1、UDC 620.193.4 : 669 DEUTSCHE NORM January 1987 Loss in mass Density Corrosion of metals Corrosion testing I*ml g e g . cm-3 Depth of attack Number of localized corrosion attacks 1 mm ni - I 50905 Corrosion characteristics under nonuniform and localized corrosion attack without mechanical stress Part
2、 3 Korrosion der Metalle; Korrosionsuntersuchungen; Korrosionsgren bei ungleichmiger und rtlicher Korrosion ohne mechanische Belastung Supersedes January 1975 edition 1 Scope and field of application The corrosion of metals can produce a great variety of corrosion effects (.e. forms of corrosion). T
3、his standard deals specifically with the corrosion characteristics which may be used to describe corrosion behaviour under non- uniform or localized corrosion attack without simulta- neous mechanical stress. It applies essentially to the follow- ing forms of corrosion: a) wide pitting; b) deep pitti
4、ng; c) selective corrosion; d) intergranular corrosion. See DIN 50905 Part 2 for corrosion characteristics under uniform 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 i
5、n the properties of the material (cf. DIN 50 905 Part 1). Table 1. Measured quantities unit Quantity Area exposed to corrosion S mm Specimen thickness before corrosion test Minim um spec men thickness after corrosion test mm Smin ItIh Exposure time I AI I cm2 Area of localized corrosion attack 2 Mea
6、sured quantities and their determination The measured quantities given in table 1 shall be used to calculate corrosion characteristics. Which measured quantities are to be determined depends on the form of corrosion; see table 2. Table 2. Measured quantities for calculating corrosion characteristics
7、 as a function of the form of corrosion I Measured quantities I Depth of attack, 1 Number of loca I zed corrosion attacks, nl Area of localized corrosion attack, Al Form of corrosion Loss in corrosion, Exposure time, t X Wide pitting Deep pitting X X X I- 1-1 Selective corrosion Intergranular corros
8、ion X x = determination of quantity is re uired (x) = determination of quantity may%e appropriate. - = determination of quantity is not necessary. I Continued on pages 2 to 4 Beuth Verlag GmbH. Berlin. has the exclusive right of sale for German Standards (01N-Normen). 01.90 Sales No. 0105 DIN 50 905
9、 Part 3 Engl. Price group Page 2 DIN 50905 Part 3 2. Area exposed to corrosion The surface area exposed to corrosion,A, shall be meas- ured before the corrosion test is commenced. 2.2 Specimen thickness before corrosion test The specimen thickness, s, shall generally be measured before the corrosion
10、 test with the aid of a precision instrument. 2.3 Loss in mass The loss in mass, I Am I, shall generally be determined by weighing before and after the corrosion test. See DIN 50905 Part 1 for specimen treatment before initial weighing and for the removal of adheringcorrosion prod- ucts before final
11、 weighing. Note. The loss in mass may also be established by other methods, e.g. by determining the quantity of hydrogen generated or by quantitative deter- mination of the metal ions passing into solution. These methods are more complicated than deter- mining the loss in mass by weighing, but are u
12、sed in special corrosion tests. 2.4 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. 2.5 Exposure time The exposure time, t, is the
13、time elapsing between the beginning and end of the corrosion test, less the duration of any interruptions. Note. As a rule, heating and cooling times shall be deducted. 2.6 Depth of attack 2.6.1 Depth of attack in the case of wide pitting To determine the depth of attack, 1, in the case of wide pitt
14、ing, parts of the original surface must be preserved as reference areas, as otherwise the maximum depth of attack cannot be established. The corrosion test shall thus be set up so that the material is exposed to corrosion attack on one side only, or so that parts of the original surface are protecte
15、d against corrosion by masking. Figure 1 illustrates how the depth of attack is to be measured. Unattacked surface In case where corrosion is limited to one side (see figure 11, the depth of attack is obtained from the equation 1 = s - Srni“ (1) where s- is the minimum specimen thickness after the c
16、orrosion test. When both sides are exposed to corrosion (see figure 2), the depth of attack, 1, can be measured directly on both sides of the specimen, the protected surface area being taken as reference areas (see figure 2). Any adhering corrosion products shall be removed before s- and the depths
17、of attack, 21 and 12, are determined, measurements preferably being made with the aid of a precision gauge or by optical focusing. See DIN 50 905 Part 1 for the method of removing adhering corrosion products. 2.6.2 Depth of attack in the case of deep pitting In the case of deep pitting, the depth of
18、 attack shall be measured directly with the aid of a precision gauge or by optical focusing once adhering corrosion products have been removed (see DIN 50905 Part 1). Direct measure- ment is possible as there are usually sufficiently large uncorroded areas on the surface to serve as reference areas.
19、 In cases where individual points of attack merge so as to leave no reference area, the depth of attack shall be determined as described in subclause 2.6.1. If the diameter of the pits is small, or their shape is unsuitable for measurement with the aid of a precision gauge or by optical focusing, th
20、e depth of attack can be determined on the basis of metallographic sections (micrographs). 2.6.3 Depth of attack in the case of selective corrosion The method used to determine the depth of attack depends on the form of corrosion. As a rule, it shall be measured directly on micrographs. In the case
21、of inter- granular corrosion, the corrosion attack can be more easily recognized if the specimen is first bent. If the original surface is intact, the depth of attack can be measured directly on the micrograph. If the original surface is not intact, e.g. as a result of thegranular micro- structure b
22、eing broken up under heavy intergranular corrosion, the depth of attack can be determined by measuring the unattacked core zone, and, given that Masking Masked surface Figure 1. Determining the depth of attack with exposure on one side only U Figure 2. Determining the depth of attack with exposure t
23、o corrosion on both sides DIN 50 905 Part 3 Page 3 Maximum depth of attack both sides of the specimen are exposed to corrosion and assuming that the depth of penetration of the corrosion attack is the same on both sides, calculated on the basis of equation (2): mm Lax (2) where s- is the specimen th
24、ickness unaffected by inter- granular corrosion after the corrosion test (see figure 3). S - hin I=- 2 2.7 Number of localized corrosion attacks The number of localized corrosion attacks, nl, can only be determined if the pit nucleating sites remain distinct from each other. That is a characteristic
25、 feature of deep pitting, but may also occur in other forms of corrosion. Original surface ,Surface after exposure to corrosion i 1 Unattacked zone Zone of intergranular attack Figure 3. Determining the depth of attack in the case of selective corrosion, taking intergranular corrosion as an example
26、If the number of pit nucleating sites is small, all such sites on the specimen surface shall be counted, the cut surfaces of the specimen being disregarded. In the case of speci- mens with a large surface area, or exhibiting a large number of points of attack, counting may be confined to a part of t
27、he surface representative of the whole. 2.8 Area of localized corrosion attack Planimetric methods may be used to determine the area of localized corrosion attack,Al, if the form of corrosion so permits. 3 Corrosion characteristics and their calculation 3.1 Corrosion characteristics in the case of w
28、ide pitting The following corrosion characteristics can be calculated as specified in subclauses 3.1 .I to 3.1.4 on the basis of the relevant measured quantities given in tables 1 and 2. 3.1.1 Average reduction in thickness The average reduction in thickness, AC, of the specimen, in mm, shall be cal
29、culated from the absolute value of the loss in mass, I Am I, the surface area exposed to corrosion, A, and the density, , on the basis of equation (3): (3) The average reduction in thickness is an operand used for determining the depth factor (see subclause 3.1.4) and corresponds to the reduction in
30、 thickness which would result from the same loss in mass if the corrosion attack were uniform. Table 3. Corrosion characteristics in the case of wide pitting I Conventional unit Characteristic Symbol Average reduction I AS I. mm in thickness Maximum rate WI. max mm.a-*) of penetration I I Depth fact
31、or IfI- *) la = 365 days. 3.1.2 Maximum depth of attack The maximum depth of attack, I,. is the greatest measured depth of attack. 3.1.3 Maximum rate of penetration The maximum rate of penetration, wl, max, shall be calculated from the maximum depth of attack, I, and the exposure time, t, on the bas
32、is of equation (4): (4) The maximum rate of penetration calculated in this way is an integral corrosion characteristic (see DIN 50905 Part 2). If the rate of of penetration is approximately constant over time, or if the maximum depth of penetration increases in accordance with a known time curve, th
33、e depth of penetration can be extrapolated to an extended exposure time and used as a criterion for assessing the service life of a component. If this is the case, it shall be verified by carrying out a series of appropriate tests (see DIN 50 905 Part 1). 3.1.4 Depth factor The depth factor, f, shal
34、l be calculated from the maximum depth of attack, E, and the average reduction in thick- ness, AS, on the basis of equation (5): lmax f=-r As (5) The depth factor, f, is a ratio of quantities which gives the factor by which the reduction in thickness in the deepest pit exceeds the reduction in thick
35、ness which would result from the determined loss in mass if the corrosion attack were uniform. By definition, the depth factor is not less than unity. The depth factor, f, approaches unity, when wide pitting merges into uniform corrosion. 3.2 Corrosion characteristics in the case of deep pitting The
36、 following corrosion characteristics can be calculated as specified in subclauses 3.2.1 to 3.2.3 on the basis of relevant quantities given in tables 1 and 2. Page 4 DIN 50905 Part 3 Table 4. Corrosion characteristics in the case of deep pitting In comparison tests, the pit density may be an indicati
37、on of the susceptibility of the material to deep pitting. Characteristic Maximum depth of attack Maximum rate of penetration Pit density Proportional area of localized corrosion attack *) la = 365 days. I Symbol I conventional unit 3.2.1 Maximum depth of attack The maximum depth of attack, l, is the
38、 greatest measured pit depth. 3.2.2 Maximum rate of penetration The maximum rate of penetration, zul.-, shall be calculated from the maximum depth of attack, 1-, and the exposure time, t, on the basis of equation (4). 3.2.3 Pit density The pit density,z, shall be calculated from the number of locali
39、zed corrosion attacks, ni, and the surface area exposed to corrosion,A, on the basis of equation (6): In the case of specimens with a large surface area, or exhibiting a large number of points of attack, evaluation may be confined to a part of the surface representative of the whole. 3.2.4 Proportio
40、nal area of localized corrosion attack The proportional area of localized corrosion attack is obtained by dividing the value for the area of localized corrosion attack, Al, by the value for the surface area exposed to corrosion, A. 3.3 Corrosion characteristics The following corrosion characteristic
41、s can be calculated as specified in subclauses 3.3.1 and 3.3.2 on the basis of the relevant measured quantities given in tables 1 and 2. in the case of selective corrosion Table 5. Corrosion characteristics in the case of selective corrosion I Conventional unit I Characteristic I Maximum depth I of
42、attack Maximum rate mm a- *) ! of penetration Wi. max I Wi. max I of penetration I *) la = 365 days. 3.3.1 Maximum depth of attack The maximum depth of attack, E, is the greatest measured depth of attack. 3.3.2 Maximum rate of penetration The maximum rate of penetration, w, shall be cal- culated fro
43、m the maximum depth of attack, I, and the exposure time, t, on the basis of equation (4). Standards referred to DIN 1306 Density; concepts and presentation of values DIN 50905 Part 1 DIN 50 905 Part 2 Corrosion of metals; corrosion testing; principles Corrosion of metals; corrosion testing; corrosio
44、n characteristics under uniform corrosion attack 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 3: 01.75. Amendments In comparison with the January 1975 edition, the content and layout of the standard have been revised. Explanatory notes This standard has been prepared by Technical Committee 17 1 Korrosion und Korrosionsschutz of the NormenausschuR Materialprfung (Materials Testing Standards Committee) of DIN. International Patent Classification C23F11 C23F13 C23F15 GOlN17/00
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