 4:01 Page 72 metals for different corrosivity categories. The establishment of corro- sion rates is complicated by the fact that these rates are not linear with time. For this reason, initial rates after 1 year and stabilized longer- term rates have been included for the different metals in the ISO methodology. In situations in which TOW and pollution levels cannot be deter- mined conveniently, another approach based on the exposure of stan- dardized coupons over a 1-year period is available for classifying the atmospheric corrosivity. Simple weight loss measurements are used for determining the corrosivity categories. The nature of the specimens used is discussed more fully in a later section of this chapter. Although the ISO methodology represents a rational approach to cor- rosivity classification, it has several inherent limitations. The atmos- pheric parameters determining the corrosivity classification do not include the effects of potentially important corrosive pollutants or impurities such as NO x , sulfides, chlorine gas, acid rain and fumes, deicing salts, etc., which could be present in the general atmosphere or be associated with microclimates. Temperature is also not included as a variable, although it could be a major contributing factor to the high corrosion rates in tropical marine atmospheres. Only four standardized pure metals have been used in the ISO testing program. The method- ology does not provide for localized corrosion mechanisms such as pit- ting, crevice corrosion, stress corrosion cracking, or intergranular corrosion. The effects of variables such as exposure angle and shelter- ing cannot be predicted, and the effects of corrosive microenvironments and geometrical conditions in actual structures are not accounted for. Dean 13 has reported on a U.S. verification study of the ISO method- ology. This study was conducted over a 4-year time period at five expo- sure sites and with four materials (steel, copper, zinc, and aluminum). Environmental data were used to obtain the ISO corrosivity classes, and these estimates were then compared to the corrosion classes obtained by direct coupon measurement. Overall, agreement was found in 58 percent of the cases studied. In 22 percent of the cases the estimated corrosion class was lower than the measured, and Environments 73 TABLE 2.5 ISO 9223 Corrosion Rates after One Year of Exposure Predicted for Different Corrosivity Classes Steel, Copper, Aluminum, Zinc, Corrosion category g/m 2 иyear g/m 2 иyear g/m 2 иyear g/m 2 иyear C 1 Յ10 Յ0.9 Negligible Յ0.7 C 2 11–200 0.9–5 Յ0.6 0.7–5 C 3 201–400 5–12 0.6–2 5–15 C 4 401–650 12–25 2–5 15–30 C 5 651–1500 25–50 5–10 30–60 0765162_Ch02_Roberge 9/1/99 4:01 Page 73 in 20 percent of the cases it was higher. It was also noted that the selected atmospheric variables (TOW, temperature, chloride deposi- tion, sulfur dioxide deposition, and exposure time) accounted for a major portion of the variation in the corrosion data, with the excep- tion of the data gathered for the corrosion of aluminum. Further refinements in the ISO procedures are anticipated as the worldwide database is developed. ISO corrosivity analysis at two air bases. Use of the ISO methodology can be illustrated by applying it to a corrosivity assessment performed for two contrasting air bases: a maritime base in Nova Scotia and an inland base in Ontario (Fig. 2.8). The motivation for determining atmospher- ic corrosivity at these locations can be viewed in the context of the ideal- ized corrosion surveillance strategy shown in Fig. 2.9. Essentially this scheme revolves around predicting where and when the risk of corro- sion damage is greatest and tailoring corrosion control efforts accord- ingly. The principle and importance of linking selected maintenance and inspection schedules to the prevailing atmospheric corrosivity has been described in detail elsewhere. 14 An underlying consideration in these recommendations is that military aircraft spend the vast major- ity of their lifetime on the ground, and most corrosion damage occurs at ground level. The ISO TOW parameter could be derived directly from relative humidity and temperature measurements performed hourly at the bases. The average daily TOW at the maritime base is shown in Fig. 2.10, together with the corresponding ISO TOW categories, as deter- mined by the criteria of Table 2.2. The overall TOW profile for the inland base was remarkably similar. In the case of the air bases, no directly measured data were avail- able for the chloride and sulfur dioxide deposition rates. However, data pertaining to atmospheric sulfur dioxide levels and chloride levels in precipitation had been recorded at sites in relatively close proximity. On the basis of these data, the likely ISO chloride and sulfur dioxide categories for the maritime base were S 3 and P 0 –P 1 , respectively. Under these assumptions, the applicable ISO corrosivity ratings are at the high to very high levels (C 4 to C 5 ) for aluminum. Using ISO chlo- ride and sulfur dioxide categories of S 0 and P 0 –P 1 , respectively, for the inland air base, the corrosivity rating for aluminum is at the C 3 level. The step-by-step procedure for determining these categories and the different corrosion rates predicted for aluminum at the two bases are shown in Fig. 2.11. The main implications of the analysis of atmospheric corrosivity at the maritime air base are that aircraft are at considerable risk of cor- rosion damage in view of the high corrosivity categories and that the 74 Chapter Two 0765162_Ch02_Roberge 9/1/99 4:01 Page 74 Environments 75 Maritime Air Base Eastern Ontario Kingston USA Nova Scotia Atmospheric Monitoring Station Atmospheric Monitoring Station Atmospheric Monitoring Station Inland Air Base (b) (a) Lake Ontario (fresh water) Figure 2.8 Geographical location of two Canadian air bases: (a) a maritime air base on the Bay of Fundy; (b) an inland air base on the shore of Lake Ontario. 0765162_Ch02_Roberge 9/1/99 4:01 Page 75 Corrosion Sensors The Base Micro-Environment on-board smart structure ground level temperature humidity rainfall pollution wind direction & speed seasonal fluctuations Corrosion Signals Climate and Weather Data Management Information for Optimized Corrosion Control Figure 2.9 An idealized corrosion surveillance strategy. Jan Mar May Jul Sep Nov Time of Year Average Daily Fractional TOW 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 T3 T4 T5 Figure 2.10 Average time of wetness (TOW) at a maritime air base. 76 Chapter Two 0765162_Ch02_Roberge 9/1/99 4:01 Page 76 fluctuations in corrosivity with time deserve special attention. Present “routine” maintenance and inspection schedules and corrosion control efforts do not take such variations into account. As a simple example of how corrosion control could be improved by taking such variations into account, the effects of aircraft dehumidifi- cation can be considered. It is assumed that dehumidification would be applied only on a seasonal basis, when the T 4 TOW category is Environments 77 Maritime Air Base Inland Air Base Determine ISO TOW categories from temperature and humidity data T4 (summer) T3 (winter) T4 (summer) T3 (winter) Estimate chloride deposition rates from atmospheric data and determine ISO categories S3 S0 Estimate sulfur dioxide deposition rates from atmospheric data and determine ISO categories P0-P1 P0-P1 Use ISO 9223 to determine corrosivity categories for aluminum C5 (summer) C4 (winter) C3 (summer, winter) Use ISO 9223 to estimate first year uniform corrosion rates for aluminum >5g/m year (summer) 2-5 g/m 2 year (winter) 2 0.6-2 g/m 2 year (summer,winter) Figure 2.11 Detailed procedure for determining the ISO corrosivity categories. 0765162_Ch02_Roberge 9/1/99 4:01 Page 77 reached on a monthly average (refer to Fig. 2.10). It is further assumed that the time of wetness can be reduced to an average T 3 level in these critical months by the application of dehumidification systems. The emphasis in dehumidification should be placed on the nighttime, on the basis of Fig. 2.12. The projected cumulative corro- sion rates of aluminum with and without this simple measure, based on ISO predictions, are shown in Fig. 2.13. The S 3 chloride and P 1 sul- fur dioxide categories were utilized in this example, together with the most conservative 12-month corrosion rates of the applicable ISO cor- rosivity ratings. The potential benefits of dehumidification, even when it is applied only in selected time frames, are readily apparent from this analysis. Aircraft dehumidification is a relatively simple, practical procedure utilized for aircraft corrosion control in some countries. Dehumidified air can be circulated through the interior of the aircraft, or the entire aircraft can be positioned inside a dehu- midified hangar. It should be noted that the numeric values for uni- form corrosion rates of aluminum predicted by the ISO analysis are not directly applicable to actual aircraft, which are usually subject to localized corrosion damage under coatings or some other form of cor- rosion prevention measures. Corrosivity classification according to PACER LIME algorithm. An environ- mental corrosivity scale based on atmospheric parameters has been developed by Summitt and Fink. 15 This classification scheme was developed for the USAF for maintenance management of structural air- craft systems, but wider applications are possible. A corrosion damage algorithm (CDA) was proposed as a guide for anticipating the extent of corrosion damage and for planning the personnel complement and time required to complete aircraft repairs. This classification was developed primarily for uncoated aluminum, steel, titanium, and magnesium air- craft alloys exposed to the external atmosphere at ground level. The section of the CDA algorithm presented in Fig. 2.14 considers distance to salt water, leading either to the very severe AA rating or a consideration of moisture factors. Following the moisture factors, pollutant concentrations are compared with values of Working Environmental Corrosion Standards (WECS). The WECS values were adopted from the 50th percentile median of a study aimed at determining ranges of environmental parameters in the United States and represent “averages of averages.” For example, if any of the three pollutants sulfur dioxide, total suspended particles, or ozone level exceeds the WECS values, in combination with a high moisture factor, the severe A rating is obtained. An algorithm for air- craft washing based on similar corrosivity considerations is presented in Fig. 2.15. 78 Chapter Two 0765162_Ch02_Roberge 9/1/99 4:01 Page 78 Environments 79 February August Hour of day Fractional TOW 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 3 5 7 9 11 13 15 17 T3 T4 T5 19 21 23 Figure 2.12 Relative TOW as a function of time of day for a dry month (February) and a humid month (August) at a maritime air base. Cumulative corrosion rate (g m ) -2 Month of the year With Dehumidification in Critical Months No Dehumidification 1 0 0.5 1 1.5 2 2.5 3 3.5 4 23 45678 9101112 Figure 2.13 Projected cumulative corrosion rates of aluminum with and without dehu- midification. 0765162_Ch02_Roberge 9/1/99 4:01 Page 79 [...]... magnesite, it has properties similar to those of calcium carbonate 076 516 2_Ch02_Roberge 9 /1/ 99 4: 01 TABLE 2 .11 Comparison of Hardness Units mval/L 50 mg CaCO3 1 mval/L 1 °dH 1 °fH 1 °eH 1 ppm 1 mmol/L 1 0.357 0.2 0.285 0.02 2 °dH (German) 10 mg CaO 2.8 1 0.5599 0.7999 0.056 5.6 °fH (French) 10 mg CaCO3 5 1. 786 1 1 .42 9 0 .1 10 °eH (British) 14 .3 mg CaCO3* 3. 51 1.25 0.7 1 0.07 7 *One grain CaCO3 per gallon †The... (American) 1 mg CaCO3 50 17 .86 10 14 .29 1 100 mmol/L (international)† 10 0 mg CaCO3 0.5 0 .17 86 0 .1 0 . 14 29 0. 01 1 Page 95 Hardness units per liter water 95 076 516 2_Ch02_Roberge 96 9 /1/ 99 4: 01 Page 96 Chapter Two 4 Magnesium bicarbonate [Mg(HCO3)2] bonate in its properties Similar to calcium bicar- Permanent hardness salts 1 Calcium sulfate (CaSO4 ) Also known as gypsum, used to make plaster of paris Gypsum... types of algae and slimeforming bacteria This topic is covered in Sec 2.6 076 516 2_Ch02_Roberge 92 9 /1/ 99 4: 01 Page 92 Chapter Two TABLE 2 .10 Typical Water Analyses (Results in ppm) A pH 6.3 Alkalinity 2 Total hardness 10 Calcium hardness 5 Sulfate 6 Chloride 5 Silica Trace Dissolved solids 33 B C D E F 6.8 38 53 36 20 11 0.3 88 7 .4 90 12 0 85 39 24 3 18 5 7.5 18 0 230 210 50 21 4 332 7 .1 250 340 298 17 4. ..076 516 2_Ch02_Roberge Page 80 Chapter Two -3 Humidity or Rain _ < 12 5 cm/yr < < < B 4. 5 km 7 .1 g m -3 12 5 cm/yr -3 43 µg m -3 61 µg m 36 µg m-3 SO2 TSP O3 SO2 TSP O3 _ < 43 µg m _ < 61 µg m _ < 36 µg m -3 -3 -3 43 µg m -3 61 µg m-3 36 µg m _ . 3 4 P 3 3 34 S 3 P 1 3 4 3 4 P 2 3 4 3 4 P 3 4 3 4 4 T 3 S 0 or S 1 P 1 2–3 3 3 P 2 3 4 3 3 P 3 4 3 3 4 S 2 P 1 3 4 3 3 4 P 2 3 4 3 4 4 P 3 4 5 3 4 4–5 S 3 P 1 4 3 4 4 P 2 4 5 4 4–5 P 3 545 T 4 S 0 or S 1 P 1 333 P 2 4. S 1 P 1 111 P 2 11 1 P 3 1 2 1 1 S 2 P 1 112 P 2 11 2 P 3 1 2 1 2 2–3 S 3 P 1 1–2 1 2 P 2 1 2 1 2 2–3 P 3 223 T 2 S 0 or S 1 P 1 111 P 2 1 2 1 2 1 2 P 3 2 2 3 4 S 2 P 1 2 1 2 2–3 P 2 2–3 2 3 4 P 3 3 34 S 3 P 1 3 4. S 1 P 1 333 P 2 4 3 4 3 4 P 3 5 4 5 4 5 S 2 P 1 4 4 3 4 P 2 44 4 P 3 555 S 3 P 1 555 P 2 555 P 3 555 T 5 S 0 or S 1 P 1 3 4 3 4 4 P 2 4 5 4 5 4 5 P 3 555 S 2 P 1 555 P 2 555 P 3 555 S 3 P 1 555 P 2 555 P 3 555 076 516 2_Ch02_Roberge