Addition Agent Behaviour in Acid Tin Plating

Abstract

Dihydroxy diphenyl sulphone (DDS) and sulphonated diphenylol propane (DPP), the addition agents used commercially in the high speed tin plating of steel strip were separated into their constituent compounds. To identify the active constituents tin was electrodeposited from a series of solutions containing the separate compounds and the structure of the deposits examined. The solid addition agent DDS was separated into two main constituents; by means of infra-red and nuclear magnetic resonance spectroscopy it was shown that the two compounds separated were structural isomers. Using mass spectroscopy the separation of the 22' and 44" isomers of dihydroxy diphenyl sulphone was confirmed since the compounds had the same molecular weight and fragmentation pattern. Commercial DDS was shown to contain about 8 - 10% of the 22' isomer. The liquid addition agent DPP could not be separated into its constituent chemical compounds but the mixture was separated into acid and non-acid components and also into water soluble and ether soluble components. Each of the components separated was used separately as an addition agent in the electrodeposition of tin and topography of the tin deposited examined by scanning electron microscopy. It was found that some constituents had no beneficial properties on the tin deposited and their presence merely diluted the effectiveness of the active constituents. The two isomers of DDS had different but complementary effects on the structure of tin deposited. The 22" isomer increased the density of nucleation sites initially formed but when used alone dendritic growth commenced after about 30 seconds plating time. The 44' isomer when used alone was totally unsatisfactory, giving very poor covering and crystals which very soon showed dendritic growth. However, when the two isomers were used together it would appear that the high nucleation rate produced by the 22" isomer results in a large number of small crystals which in the presence of the 44' isomer did not develop dendritic growth until at least 60 seconds plating time. The two isomers appeared to act synergistically. By altering the ratio of 22' to 4a" isomer present, the structure of the tin deposited could be improved. Using the components of DPP separately as addition agents it was possible to identify two active constituents. The water soluble compounds which improved the structure of the tin deposited were only active when present as acids. As salts they were inactive. It was thought possible that both of the active components separated contained the same functional group and that the difference in solubility may have been due to a difference in molecular weight. It was found impractical to separate the compounds or their degradation products from the electrolyte used. However the effect of the break down products formed on the © tin deposited was investigated satisfactorily. By analysis it was shown that over a ten hour plating period the concentration of tin and phenol sulphonic acid varied only marginally and could easily be held constant. By plating from the electrolyte continuously for up to eight hours the break down of the addition agents could be charted. Test pieces were plated in the normal way at hourly intervals throughout this period. The break down products from DPP were not detrimental to the tin deposited over periods of time up to five hours. The tin deposited from solution containing DDS as addition agent, however, showed a change in structure after a short plating period.