Improvement of the Corrosion Resistance of Aluminium Alloys Applying Diff erent Types of Silanes - Advanced Materials for Agriculture, Food and Environmental Safety - page 30

rapid migration of Copper into the electrolyte, Cu 2+ ions might therefore
also be present in the pores of the surface i lm together with the acid resid-
uals at the present pH. So in the anodic i lm formed, copper islands will
mainly be dissolved in the electrolyte. h ese Cu 2+ cations in the solution
would re-precipitate at this pH (≈ 5) but are bound in chelate complexes by
the tartaric acid. So, tartrate or Htart − ions might be able to remove Cu 2+
not only directly from the pore surfaces but also through migration from
the pores themselves. h is reduces the heterogeneity of the formed anodic
i lm and as a result leads to a more corrosive resistive layer [10].
Another alternative to Cr(VI) for the corrosion protection of alumin-
ium and its alloys is the application of dif erent silanes. Silane-based coat-
ings usually exhibit good barrier properties due to the development of
dense -Si-O-Si networks [11, 12], which inhibit the penetration of aggres-
sive species towards the metallic substrate. h us, the ei ciency of the metal
surface pretreatment based on silane coatings is strongly dependent on the
barrier properties of the formed surface i lm [13, 14]. But even a small
defect in the barrier layer may cause a localized corrosion process. Even
for improved corrosion protection an additional sealing treatment of the
formed porous oxide layer is required.
h e purpose of sealing an anodic coating is to close the pores of the formed
anodic layer, thereby at least causing the layer to become chemically inert.
h is is accomplished by a chemical change of the surface structure by hydra-
tion—experimentally realized by a cold seal and/or a hot seal procedure.
h e process which takes place during hot water sealing (HTS) involves
(1) the dissolution of anhydrous alumina from the pore walls, (2) the for-
mation of a hydrated alumina gel inside the pores, (3) the saturation and
subsequent precipitation of alumina hydrates—which eventually plug the
entire length of the pores—and (4) the i nal process of crystallization and
agglomeration, which might continue over months and years.
In detail the HTS process involves a complex reaction cascade leading to
the formation of dif erent (ad) layers in the surface i lm, namely:

• an outer layer of formed acicular boehmite crystals

• an intermediate layer of a high density hydrated oxide

• a near bulk and much thicker layer with a porous cell struc-

ture; and

• the innermost layer, also known as the barrier layer, being

extremely thin (≈ 25 nm) and dense pore free.

However, in order to “close” very ef ectively (seal) the pores in the
anodized layer to obtain an improved corrosion protection a process