1、UDC 620.193.4 : 669 DEUTSCHE NORM January 1987 Corrosion of metals Corrosion testing Corrosion characteristics under uniform corrosion attack DIN 50 905 Part 2 Korrosion der Metalle; Korrosionsuntersuchungen; Korrosionsgroen bei gleichmiger Flchenkorrosion Quantity 1 Scope and field of application T
2、he corrosion of metals can produce a great variety of corrosion effects (.e. forms of corrosion). This standard deals specifically with the corrosion characteristics which may be used to calculate the rate of corrosion in cases where surface removal isvirtually uniform. See DIN 50905 Part 3 for corr
3、osion characteristics under nonuniform and localized corrosion attack. Besides establishing the values of the measurands and calculating corrosion characteristics, it may in some cases be of significance to determine other changes in the properties of the material (cf. DIN 50905 Part 1 ). See DIN 50
4、905 Part 1 for other relevant standards. Conventional unit Symbol 2 Measured quantities and their determination Since corrosion characteristics are generally calculated on the basis of changes in mass, the following measured quantities must be determined. Surface area ex- posed to corrosion Change i
5、n mass Table 1. Measured quantities A m2 I AM I 9 Density e g.rn- Exposure ti me t h 2.1 Surface area exposed to corrosion The surface area exposed to corrosion, A, shall be measured before the corrosion test is commenced. 2.2 Change in mass The change in mass shall be determined by weighing. It is
6、equal to the difference in the mass of the specimen before and after the corrosion test. If no corrosion products adhere to the specimen, its change in mass is always negative and corresponds to the loss in mass which has occurred during corrosion. Supersedes January 1975 edition. If all corrosion p
7、roducts produced adhere, the change in mass is always positive, and is equal to the combined mass of the oxidizing agent and the components of the corrosive medium (agent) which have been consumed in producing the corrosion products. If only some of the corrosion products adhere to the specimen, the
8、 change in mass may be positive or negative, and cannot be used directly for calculating the corrosion rate. 22.1 Loss in mass The loss in mass is equal to the negative value of the change in mass, I Am I. See DIN 50 905 Part 1 for treat- ment of the specimens before initial weighing and for the rem
9、oval of adhering corrosion products before final weighing. Note. The loss in mass may also be determined by other methods, e.g. from the quantity of hydrogen generated, the quantity of oxidizing agent (e.g. oxygen) consumed or by quantitative deter- mination of the metal ions passing into solution.
10、These methods are more complicated than determining the loss in mass by weighing, but are used in special corrosion tests. 2.2.2 Gain in mass The gain in mass is equal to the positive value of the change in mass. See DIN 50905 Part 1 for determination of the loss in mass from the measured gain in ma
11、ss in cases where the chemical composition of the corrosion product is known. 2.3 Density For most materials, tables giving the density, are available. If not, it shall be obtained by calculation. In the case of porous solids, the apparent density shall be taken as the density value; see DIN 1306. 2
12、.4 Exposure time The exposure time, t, is the time elapsing between the beginning and the end of the corrosion test, less the duration of any interruptions. Note. As a rule, heating and cooling times shall be sub- tracted. 3 Corrosion characteristics and their calculation The following corrosion cha
13、racteristics can be calculated from the measured quantities. Continued on pages 2 to 4 euth Verlag Gmbii. Berlin. has the exclusive right of sale for German Standards (DIN-Normen) 01.90 DIN 50 905 Part 2 Engi. Price group 5 Sales No. O1 05 Page 2 DIN 50905 Part 2 Table 2. Corrosion characteristics R
14、ate of removal I Conventional Characteristic ZIJ mm I a.- *) I I ma 1 g .m-* Loss in mass per unit area Gain in mass per unit area Reduction in thickness I g . n-2. h- I z Rate of loss in mass per unit area 3.1 Loss in mass per unit area The loss in mass per unit area, ma, is the absolute value of t
15、he loss in mass, I Am I, divided by the value of the surface area exposed to corrosion, A. The loss in mass per unit area shall not be used to compare directly the corrosion behaviour of metals of different densities. 3.2 Gain in mass per unit area The gain in mass per unit area, m, is the absolute
16、value of the gain in mass, IArnl, divided by the value of the surface area exposed to corrosion,A. The gain in mass per unit area shall not be used to compare directly the corrosion behaviour of metals of different densities. 3.3 Reduction in thickness The reduction in thickness, As, in mm, is calcu
17、lated from the loss in mass, IArnl, the surface area exposed to corrosion, A, and the density, e, in accordance with equation (I): The reduction in thickness allows a comparison to be made between the corrosion behaviour of metals of different densities exposed to corrosion for the same period. 3.4
18、Rate of loss in mass per unit area The rate of loss in mass per unit area, u, in g. m-2 . h-1, is obtained from the absolute value of the loss in mass, iam I, the surface area exposed to corrosion,A, and the exposure time, t, in accordance with equation (2): The rate of loss in mass per unit area sh
19、all not be used to compare metals of different densities. In such cases, the metals in question are best compared on the basis of the difference in reduction in their thickness, with due allowance being made for density (see subclause 3.31, over the same exposure time, or on that of the different ra
20、te of removal. Particularly when studying prolonged corrosion processes, allowance shall be made for the dependence of the corrosion on the exposure time by plotting loss in masdtime curves; these may vary quite considerably (see figure 1). The values obtained for the loss in mass per unit area may
21、be evaluated in three ways, as indicated under items a) to c) below and illustrated in figures 2a to 2c. a) Integral rate of loss in mass per unit area, uht (see figure 2a!, which is given by: ma Vi” = - 1 (3) (ubt corresponding to tan CY in figure 2a). b) Differential rate of loss in mass per unit
22、area, um (see figure 2b), which is given by: (uds corresponding to tan Bin figure 2b). c) Linear rate of loss in mass per unit area, vh (see figure Zc), which is given by: rn,(t2) - ma(tl 1 (t2 - t, 1 Illin = (uu, corresponding to tan y in figure 2c). In the above equations, ma (tl ) and ma (tp) are
23、 the loss in mass per unit area after exposure times tl and t2 respeciveiy. ulin is a specii case of a rate of loss in mass per unit area for the region where the curve of loss in mass over time follows a straight line. To determine this value, at least three measured points on the curve shall be in
24、 a straight line (see figure 212). The time intervals between the indi- vidual points shall follow an arithmetical or geometrical progression. The linear rate of loss in mass per unit area, uh, thus calculated allows a simple extrapolation to be made to longer exposure times, and hence an assessment
25、 to be made of the service life of engineering components. Note. If the variation over time is nonlinear, the rate of loss in mass can only be extrapolated if the type of variation curve is known (e.g. a parabolic curve in the case of the oxidation of metals in hot gases). 3.5 Rate of removal The ra
26、te of removal, w, in mm per year, is obtained from the rate of loss in mass per unit area, u, and the density, e, in accordance with equation 16): (6) By analogy with the various rates of loss in mass per unit area, the following rates of removal can be defined: a) integral rate of removal, wht; b)
27、differential rate of removal, wdif; c) linear rate of removal, %. These rates may be represented by diagrams similar to those illustrated in figures 2a to 2c. u w=- e DIN 50 905 Part 2 Page 3 The integral, differential and linear rates of removal are obtained from equation (6) by substituting the va
28、lues for u, vdir and u, for u. The linear rate of removal, wh, is of particular interest because, if extrapolated to an extended exposure time, it allows a comparison to be made between the service lives of engineering components made of different materiais. wint and wa are of minor importance in as
29、sessing corrosion behaviour. Note. The service life of an engineering component can also be estimated by extrapolation to an extended exposure time in cases where the loss in mass per unit area is nonlinear, but follows a known type of function (e.g. parabolic function for the oxidation of metals in
30、 hot gases), and if this variation over time of the loss in mass per unit area, or the reduction in thickness, is taken into account. Exposure time, t, in h Figure 1. Diagram showing loss in mass over time of exposure for three corrosion processes depending to different extents on the exposure time
31、E m m .- N e w 51 LE QU E m. v roc m .- - t Exposure time, t, in h a) Integral rate of loss in mass per unit area E Exposure time, t, in h t b) Differential rate of loss in mass per unit area 2 Exposure time, t, in h c) Linear rate of loss in mass per unit area Figure 2. Rates of loss in mass per un
32、it area Page 4 DIN 50 905 Part 2 Standards referred to DIN 1306 Density; concepts and presentation of values DIN 50905 Part i DIN 50 905 Part 3 Corrosion of metals; corrosion testing; principles Corrosion of metals; corrosion testing; corrosion characteristics under nonuniform and localized corrosio
33、n attack without mechanical stress Previous editions DIN 4850: 10.37; DIN 4851: 10.37; DIN 4852: 08.39; DIN 50901: 08.57; DIN 50906: 10.58; DIN 50905: 11.52; DIN 50905 Part 2: 01.75. Amendments In comparison with the January 1975 edition, form and content of the standard have been revised. Explanato
34、ry notes This standard has been prepared by Technical Committee 171 Korrosion und Korrosionsschutz of the Normenausschu Materialprfung (Mater i a Is Testi ng Standards Co mm it tee I. The classification of the linear rate of removal by characteristic values which was introduced in the January 1975 edition of DIN 50905 Part 2 has not been included in the new version, as it has not found general acceptance in industry. International Patent Classification C23F 11 C23F13 C23F15 G O1 N 17/00
