The phenomenon of pitting corrosion attack on the milled aluminium alloy Al 2618 plate during surface preparation through sulphuric acid anodising Handoko Subawi1, a, Sutarno2 1,2 Indonesia Aerospace, Jl. Pajajaran, No. 154 Bandung – Indonesia a
[email protected]
Abstract. This study purposed to investigate corrosion characteristic on aluminium alloy by considering parameters that involved metal preparation, different surface treatment, and alloy types. Through series of the salt spray test, the rolled aluminium sheet revealed higher resistance to surface corrosion rather than milled aluminium plate. However trace elements, as reinforced filler in the metal alloy, may contribute to possible pitting corrosion. By employing sulphuric acid anodising, it revealed higher probability of pitting corrosion to attack the milled aluminium plate surface compared to rolled aluminium sheet. The surface pitting corrosion on the anodized aluminium alloy Al 2618 plate was observed through enlargement of pitting diameter and additional new pitting holes during 500 hours corrosion test. The corrosion propagation grew sharply during 500 hours test and it increased slowly after 750 hours. This study did not evaluate further variables either alloy composition, metal processing, or operation condition in anodising process.
I. INTRODUCTION
Pitting corrosion mechanism
Corrosion on aluminium
A corrosion is an electrochemical process, it relies on electron transfer. Corrosion of aluminium can be split into an anodic and cathodic part reaction which can occur at separate places on the metal surface provided that they are in electrical contact.
The corrosion attack on aluminium surface penetrates relatively slowly, but is speeded up in the presence of dissolved salts. There are three kinds of attack on aluminium alloys i.e. 1) the penetrating pit type corrosion through the walls of aluminium tubing, 2) stress corrosion cracking of materials under sustained stress, 3) intergranular corrosion The Al-oxide film is stable over the range of pH 4.5 to 8.5. When the oxide film dissolves, either in strong acids or alkalis, dissolution of the metal occurs too, or in other word, the metal corrodes. As aluminium is an amphoteric metal, the presence of certain anions such as chloride or cations such as copper also has an influence on the stability of the oxide film.
The anodic reaction is an oxidation process and releases electrons: 2 Al 2 Al3+ + 6 e (in acid solutions) 4Al + 4 OH Al(OH) + 3 e (in neutral and alkaline solutions) The cathode reaction is a reduction process and accepts electrons: 6 H+ + 6 e- 3 H2 (in acid solutions) 3 H2O + 3 e 3 OH + 3/2 H2 (in alkaline solutions) O2 + 2 H2O + 4 e 4 OH (in aerated solutions)
II. KONSEP RANCANG BANGUN BILAH KOMPOSIT Fig.1. Potential-pH diagram for aluminium with oxide film. Source : M.Pourbaix (1966). In EAA (2002).
.
a. The diagram shows the regions of immunity and passivity as a function of pH-value and electrode potential. b. Potential values are for the standard hydrogen electrode. c. The corrosion resistance of the aluminium alloys is the important characteristics. d. However, under certain conditions of elevated temperature exposure, alloys containing higher amounts of 3% magnesium may become susceptible to intergranular corrosion, exfoliation, or stress corrosion cracking. e. Pitting corrosion is a form of extremely localized attack that results in holes in the metal. f. Pits can be isolated, or close together that they look like a rough surface. Most pits develop and grow downward, in the direction of gravity, from a horizontal surface. g. Pitting failures are commonly caused by electrolytes containing chloride or chlorine-containing ions. h. Generally the most susceptible to pitting damage include stainless steels, aluminium, magnesium, and copper. i. In anodizing system, the time to deposit of each 1 m thickness through chromic acid anodising about 15 minutes, was twice than sulphuric acid anodising at about 7 minutes.
Fig.2 The pitting corrosion mechanism of aluminium Source: J.Gunson (2010) The rate of electron transfer between the anodic and cathodic reactions is a measure of the corrosion rate, or often called corrosion current. Pitting is an autocatalytic process, meaning once a pit begins to propagate, the resultant chemistry that is developed in the surrounding area is such that continued pit growth is sustained. In contact with neutral, weak acidic or alkaline solutions the metal loss is only small. Furthermore, often un-dissolvable corrosion products are formed that hinder further corrosive attack. II. EXPERIMENTAL PROCEDURE Materials These experiments employed aluminium alloys materials i.e.: (a) Rolled aluminium Al2024 T3 (Al-Cu) sheet, (b) Rolled aluminium Al 7075 T6 (Al-Zn) sheet, (c) Milled aluminium Al7075 T6 (Al-Zn) sheet, (d) Milled aluminium Al 7175 (Al-Zn) plate, (e) Milled aluminium Al 2618 (Al-Cu) plate.
Sample preparation 1. These specimens were treated through chromic acid anodizing, sulfuric acid anodizing and alodine processes to ensure corrosion resistivity level among them. 2. In the chromic acid anodizing, after precleaning the specimens were anodized in the 75 g/l chromic acid. The contaminants such as dissolved aluminium , sulfuric ion, and chloride ion are controlled in this solution. The solution temperature was maintained at the range of 35 to 40oC. Anodizing process was performed during 60 minutes at the voltage at 40 to 42 Volt. Before and during process, the sulfuric solution was agitated to maintain circulation. 3. In the sulfuric acid anodizing process, after precleaning the specimens were anodized in the 200 g/l sulfuric acid. The contaminants such as dissolved aluminium, chloride ion, fluoride ion are controlled in this solution. The solution temperature was maintained at the range of 10 to 20oC. Anodizing process was performed during 60 minutes at the voltage at 12 to 20 Volt. Before and during process, the sulfuric solution was agitated to maintain circulation. Laboratory Testing 1. 2. 3. 4. 5.
The spesimen after surface treated in anodizing line then prepared for salt spray test. All specimens were exposed in the salt spray chamber per ASTM B-117, with incline angle of 15o, to ensure corrosion resistivity level. The 2.5% salt solution was sprayed continually to the specimens. At certain period the specimen was taken out for witnessing whether corrosion occur on the metal surface. Some photographs were taken to the corroded point for further analysis.
RESULT AND DISCUSSION
Table.3. Corrosion potentials of intermetallic phases (ref. 0.1 N calomel electrode)
Salt spray test after 500 hours after CAA
Fig.3.a. Al 7075 sheet,no corrosion
Fig.3.b. Al 7175 plate, all corroded
Table.1. Experiment results
No
Aluminium
1
Chromic anodizing
Specimens
Rolled
Al 2024-T3 sheet
pass (3)
Alodine
pass (3)
Sulfuric anodizing
pass (3)
No
Alloying element
1
Si
Si
-0.26
cathodic
2
Ni
NiAl3
-0.52
cathodic
3
Fe
FeAl3
-0.56
cathodic
4
Cu
CuAl2
-0.73
cathodic
5
Mg + Si
Mg2Si
-0.83
neutral
6
Al
Al 99.95
-0.85
neutral
7
Mn
MnAl6
-0.85
neutral
8
Mg + Zn
MgZn2
-1.05
anodic
9
Mg
Mg5Al8 (- Mg2Al3)
-1.24
anodic
Intermetallic phase Potential (volt)
Behaviour relative to Al matrix
Source: Textor M (1995). In EAA (2002).
2
Rolled
Al 7075-T6 sheet
pass (3)
pass (3)
pass (3)
3
Millled
Al 7075-T6 plate
pass (3)
pass (3)
pass (3)
4
Millled
Al 7175-T6 plate *
fail (3)
fail (3)
fail (3)
5
Milled
Al 2618 plate
fail (3)
fail (3)
fail (30)
Pitting corrosion on Al 2618 plate
Table.2. Alloy composition factor 2024clad Element
Al 2024
Cu
3.8 - 4.9
Si
0.50
Al 1230 0.10
Al 2618
Al 5052
1.9 - 2.7
0.10
0.25 0.7
Fe
0.50
Mn
0.30 - 0.9
Mg
1.2 - 1.8
Zn
0.25
Cr
0.10
0.1
7075 clad
Al 7075
Al 7072
1.2 - 2.0
0.10
0.50 0.45
0.9 - 1.3 0.05
Al 7175
0.7 0.7
-
0.10
0.30
0.10
1.3 - 1.8
2.2 - 2.8
2.1 - 2.9
0.10
-
0.10
5.1 - 6.1
0.8 - 1.3
-
0.15 - 0.35
0.18 - 0.40
-
Ti
0.04 - 0.10
0.20
Ni
0.9 - 1.2
-
Other
0.15
0.15
0.15
0.15
0.15
0.15
Al
balance
> 99.3
balance
balance
balance
balance
Source: MIL-HDBK-694A (1966).
Fig.4. Pitting corrosion on aluminium Pitting corrosion growth a. The first pitting was firstly observed in the eleventh day with 130 microns depth. b. The pitting was being grown up during salt spray test period of the 500 hours up to 640 microns depth. c. The depth of pitting hole on aluminium Al 2618 plate tends to grow fastly during 500 hours and tends to slow after 750 hours. d. However, the study did not investigate the parameters influence the order of acceleration growth. e. The existing growth order may change and influence by several factors either, trace elements concentration, milling process situation, sulfuric anodizing operating condition or others.
Fig.5. Corrosion rate trend a. Through the corrosion test, it was observed that sulfuric anodizing provide good result for Al 7075 sheet. b. On the area being intentionally scratched (marked) there was no corrosion tendency along this marking. c. The sulfuric anodizing provide good result and may not correlate directly with the corrosion on aluminium plate. d. The evaluation on milled aluminium Al 2618 plate still showed corrosion evidence using the process with sulfuric anodizing. e. The rolled aluminium is relatively more resist to corrosion than milled aluminium plate. f. The evaluation turned to possibly root cause of trace elements existence in aluminium Al 2618 alloy. The exposure of metal surface during milling whether any influences to the corrosion attack.
CONCLUSION 1. The pitting corrosion was observed on milled aluminium alloy Al 2618 plate through the corrosion spray test within 500 hours. However, all specimens made of the rolled aluminium sheet Al 2024 T3, and Al 7075 T6 provided negative result during corrosion test. 2. Both of chromic acid anodising and sulphuric acid anodising showed similar influence to all specimens. 3. The milled aluminium alloy tends easier to corrosion attack than rolled aluminium sheet. Trace elements in aluminium alloy as the metal reinforcement in parallel may also contribute to the pitting corrosion tendency. ACKNOWLEDGEMENT We would like to thank Indonesian Aerospace Ltd for technical and financial supports in this research package. REFERENCES [1] FAA Advisory Circular, Corrosion control for aircraft, FAA (AC) 43-4A; in the Aircraft cleaning and corrosion control, Chapter 6, FAA-8083-30, (cited 2013), 6.1-26. [2] European Aluminium Association, Materials microstructure and properties, Aluminium Automotive Manual, (2002). [3] Pourbaix, M., Atlas of electrochemical equilibria in aqueous solutions, Pergamon Press, (1966), 171. [4] Francine S. Bovard, Sensitization and environmental cracking of 5xxx aluminum marine sheet and plate alloys, Alcoa, PA 15069, (cited 2013). [5] Donald O. Sprowls, Progress toward standardization of stress corrosion crackingtest techniques by the National Association of Corrosion Engineers and the Aluminium Association, Alcoa, Structures and Material Panel 33rd Meeting, Brussel, (1971). [6] NAVAIR, Cleaning and corrosion control, Volume I Corrosion Program and Corrosion Theory, NAVAIR 01-1A-509-1, TM 1-1500-344-23-1, TO 1-1-689-1, (2005). [7] HandokoSubawi, the Painting preparation for aluminium and steel, 1st International Conference of Protective and Decorative Coatings,Jakarta, (2007). [8] James Gunson, Effect of sensitisation on the corrosion fatigue properties of AA5456-H116, Masters thesis, Materials and Metallurgy, University of Birmingham, (2010). [9] Textor M., Néma P., and Timm J., Car body alloys and methods of corrosion protection on aluminium sheet, Werkstofftech, 26, (1995), 318-326. [10] MIL-HDBK-694A, Aluminum and aluminum alloys, Military Standardiza-tion Handbook, DoD, Washington D. C., (1966), 82-106.
Further reading: [1] Handoko Subawi, and Sutarno, The optimisation of processing cycle parameters to alleviate pitting corrosion on anodised aluminium alloy surfaces, Corrosion Management,ICORR, U.K., August 2013, pp.10-12.
