Even if the copper sample has been treated with amidosulfonic acid
to remove residual surface oxides before exposure, the surface is
nevertheless covered by a thin hydroxylated cuprous oxide layer under
ambient conditions [34]. This is schematically illustrated in Fig. 8a) atthe beginning of the exposure, marked with native Cu2O, and no other
corrosion products yet formed. The active species in the exposure
chamber result in an aqueous adlayer consisting of some monolayers of
water molecules on the surface, which acts as an ultrathin electrolyte for
further electrochemical reactions. For clarity reasons, this adlayer is not
shown in the forthcoming illustrations of Fig. 8.
Upon exposure and the incorporation and dissociation of formic acid
into the adlayer, two different dissolution mechanisms of the Cu2O-layer
have previously been identified, both experimentally and computationally: a ligand- and a proton- promoted dissolution process, both of
which result in more general or more local dissolution of parts of the
Cu2O-layer [5]. Because of the solid-state growth of the Cu2O layer
during exposure, as seen in Fig. 1 and studied more in detail elsewhere
[31], the result is a thickened oxide layer exhibiting increased surface
topography, depicted in Fig. 8b).
From the detailed analysis of, e.g. Fig. 5g) and Fig. 7f), one can
discern that the formation of Cu(OH)(HCOO) seems to follow two
different reaction routes. The first route consists of partial dissolution of
Cu2O over larger parts of the oxide surface and concomitant release of
Cu+ ions which immediately oxidise to Cu2+ to form Cu(OH)(HCOO)
together with dissolved HCOO- and OH- ions, depicted in Fig. 8c).
The second reaction route to form Cu(OH)(HCOO) is considered
next. As shown in the cross-section displayed in Fig. 7f), local parts of the
Cu2O-layer will be eventually completely dissolved, allowing the
aqueous adlayer to be in direct contact with the copper substrate,
depicted in Fig. 8c).
The exposure of the bare copper substrate and the higher nobility of
Cu2O compared to Cu [4] triggers the electrochemical reactions and the
formation of a corrosion cell:
Anodic reaction: Cu → Cu+ + e- (1)
Cathodic reaction: O2 + 4e- + 2 H2O → 4OH- (2)
An immediate implication of these reactions is the formation of Cu2O
over parts of the surface, as seen in several of the images in Fig. 3, Fig. 5,
and Fig. 7, according to:
2Cu+ + 2OH- → Cu2O + H2O (3)
In Fig. 5e) for the 48 h sample and Fig. 7f) for the 72 h sample, shown
with a dashed yellow rectangle, one can discern further growth of Cu2O
next to the Cu(OH)(HCOO) region, suggesting that Cu2O has been
formed as a result of the cathodic reaction. The fact that the overall layer
thickness of Cu2O eventually reached an approximately constant
thickness according to the IRRAS results shown in Fig. 1, suggests that
both the dissolution and formation of Cu2O take place so that an overall
steady-state thickness was reached after 24 h or more.
A conclusion from the GILDES calculations [31] is the immediate
oxidation of released Cu+ to Cu2+ because of the oxidative conditions in
the current atmospheric environment. The end reaction product found,
besides Cu2O, is Cu(OH)(HCOO) which previously has been identified in
laboratory exposures [35]. In the presence of released Cu2+ and OH- and
dissolved HCOO-, the conditions are favourable to form Cu(OH)(HCOO)
at local areas adjacent to areas where the Cu2O-layer has been dissolved
locally, with the following overall reaction:
Overall reaction: Cu2+ + OH- + HCOO- → Cu(OH)(HCOO) (4)
Findings from the current study regarding the local formation of
Cu2O and of Cu(OH)(HCOO) have been schematically illustrated in
Fig. 8d). Route 2 in this figure forms evidence of a corrosion cell. This is
supported by the local formation of Cu(OH)(HCOO) at the anodic site
(including reactions 1 and 4) and nearby local formation of Cu2O at the
cathodic site (reactions 2 and 3). The fact that extremely confined areas
of copper formate can be seen in Fig. 6 may support the conclusion that
corrosion cells have been formed at many parts of the corroding copper
surface.