Final Report - Strategic Environmental Research and Development ...

5.3 Task 3: Active inhibition, barrier properties and adhesion
Objectives
The objective of this task was to use a range of experimental and modeling approaches to
evaluate the efficacy of active inhibitor species in emergent chromium-free coating systems for
the purpose of understanding their role in promoting corrosion protection. A further objective is
to study the tradeoffs between active inhibition and other elements of corrosion protection.
5.3.1 Corrosion inhibition of AA 2024-T3 alloy by Lanthanide cations – Ce 3+ , Pr 3+ , La 3+
5.3.1.1 Introduction
AA 2024-T3 is a solution heat treated, cold worked and naturally aged alloy commonly used in
the aerospace industry. Its composition by weight percent is 3.8-4.9 Cu, 1.2-1.8 Mg, 0.3-0.9 Mn,
and small quantities of Si, Fe, Zn, Cr and Ti with the balance being aluminum [1]. Elemental
alloy additions result in superior mechanical properties and a heterogeneous microstructure. This
gives high strength but this leads to a significant decrease in corrosion resistance [1]. As a result
of mircostructural heterogeneity, localized attack occurs due to a fluctuating galvanic interaction
with the matrix. This manifests in the form of pitting, crevice, intergranular and filiform
corrosion [2-8]. It is therefore of considerable importance to understand the corrosion of these Al
alloys and develop effective strategies to inhibit corrosion in aggressive environments.
The copper-containing second phase particles are nobler than the surrounding matrix and act as
local cathodes causing pitting [3, 7, 8]. The constituent particles include Al 2 CuMg,
Al 6 (Fe,Mn,Cu), (Al,Cu) 6 Mn, Al 7 Fe 2 Cu among which the S phase particles are the most abundant.
S phase particles are first anodic to the matrix and undergo selective dissolution leading to a rich
copper remnant and a non-faradaic redistribution of copper on the surrounding matrix [8]. The
redistribution of copper is by its oxidization and reduction on the matrix and this serves to
initiate new pits. The intact particle remnant is noble to the matrix and stimulates peripheral
pitting [8]. Hence, suppression of localized attack can be achieved by protecting the Al 2 CuMg
precipitates from dissolution.
Chromates have been historically used to prevent the corrosion of these alloys [9, 10]. The
toxicity and carcinogenic nature of the chromate ion has led to the development of alternative
pigments that are environmentally acceptable and offer corrosion resistance to the high strength
aluminum alloys. Among the candidate inhibitors are rare earth metal (REM) compounds, which
have received much of attention over the past two decades. The early work by Hinton et al. by
immersion in various concentrations of the rare earth chloride solutions to reduce the corrosion
of high strength AA 7075 (Al-Zn-Mg-Cu), demonstrated that they can be good aqueous
corrosion inhibitors [11, 12]. It was shown that the maximum inhibition of corrosion of AA 7075
in 0.1M NaCl occurs at a concentration of 100 ppm of the Ce 3+ , La 3+ , Y 3+ chlorides and it
remains constant at concentrations above this. These results were obtained from weight loss
immersion tests for 21 days. Evidence of a colored film was seen after 21 days without any
pitting while the substrate immersed in uninhibited NaCl solution was deeply pitted [11].
Potentiodynamic polarization of AA 7075 in 0.1M NaCl with 1000 ppm of chloride salts of Ce 3+ ,
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