Page 310 - Failure Analysis Case Studies II

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Maximum temperature at flow elbow x 90 “C.
Maximum temperature drop across tube wall= 10 “C.
Reynolds number at 80 “C ~20,000 (fully turbulent).
The tubes were made from wrought aluminium-magnesium-silicon alloy 6063. The specified com-
position is shown in Table 1.
A number of failures were reported where the heat-exchanger tubes had perforated from the
inside. The water had been treated with a liquid inhibitor package containing sodium nitrite, sodium
borate, sodium silicate, sodium hydroxide, and “organic scale and corrosion inhibitors”. When the
water contained less than 50mgI-’ of chloride, the recommended dose of sodium nitrite was
1250mgl-’. This was achieved by adding 5 parts by volume of the inhibitor liquid to 1000 parts by
volume of water. The organic corrosion inhibitor was presumably added to protect any copper in
the system; it could well have been benzotriazole. The neat inhibitor had a pH of 11.5, so the pH of
the diluted solution was probably around 9.

7.2. Failure analysis
7.2.1. Corrosion resistance of aluminium. The Pourbaix diagram for aluminium is given in Appen-
dix A. It shows that a protective film of aluminium oxide can form on the surface of the metal as
long as the pH of the water is between 4 and 8.5. The diagram does not give any information about
the effectiveness of this barrier layer. When aluminium is exposed to oxygen in the atmosphere, it
immediately forms a thin invisible film of aluminium oxide [I 1-1 31. The film is bonded firmly to the
surface, and is an excellent electrical insulator. Because of this, aluminium (a very reactive metal
which would otherwise oxidize very rapidly) is widely used as a corrosion-resistant material. Even
when the surface is damaged by mechanical abrasion, the oxide film reforms immediately. Although
this repaired film is very thin to begin with, it is still a good barrier to corrosion. On further exposure
to the air, the film thickens, typically by 100 times, and the barrier becomes more effective. Because
of this film, aluminium does not corrode in water where the pH is in the range 4-8.5.
Outside this pH range, the oxide film is unstable, and the aluminium corrodes: the rate of uniform
corrosion increases by roughly 10 times for every unit increase in pH above 8.5 [l]. The film can,
however, be stabilized by adding chemicals to the water. At low pH, aluminium is hardly affected
by dilute or concentrated nitric acid or dilute sulphuric acid because the oxidizing action of these
chemicals provides a strong film-forming tendency [14]. At high pH, the film is stabilized by silicates
[6, 15, 16]--compounds having the variable composition nNazO.mSiO2. These are most effective on
aluminium when the module of the silicate (the ratio m/n) is high (typically x3-3.5) [9]. But even
sodium disilicate (Na2Si20J; module=2) can stop uniform corrosion at a pH of z 11.5 [12].

7.2.2. Aluminium alloy 6063.6063 alloy is one of a series of wrought aluminium alloys containing
silicon and magnesium. They have good corrosion resistance, especially in alkaline solutions [l 11.
Alloying elements, mainly iron and copper, can lead to local weaknesses in the oxide film, and can
cause pitting [IO, 11. 6063 resists corrosion well because the concentration of the alloying elements
(silicon and magnesium) is small, and the harmful elements (iron and copper) are present only as
low-level impurities. In contrast, the corrosion resistance of the 2000 series alloys (which contain 2-
7% copper as an alloying element) is relatively poor. Minute particles or films of copper can deposit
on these alloys as a product of corrosion, and the galvanic cells which are created attack the film
[12]. 6063 alloy was therefore a good choice for the heat-exchanger tubes. High-purity aluminium
would have been better still, although it might not have had the neccessary mechanical properties.

Table 1. Composition of aluminium alloy 6063
Element Weight % Element Weight % Element Weight %
Silicon 0.204.6 Magnesium 0.45-0.9 Tin 0.05 maximum
lron 0.35 maximum Zinc 0.10 maximum Lead 0.05 maximum
Copper 0.10 maximum Chromium 0.10 maximum Aluminium Balance
Manganese 0.10 maxlmum Titanium 0.10 maximum