Environmental Engineering Reference
In-Depth Information
2.1 Introduction
With 8.1% by weight, aluminium is the third most abundant element in
the Earth's crust, but due to its reactivity it is not found in an elemental
state in nature. Aluminium and its alloys have been extensively studied
due to their wide range of commercial applications in the aerospace, archi-
tecture, transportation, and manufacturing industries. One of the most
prominent reasons for the applicability of aluminium alloys in aircrat
construction are their low density and good mechanical characteristics.
To improve the mechanical strength of aluminium, further alloying with
dif erent elements such as copper, zinc, magnesium and others is neces-
sary. While this alloying increases the mechanical strength, it decreases
the resistivity against localized corrosion attacks—especially in chloride-
containing environments. h e native oxide i lm formed when exposed to
ambient atmospheres protects aluminum and its alloys to a certain degree,
but it is heterogeneous and does not provide adequate corrosion resistance
in many environments. h us, in the aeronautic industry, the corrosion
protection of structural aluminium alloys, such as the 2XXX and 7XXX
series, requires an additional surface treatment which normally involves
the use of Cr(VI) salts to obtain a high corrosion resistance. Anodic i lms
formed under these conditions exhibit a good corrosion performance and
Cr(VI) species are considered as the “standard” corrosion inhibitors, even
if they are very toxic, carcinogenic and allergenic. An alternative route to
improve the anticorrosive properties of aluminium and its alloys would be
the sample anodization [1]. In general, the anodizing processes in acidic
media are based on the use of electrolyte baths composed of mixtures of
inorganic acids or mixtures of organic/inorganic acids [2]. h e anodizing
process and the properties of the thereby obtained anodic i lms are also
inl uenced by the alloying elements of the aluminium itself. During the
past two decades, a lot of work has been carried out to develop process
alternatives to CAA (chromic acid anodization) based on sulphuric acid,
sulphuric-boric acid, sulphuric-tartaric acid, malonic and oxalic acid for-
mulations [3-5].
Addition of dif erent weak acids such as boric acid (Boening Co., EP
0,405,624 (1991) or tartaric acid (US patent 2002/0157961) to sulphuric
anodizing baths has been proposed as one possible alternative. Alenia
Aeronautica S.P.A has proposed a new anodizing procedure involving the
addition of tartaric acid in diluted sulphuric acid electrolyte, i.e., tartaric-
sulphuric acid (TSA) anodizing [6]. In particular, anodizing of aluminium
in a mixture of sulphuric and L(+)-tartaric acid (TSA) has proven to pro-
duce oxide layers comparable to those formed in chromic acid baths [7].
Search WWH ::
Custom Search