the anode for chromium plating according to the present invention is characterized as having an electrode catalyst coating comprising platinum and an oxide of tin dispersed in the platinum . although the electrode catalyst coating contains an oxide of tin , the ability of platinum to anodize trivalent chromium ions into hexavalent chromic acid can be fully maintained because the tin oxide itself is low in electrochemical catalytic activity , and hence has little influence on the platinum electrode . it is also possible , according to the present invention , to further diminish the consumption of platinum by dispersing iridium oxide , along with the tin oxide , into the platinum . in producing the anode of the present invention , the electrode catalyst coating comprising these components is formed on a substrate made of a valve metal such as titanium , tantalum , niobium , zirconium , hafnium , or an alloy thereof . however , since direct covering of the substrate with the electrode catalyst coating results in insufficient electrode performance , the coating is formed on the substrate not directly but through the medium of intermediate layers . that is , before the electrode catalyst coating is formed , a first intermediate layer comprising an oxide of tin is formed by coating the substrate with a solution containing a tin compound and then heat - treating the coating in an oxidizing atmosphere , and , furthermore , a second intermediate layer containing an oxide of tin and platinum metal is formed on the first intermediate layer . the first intermediate layer serves mainly to improve the adhesion of an electrode catalyst coating to the substrate and increase the electrical conductivity between the coating and the substrate . preferable effects can be obtained when the amount of the tin oxide fixed on the electrode substrate is from 0 . 5 g / m 2 to 30 g / m 2 . a more preferred range of the tin oxide amount is from 0 . 5 g / m 2 to 10 g / m 2 . amounts of the tin oxide covering the substrate exceeding 30 g / m 2 are not preferable because such amounts lead to an increase in electrode potential . on the first intermediate layer , a second intermediate layer comprising platinum metal and an oxide of tin is formed by coating the first intermediate layer with a solution containing a platinum compound and a tin compound , and then heat - treating the coating in an oxidizing atmosphere . this second intermediate layer may further contain iridium oxide as the third component in addition to the two components , platinum metal and tin oxide . such a three - component covering layer can be formed by coating a solution containing a platinum compound , a tin compound , and an iridium compound , and then heat - treating the coating in an oxidizing atmosphere . in the case where the second intermediate layer is constituted by two components , i . e ., platinum metal and tin oxide , the proportion of the former to the latter component is preferably from 30 : 70 to 60 : 40 by mol . part of the platinum contained in an amount in the above - specified range may be replaced with iridium oxide in an amount so as to result in an iridium oxido content of 10 mol % or less based on the total amount of the platinum metal , tin oxide , and iridium oxide . if the content of iridium oxide in the second intermediate layer exceeds 10 mol %, an oxygen - evolving reaction takes place on the second intermediate layer because of the significantly high electrochemical activity of iridium oxide as compared with platinum metal , and , as a result , the electrode potential rises disadvantageously in a short period of time . the second intermediate layer is exceedingly effective in improving the adhesion of an electrode catalyst coating as a surface layer to the first intermediate layer and the electrical conductivity between the two layers . on the second intermediate layer , a surface layer which comprises platinum metal and an oxide of tin and may further contain iridium oxide is formed by coating the second intermediate layer with a solution which contains a platinum compound and a tin compound and may further contain an iridium compound and in which the relative amounts of the ingredients are different from those for the second intermediate layer , and then heat - treating the coating in an oxidizing atmosphere . in the case where the surface layer is constituted by two components , i . e ., platinum metal and tin oxide , the proportion of the former to the latter component is preferably from 70 : 30 to 90 : 10 by mol . part of the platinum the relative amount of which to the tin oxide is in the above - specified range may be replaced with iridium oxide in an amount so as to result in an iridium oxide content of 10 mol % or less based on the total amount of the platinum , tin oxide , and iridium oxide , thereby to form a three - component surface covering layer . if the platinum content in the surface layer is below 70 mol %, electrode potential increases in a short period of time . if the platinum content is above 90 mol %, the platinum is consumed at an increased rate . further , if iridium oxide is incorporated in an amount exceeding 10 mol %, the ability of electrode to anodize trivalent chromium ions formed by cathodic reduction into hexavalent chromium - based chromic acid is weakened because iridium oxide has a low oxygen - evolving potential . for this reason , iridium contents exceeding 10 mol % are not preferred . in order to obtain the desired thickness for each covering layer of the electrode , the coating and heat - treatment operations as described hereinabove may be conducted repeatedly . the anode for chromium plating according to the present invention , which comprises an electrically conductive substrate comprising a valve metal or an alloy thereof , a first intermediate layer formed on the substrate and comprising an oxide of tin , a second intermediate layer formed on the first intermediate layer and comprising either platinum metal and an oxide of tin or platinum metal , an oxide of tin , and iridium oxide , and a surface layer formed on the second intermediate layer and comprising either platinum metal and an oxide of tin or platinum metal , an oxide of tin , and iridium oxide , and in which the composition of the surface layer is different from that of the second intermediate layer , shows exceedingly good corrosion resistance , particularly when it is used in chromium - plating baths containing additives of organic materials . the present invention is explained in more detail by reference to the following examples , which should not be construed as limiting the scope of the invention . using stannic chloride , platinum chloride , and iridium chloride as raw materials for electrode coating ingredients , various solutions for forming first intermediate layers , second intermediate layers , and surface layers were prepared by dissolving these compounds in hydrochloric acid . the first intermediate layer - forming coating solution was coated by brushing it on a titanium plate cleaned with hot oxalic acid , and the coating was dried and then heat - treated at 550 ° c . in an oxidizing atmosphere . the above procedure of coating , drying , and heat treatment was repeated to form a first intermediate layer having a desired thickness on the titanium substrate . subsequently , a second intermediate layer and a surface layer were formed on the first intermediate layer using a second intermediate layer - forming solution and a surface layer - forming solution , respectively , in substantially the same manner as that for the first intermediate layer . likewise , a total of eight kinds of electrodes were prepared in each of which the first intermediate layer , second intermediate layer , and surface layer had respective compositions as shown in table 1 . table 1______________________________________ fixed sno . sub . 2 molar ratio of amount in the pt : sn : ir in the molar ratio first second pt : sn : ir inelectrode intermediate intermediate the surfaceno . layer ( g / m . sup . 2 ) layer layer______________________________________1 0 . 5 30 : 70 : 0 90 : 10 : 02 5 25 : 70 : 5 65 : 30 : 53 30 40 : 60 : 0 78 : 20 : 24 20 38 : 60 : 2 70 : 30 : 05 5 50 : 50 : 0 74 : 20 : 66 20 49 : 50 : 1 80 : 10 : 107 10 60 : 40 : 0 80 : 20 : 08 10 50 : 40 : 10 63 : 30 : 7______________________________________ using each of the thus - obtained electrodes of the present invention , which were different in coating layer composition , as an anode , and using a copper plate as a cathode , continuous electrolysis was conducted at 60 ° c . in a chromium - plating bath containing 40 ml / l of organic type additive mi - 40 ( manufactured by canning co ., u . k .) at an anode current density of 30 a / dm 2 . the time period in which the anode potential rose by 1 v from its initial value at the beginning of the electrolysis was measured for each anode and taken as lifetime . as a result , the anodes were found to have lifetimes of 2 , 000 hours or more . in the same manner as in example 1 , electrode nos . 9 to were prepared which were the same as those prepared in example 1 , except that they differed from the electrodes of example 1 in fixed tin oxide amount for the first intermediate layer and in the composition of the second intermediate layer and surface layer . the compositions of the first intermediate layer , second intermediate layer , and surface layer are shown in table 2 . table 2______________________________________ fixed sno . sub . 2 molar ratio of amount in the pt : sn : ir in the molar ratio first second pt : sn : ir inelectrode intermediate intermediate the surfaceno . layer ( g / m . sup . 2 ) layer layer______________________________________ 9 0 30 : 70 : 0 90 : 10 : 010 0 . 5 20 : 80 : 0 80 : 20 : 011 0 . 5 70 : 30 : 0 80 : 20 : 012 0 . 5 43 : 40 : 17 80 : 20 : 013 0 . 5 15 : 70 : 15 80 : 20 : 014 0 . 5 40 : 60 : 0 100 : 0 : 015 0 . 5 40 : 60 : 0 60 : 40 : 016 0 . 5 40 : 60 : 0 72 : 10 : 1817 0 . 5 40 : 60 : 0 56 : 30 : 1418 40 40 : 60 : 0 80 : 20 : 0______________________________________ using each of the thus - obtained electrodes as an anode for chromium plating , electrolysis was conducted under the same conditions as in example 1 . as a result , the lifetimes of electrode nos . 9 to 14 were 1 , 000 hours or less , while those of electrode nos . 15 to 18 were between 1 , 000 hours and 2 , 000 electrode nos . 19 to 26 were prepared under the same conditions as in electrode nos . 1 to 8 of example 1 . using each of the thus - obtained electrodes as an anode and using a copper plate as a cathode , 100 - hour continuous electrolysis was conducted in the same chromium - plating bath as in example 1 at an anode current density and cathode current density of 30 a / dm 2 . after completion of each electrolysis , the concentration of trivalent chromium ions in the resulting chromium plating bath was measured by redox titration . the results obtained are shown in table 3 , from which it is seen that the trivalent chromium ion concentration for each electrolysis was so low that the chromium plating was never impeded by the chromium ions . table 3______________________________________ fixed sno . sub . 2 molar ratio of molar trivalent amount in pt : sn : ir in ratio of chromiumelec - the first the second pt : sn : ir in ion con - trode intermediate intermediate the surface centrationno . layer ( g / m . sup . 2 ) layer layer ( g / l ) ______________________________________19 0 . 5 30 : 70 : 0 90 : 10 : 0 2 . 920 5 25 : 70 : 5 65 : 30 : 5 3 . 221 30 40 : 60 : 0 78 : 20 : 2 3 . 022 20 38 : 60 : 2 70 : 30 : 0 3 . 523 5 50 : 50 : 0 74 : 20 : 6 3 . 624 20 49 : 50 : 1 80 : 10 : 10 3 . 325 10 60 : 40 : 0 80 : 20 : 0 3 . 326 10 50 : 40 : 10 63 : 30 : 7 3 . 7______________________________________ electrode nos . 27 and 28 were prepared under the same conditions as in electrodes of example 1 except that the molar proportion of iridium oxide in the surface layer was increased . electrolysis was conducted and the concentration of trivalent chromium ions in the resulting plating bath was then measured under the same conditions as in example 2 . the results obtained are shown in table 4 , from which it is seen that the trivalent chromium ion concentrations were unfavorably high for chromium plating . table 4______________________________________ fixed sno . sub . 2 molar ratio of molar trivalent amount in pt : sn : ir in ratio of chromiumelec - the first the second pt : sn : ir in ion con - trode intermediate intermediate the surface centrationno . layer ( g / m . sup . 2 ) layer layer ( g / l ) ______________________________________27 0 . 5 40 : 60 : 0 72 : 10 : 18 13 . 428 0 . 5 40 : 60 : 0 56 : 30 : 14 16 . 2______________________________________ as described above , the anode for chromium plating according to the present invention , which comprises an electrically conductive substrate comprising a valve metal or an alloy thereof , a first intermediate layer formed on the substrate and comprising an oxide of tin , a second intermediate layer formed on the first intermediate layer and comprising either platinum metal and an oxide of tin or platinum metal , an oxide of tin , and iridium oxide , and a surface layer formed on the second intermediate layer and comprising either platinum metal and an oxide of tin or platinum metal , an oxide of tin , and iridium oxide , and in which the composition of the surface layer is different from that of the second intermediate layer , shows excellent durability when used in organic ingredient - containing plating baths that are advantageous in providing chromium deposits having excellent properties , so that the consumption of the anode can be one - tenth to one - twentieth as large as that of conventional platinum - plated electrodes and the anode enables chromium plating to be conducted stably over a prolonged period of time . while the invention has been described in detail and with reference to specific embodiments thereof , it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof .