Accelerated corrosion test and corrosion failure distribution model of aircraft structural aluminum alloy
LIU Wen-lin(柳文林)1, MU Zhi-tao(穆志韬)2, JIN Ping(金 平)2, 3
1. Airborne Vehicle Engineering, Naval Aeronautical Engineering Academy, Yantai 264001, China;
2. Qingdao Branch, Naval Aeronautical Engineering Academy, Qingdao 266041, China;
3. School of Aeronautics Science and Technology, Beijing University of Aeronautics and Astronautics,
Beijing 100081, China
Received 28 July 2006; accepted 15 September 2006
Abstract: Based on corrosion damage data of 10 years for a type of aircraft aluminum alloy, the statistical analysis was conducted by Gumbel, Normal and two parameters Weibull distribution function. The results show that aluminum alloy structural member has the corrosion history of pitting corrosion—intergranular corrosion—exfoliation corrosion, and the maximum corrosion depth is in conformity to normal distribution. The accelerated corrosion test was carried out with the complied equivalent airport accelerated environment spectrum. The corrosion damage failure modes of aluminum alloy structural member indicate that the period of validity of the former protective coating is about 2.5 to 3 years, and that of the novel protective coating is about 4.0 to 4.5 years. The corrosion kinetics law of aluminum spar flange was established by fitting corrosion damage test data. The law indicates two apparent corrosion stages of high strength aluminum alloy section material: pitting corrosion and intergranular corrosion/exfoliation corrosion. The test results agree with the statistical fit result of corrosion data collected from corrosion member in service. The fractional error is 5.8% at the same calendar year. The accelerated corrosion test validates the corrosion kinetics law of aircraft aluminum alloy in service.
Key words: aircraft structure; aluminum alloy; corrosion damage; probability density function; corrosion kinetics law
1 Introduction
Naval aviation operations routinely expose structural components to salt spray and high loads, especially during landing. There are many alloys used in naval aircraft structures. The high strength aluminum alloys, magnesium alloys and steel alloys, especially aluminum alloys, were apt to initiation of corrosion [1-2]. In general investigation of a type of naval aircraft, more than 75% corrosion members are aluminum alloys [3]. The major aluminum alloys used in naval aircraft are LY12, Al-Zn-Mg-Cu alloy, LC4, LC9, ZL, LD2, LD5 and LD10. LY12CZ and LC4 are sensitive to intergranular corrosion [4-5].
The distribution of corrosion damage is always dependent on the failure mode and the failure mechanism of material. Over the past several years, many scholars and engineers studied the maximum corrosion depth distribution. There are Normal distribution [6], Gamma distribution [7], Gumbel distribution [8] and Weibull distribution etc [1, 9, 10]. Limited study showed that it might be necessary to use several distribution functions rather than a single distribution function to corrosion according to different corrosion stages [11].
Some research showed that the corrosion fatigue life of aluminum alloy was greatly improved by protective coating [12]. But the incipient corrosion kinetic law and the effective lifetime of protective coating are not obvious.
In this study, firstly, the distribution type of the corrosion damage of longeron(LY12CZ) was conformity to verify and the corrosion kinetics law was established. Secondly, the calendar year of the former and novel protective coating in service by accelerated corrosion test was evaluated. Thirdly, the incipient corrosion kinetics law of aircraft aluminum alloy in service was validated.
2 Corrosion failure distribution model
It was studied that the maximum corrosion depth for LY12CZ and LC4CS was in conformity to Gumbel distribution in pitting corrosion stage [8-9]. Due to general corrosion investigation for a type of aircraft aluminum alloy longeron (LY12CZ), the aluminum alloy member has the corrosion damage history of pitting corrosion, intergranular corrosion and exfoliation corrosion, based on corrosion damage data of 10 years, the fitting relationship with Gumbel, normal and two parameters Weibull distribution functions are shown in Figs.1-3, respectively [13].
The fitting linear equation may be expressed as
Gumbel 
Normal 
Weibull 
where d is corrosion depth, mm.
Three probability density functions (PDF) may be expressed as
Gumbel:
Normal:
Weibull:
Fig.1 Fitting test by Gumbel distribution
Fig. 2 Fitting test by Normal distribution
Fig. 3 Fitting test by Weibull distribution
The corrosion depth PDF curves are shown in Fig.4. The analysis linearly dependent coefficient and standard deviation are summarized in Table 1.
Figs.1-3 show the linear fitting results by Least squares procedure. Three kinds of distributions have high linear correlation. As shown in Table 1, the linearly dependent coefficient for normal distribution has the maximum value, thus it is the preferential distribution law of the corrosion depth.
Fig. 4 Curves of corrosion depth vs probability density function
Table 1 Fitting results of distribution models of maximum corrosion depth for LY12CZ (mm)
3 Deepest corrosion depth assessment
According to normal distribution, the corrosion depth has the following expression:
(1)
The probability density function is as follows:
(2)
At the confidence level of 0.95, that is
(3)
Thus,
, the quantile of standard normal distribution is 
That is
(4)
We have 
Thus,
(5)
where dm is the deepest corrosion depth, mm; μ is mean value; σ is standard error.
The estimated value for the LY12CZ longeron of 10 years by formula (5) at a confidence level of 95% is 2.2 mm. The fractional error between the measured value (2.4 mm) and the estimated value is 8.3%.
4 Accelerate corrosion test
In order to reproduce corrosion damage of aluminum alloys member in service, accelerated corrosion test is obligatory. The above mentioned analyses are all on the basis of the corrosion damage data collected from corrosion member for a certain years in service. Corrosion data, entity photo and corrosion product of these aluminum alloy members indicate that the corrosion is much serious and most of members run to intergranular corrosion/exfoliation corrosion stage, the corrosion kinetic law is also obvious. But the incipient corrosion kinetic law and the effective lifetime of protective coating are not obvious.
4.1 Test specimens
According to the method about accelerated spectrum and corrosion test described in Refs.[14-16], 86 pieces of test specimens were manufactured from the corrosion longeron (LY12CZ). 8 pieces with good former protective coating were used for the coating residual life assessment, 8 pieces with novel protective coating were used for the coating calendar life assessment, and other pieces with depainting treatment were used to establish the corrosion kinetic law without the protective coating.
4.2 Test results analysis
The maximum corrosion depth is plotted as a function of year and logistic calendar year in Figs.5-6, respectively. There are two obvious stages in corrosion development: the first is that corrosion rate is quick relatively in accelerated corrosion prestige (about 1-3 years), the second is that the corrosion rate keeps a relative stable value and the maximum depth has linear relationship with calendar year. Accelerated corrosion test indicates that the period of validity of the former anticorrosion layer is about 2.5-3 years and the novel anticorrosion layer is about 4.0-4.5 years. Based on the kinetic law established from the corrosion damage data of structural member in service, the concluded result of the maximum corrosion depth is 1.64 mm after 9 years. Compared with the accelerated corrosion test result (1.55 mm), the fractional error is 5.8%.
Fig.5 Curves of corrosion depth vs time: 1—Upper 50% prediction limit; 2—Lower 50% prediction limit; 3—Upper 95% prediction limit; 4—Lower 95% prediction limit
Fig.6 Curves of corrosion depth vs logistic year: 1—Upper 50% prediction limit; 2—Lower 50% prediction limit; 3—Upper 95% prediction limit; 4—Lower 95% prediction limit
5 Conclusions
1) Aircraft aluminum alloys member in service has the corrosion history of pitting corrosion-intergranular corrosion-exfoliation corrosion. After the corrosion course, the maximum corrosion depth is in conformity to normal distribution. The maximum corrosion depth corresponding to years can be concluded by the gained corrosion depth function.
2) Accelerated corrosion test reproduces the corrosion damage failure modes of aluminum alloy structural member and indicates that the period of validity of the former anticorrosion layer is about 2.5-3 years and the novel anticorrosion layer is about 4.0-4.5 years. The corrosion kinetic law is established.
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(Edited by LI Yan-hong)
Foundation item: Project(513270301) supported by the National Natural Science Foundation of China
Corresponding author: LIU Wen-lin; Tel: +86-532-88033179; E-mail: navylwl@yahoo.com.cn
