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.

Date of Award	1972
Original language	English