宋代银制品模拟样品在Na_2S溶液中的电化学腐蚀及其产物

Electrochemical Corrosion and Corrosion Products of Simulated Song Dynasty Silver in Na_2S Solution

研究了2008年南京长干寺出土的北宋七宝阿育王塔鎏金银基体及其腐蚀产物成分,并根据银基体成分按传统工艺自行冶炼、加工出模拟的银试样.用电化学、OM、XRD、XRF、SEM-EDS技术研究并比较了银试样在不同浓度Na2S溶液中的腐蚀电位与电流、腐蚀产物成分、形貌、晶粒度等.结果表明,Na2S浓度由20 mg/L升高到5.0×104mg/L(5%Na2S)时,自腐蚀电位Ecorr负移0.70V,自腐蚀电流密度Icorr从0.274μA/cm2升高至1.056μA/cm2,腐蚀倾向和速度大幅度增加.在20 mg/L Na2S溶液中-0.31 V^1.8 V范围内极化,银试样始终处于活化溶解状态.在5%Na2S溶液中,腐蚀活化区电流密度从1.056μA/cm2迅速升高至1.56×104μA/cm2,并以较大的腐蚀电流密度(约1.56×104μA/cm2)进入钝化区并维持钝态;分别在20 mg/L和5%Na2S溶液中活化区恒电位+0.3 V、-0.8 V极化不同时间后,腐蚀产物主要为Ag2S.随着Na2S溶液浓度的增大,试样表面S2-腐蚀活性中心增多,Ag2S晶体成核速率增大,而腐蚀产物的晶粒度则由20μm减小到1.86~5.10 nm.

Chemical composition and corrosion product of the Northem Song Dynasty Emperor Asoka Pagoda substrate were studied for the first time since it was unearthed from the Changgan Temple underground palace located at Nanjing in 2008. Silver samples based on composition of the Pagoda substrate were simulated by traditional smelting and processing technique. The corrosion potential, corrosion current of the silver sample and the composition, morpholoy, grain size of the corrosion products on the sample in corrosion medium with different concentrations of Na2S were examined by means of electrochemical technique, OM, XRD, XRF, SEM and EDS. The results showed that with the increase of Na2S concentrations from 20 mg/L to 5.0×104mg/L （i.e. 5% Na2S）,the corrosion current density Icorr increased from 0.274 μA/cm2 to 1.056 μA/cm2 and the corrosion potential Ecorr was shifted towards less value by 0.70 V. Thereby, the driving force for corrosion and corrosion rate were enhanced. Silver samples were kept in active state when polarization potential was shifted from -0.31 V to 1.8 V in 20 mg/L Na2S solution. In 5% Na2S solution, the corrosion current density for silver in the active corrosion region increased rapidly from 1.056 μA/cm2 to 1.56 × 104 μA/cm2, and then silver samples transferred to passive corrosion region and maintained the passivation with higher corrosion current density about 1.56× 10＇ μA/cm2. Corrosion products of silver were mainly Ag2S after 0.3 V （20 mg/L Na2S） or-0.8 V （5% Na2S） constant potential polarization for different time intervals, respectively. The amount of S2- as corrosion active center and the growth rate of Ag2S crystals on the silver sample surface increased with the increase of the Na2S concentrations from 20 mg/L to 5%. However, the grain size of Ag2S crystals was reduced from 20 μm to1.86-5.10 nm.