194 CORROSION OF METALS
TABLE 2--1ncidence o f stress corrosion cracking on coiled 304 spring specimens in boiling saturated calcium chloride
solution at 138~
Total Number
of Cracks, Protection Efficiency,
Protection System 4 Specimens %
None (control) 462
Silicone-alkyd paint, cured at 120~ for
1 h 21 95
Silicone-alkyd paint,
uncured 26 94
Aluminum foil 0 100
3. The particular zinc-rich paint tested reduced markedly the incidence of cracking, and depressed the natural corrosion potential by approximately 350 mV cathodically. Indeed, two of the four specimens tested exhibited no cracks at all, and it is surprising that any cracking was experienced at such low corrosion potentials.
4. Aluminum foil proved 100% efficient at preventing cracking under the test conditions, unsurprisingly in view of the very low natural corrosion poten- tials achieved, involving a depression of about 530 mV relative to the control corrosion potential. At such low potentials, the stainless steel is almost cer- tainly fully cathodically protected [4]. The beneficial effects of aluminum foil wrapping under Karnes test conditions have been confirmed recently [5].
The above data confirm the galvanic protection afforded by aluminum foil under fully flooded conditions, and its advantages over single-coat paint sys- tems. It is important to recognize that higher integrity (and costly) paint sys- tems, that is, spark or sponge tested multi-coat systems on blasted surfaces, would probably have performed as well as aluminum foil under test condi- tions. Indeed certain of the single-coat specimens achieved 100% protection in the tests quoted above, for example, one of the four springs coated with aluminum-rich silicone quoted in Table 1 exhibited no cracking after seven days. However, the purpose of the tests was to evaluate aluminum foil against some relatively cheap single-coat site application procedures, and its advan- tages in this respect have always been confirmed in laboratory testing.
Consideration of Fire Risks
In the case of plants handling flammable fluids, which can give rise to flammable atmospheres, there are essentially three problems to address:
1. Is there an additional risk of spread of fire associated with the presence of the foil per se?
RICHARDSON AND FITZSIMMONS ON ALUMINUM FOIL 195 2. Is there an additional risk of spread of fire resulting from liquid metal embrittlement of stainless steel substrates by molten aluminum?
3. Is there an increased risk of ignition of a flammable atmosphere by in- cendive sparks resulting from a "thermite" reaction between aluminum foil and adjacent metal surfaces?
In relation to the first question, there is some dispute as to whether alumi- num foil burns in air [6]. However, if exposed in the event of a fire it could undoubtedly act as a source of hot, molten metal or oxide particles, which propelled by expanding air, could raise surrounding materials to their igni- tion temperatures [7]. But.if the foil is entirely contained beneath a lagging system, this should not be possible unless the lagging material itself is flam- mable. The presence of the foil in the context described in this paper, is thus not considered to increase the risk of spread of fire.
Regarding the second issue, it is undoubtedly true that molten aluminum can cause cracking of stressed austenitic stainless steel. However, laboratory testing suggests that cracks do not initiate very readily 2 and that the risks are relatively minor compared with zinc, where the zinc/nickel reaction favors crack initiation and propagation. There is some practical evidence to support laboratory findings. For example, during one reported refinery fire, both molten zinc and aluminum had access to stainless steel piping, but embrittle- merit problems were restricted to those caused by zinc [8]. In one instance, we have experienced melting of foil because of a temperature excursion on some lagged stainless steel ducting, without any evidence of liquid metal attack.
Again, given the present context involving foil beneath a lagging system, the likelihood of melting induced by external flame impingement would seem re- mote and the risks of spread of fire caused by embrittlement acceptably low.
Regarding the "thermite" reaction risks, it is recognized that aluminum foil if placed on rusty steel and subsequently struck by a hard object, could ignite a flammable vapor through the creation of an incendive spark [9].
However, there is no evidence that such sparks can be produced between alu- minum and a clean stainless steel surface, and in any event, there would inevi- tably be some attentuation of impact energy within the lagging system where the foil is entirely contained. It has thus been concluded that the risks associ- ated with possible aluminum foil/stainless steel surface interactions beneath a lagging system are acceptably low. However, foil is not used in locations where it could contact carbon steel surfaces in service, for example, where carbon steel backing flanges are used on stainless steel lines carrying flamma- ble fluids. In these circumstances silicone-based paints are used.
Given the qualifications outlined above, it has been concluded that the ad- ditional fire risks associated with the use of aluminum foil are acceptably low.
They must be balanced against the additional fire risks associated with exter- nal stress corrosion cracking itself, which can obviously result in the leakage of hazardous/flammable materials into the plant atmosphere.
196 CORROSION OF METALS
The above comments are not to be taken as a justification for the extensive use of aluminum alloys in high fire risk areas. ICI shares the concerns of other major operators on the latter topic, and the comments above relate strictly to the context of the paper.
Practical Application and Performance of Aluminum Foil
All ICI lagging specifications require that austenitic stainless steel surfaces operating continuously or intermittently between 60 and 500~ be wrapped in 46-SWG aluminum foil. Application to pipe and vessel surfaces presents few, if any, problems, largely because of the lightness and compliance of the foil.
Pipes are simply wrapped in the foil with 25-mm minimum overlaps, formed so as to shed water on vertical lines. The foil can be "molded" around flanges and fittings. On vessels, the foil is usually applied in bands in advance of the lagging system. There are no support problems, because given a relatively few anchor points, the foil is virtually self supporting. Obviously, there are no support problems on the top dome. On the sides, there are usually lagging support rings that provide anchor points. On the base, there are sprags or other means of lagging support that can be used to support the foil. Over- lapped, crimped joints are executed to shed water.
In the case of steam traced lines, a double wrapping of foil if specified, one directly onto the pipe beneath the tracing and the other enclosing both tracing and pipe in order to exclude unwanted insulation from the space between them. Overlaps on vertical lines are again formed so as to be water shedding.
All compressive or flanged joints in steam tracing are specified to be outside, preferably underneath, the main pipe insulation system.
Some relative typical costs of applying foil and single paint coatings are as follows:
aluminum foil, s 2
aluminum-filled silicone, s 2 silicone-alkyd, s 2
The attractions of foil are self evident. Foil has the additional advantage that its application by lagging trades is accepted, whereas the use of coatings re- quires the involvement of additional trades in the lagging process, with conse- quent additional timing/management problems.
During the approximate 15-year period since aluminum foil was first speci- fied, there have been no failures of austenitic stainless steel surfaces caused by external stress corrosion cracking where the foil had been applied to specifi- cation. Admittedly, a relatively small sample of the total lagged surface area has been available for inspection during that period, but a small number of vessels and piping systems have had their laggings removed after periods of service up to 10 years without any evidence of cracking. There have been a few instances of failure of stream traced lines during this period where the protec-