Corrosion and Thermal Insulation
in Hot areas – A New Approach
Mike Mitchell
International Protective Coatings
Stoneygate Lane, Felling,
Gateshead, Tyne & Wear, England
电信业务Fax No: + 44 (0)191 438 1709
E-mail: mike.j.mitchell@
ABSTRACT
In many process areas atmospheric corrosion problems are often most vere in areas where high temperature surfaces are subject to cyclic conditions and also where the areas are insulated. Solutions are propod to prevent this corrosion by the u of non-zinc coatings.
Also, a route to inherent corrosion protection, along with thermal insulation is prented. This also gives damage resistance, ea of application, adhesion and personnel protection.
Further derivatives of this technology, which will also be discusd, allow thermal insulation of sub-structures, even to depths in excess of 3000 metres.
1. INTRODUCTION
In many process plants it is normal procedure to insulate areas operating above 60ºC (140 ºF)., which are accessible to operators, in order to prevent burns and skin damage from contact. At higher operating temperatures it often also becomes necessary to insulate in order to prevent heat loss and to improve the efficiency of the process.
Typically, insulation ud has been bad on Rockwool, Foam Glass or Calcium Silicate. The materials have different degrees of water uptake, but all require cladding with stainless steel or binding with special tape in order to keep in place, to al from the weather and prevent water penetrating cracks and joins and reaching the steel surface.
2. BACKGROUND
Unfortunately, in almost all instances, the cladding once installed, even if initially watertight with the correct als and mastic, can be damaged by mishandling, e.g. walking across the pipes, damaging the protective cladding and thus allowing water ingress to occur. Although this may not appear immediately rious on onshore sites, especially in dry areas, if there is any water prent ideal corrosion conditions do exist, pitting can occur and, conquently, premature failure of the pipe. Offshore the situation is obviously much wor, where a water can be ud for hosing down and also possibly as part of a fire control deluge system which is often regularly tested.
As well as the insulated hot areas, on most chemical and petrochemical plants there are also high temperature areas with no insulation, such as flare stacks and exhausts. All the time the are operating at high temperature there is very little corrosion problem, but if there is cycling between high temperature and ambient then corrosion does become possible once the temperature drops below 100ºC (212ºF). The areas are normally coated with a high temperature paint system which tends to remain intact all of the time the structure operates at high temperature, but can crack and flake when subjected to temperature cycling and thus ceas to give protection at the lower temperatures where it is needed.
Traditionally, pipework on a process plant was painted in situ with oleoresinous type coatings and th
en lagged, however, this does not really fit with modern construction methods, and the advent of zinc silicate coatings with their inorganic characteristics with regards temperature resistance, excellent corrosion resistance, and resistance to mechanical damage during handling, emed to be the solution to protection in all high temperature areas.
However, in the wet situations under insulation previously described, failures started to be obrved sometimes leading to actual perforation of the pipe. There are various theories to why this vere corrosion occurs:-
•Polarity reversal in sodium chloride solution at 70-80ºC (158-176ºF), so that the steel becomes anodic and protects the zinc. This is recorded in the literature for zinc metal but experimental data on this phenomena for zinc silicate is difficult to find and we have not been able to reproduce the vere pitting in the laboratory, although slight pitting corrosion has been induced with cyclic wetness and temperature.
•The zinc is simply more soluble in the warm water prent which can be slightly acidic or alkaline (this can increa due to evaporation), and due to the amphoteric nature of the zinc any move from neutral pH will cau an increa in solubility.
Due to the problems a divergence of approaches of corrosion engineers has occurred. Many follow the NACE recommendation that zinc silicate should not be ud under insulation, in any form; even if topcoated, and others take the view that the benefits during construction of using zinc silicate outweigh the potential problems and that, in any ca, the corrosion problem can potentially be alleviated by aling off the zinc silicate with a suitable primer and thus obtaining all the benefits.
spam
3. THERMAL INSULATION
Clearly then, for most circumstances, corrosion under insulation can be prevented in two ways:-
•By using a coating system which will prevent corrosion in the potentially hot, wet conditions existing under the insulation.
•Design insulation which will not be easily damaged and will prevent water ingress, either by nature of the insulation or by an alternative more effective method of cladding.
Current insulation is difficult and time consuming to install and is very labour intensive, and typically will have to be replaced 2-3 times during the lifetime of a plant at high cost and disruption. An alternative method of insulation is therefore propod which, although initially more expensive, will n
ot require the normal maintenance and is designed to eliminate the corrosion problems. Becau the material can be either installed by spray direct to the substrate on large areas, or using pre-cast half shells on piping, aled and glued to the surface, water ingress and hence corrosion has been eliminated.
This has been achieved by utilising two pha and three pha epoxy syntactic foams. At first sight it may be thought that epoxy will not have sufficient temperature resistance, however, rearch shows that on most plants more than 90% of the hot steel is operating below 120ºC (248ºF), and thus this material can be ud in the majority of circumstances.
Long term tests (30 months) have been undertaken with a cast two pha epoxy syntactic foam on pipes containing hot oil at up to 150ºC (302º F) (Figure 1). There has been no sign of any adhesion loss and no signs of any corrosion (Figure 2). Figure 3 shows the areas repaired. Repeating the tests on pipe ctions, coated with syntactic foam and placed in ASTM B117 Salt Spray for 6000hours showed no indication of underfilm corrosion or any adhesion loss (Figures 4 to 7).
Table 1 shows general thermal, mechanical and other properties of this material.
Table 1 – Physical Properties of Epoxy Syntactic Foam Physical Property
Method Intertherm 46Density
怎么加密u盘肯麦基0.6Thermal Conductivity
ASTM C1770.118 W/M-C Specific Heat
DSC 1.25J/CM-C ASTM D6383270 p.s.i.ASTM D638740 p.s.i.Tensile Strength @25ºC (77ºF)
60ºC (140ºF)
100ºC (212ºF)
ASTM D638220 p.s.i.Tensile Strain @ 25ºC (77ºF)ASTM D6380.8%ASTM D6380.337M p.s.i.ASTM D6380.056M p.s.i.Tensile Modulus @25ºC (77ºF)
60ºC (140ºF)
100ºC (212ºF)
ASTM D6380.018M p.s.i.ASTM D6958145 p.s.i.ASTM D6952467 p.s.i.Compressive Strength @25ºC (77ºF)
60ºC (140ºF)
100ºC (212ºF)
ASTM D6951389 p.s.i.ASTM D695 5.20%ASTM D6959.20%Compressive Strain @25ºC (77ºF)
60ºC (140ºF)
100ºC (212ºF)
ASTM D6957.30%ASTM D6950.157M p.s.i.ASTM D6950.027M p.s.i.Compressive Modulus @25ºC (77ºF)
60ºC (140ºF)
100ºC (212ºF)ASTM D695
0.019M p.s.i.Hardness ASTM D-2240
60 Shore D Water Absorbtion @ 3000 p.s.i.
BS9031% (5% NaCl)
Depending upon the method of application, it is possible to vary the specific gravity as the foam moves from a two pha material bad basically on insulating beads and cured epoxy resin with glass spheres, to a three pha material also including air (or other gas) disperd through the matrix. The impact of this enables the specific gravity to be varied between 0.6 (two pha) and 0.4 (three pha), with a corresponding increa in thermal insulation efficiency.
There are a number of issues with this approach, primarily the higher cost due to lower thermal efficiency when compared to foam glass or calcium silicate, however, over a structure’s lifetime average costs per year are less.
4. HIGH TEMPERATURE ANTI-CORROSIVE COATINGS
Concurrently with the development of this insulation material, anti-corrosive primers suitable for overcoating the steel after blasting, capable of operating at the specified temperatures and allowing excellent adhesion of the insulation, have been formulated, bad on epoxy phenolic resin systems. This is one possible approach to the prevention of corrosion of hot surfaces, especially tho which are insulated, but realistically is unlikely to achieve universal acceptance in the short to medium term, conquently, a more conventional approach has also been considered which can obviously be utilid on uninsulated steel, as well as insulated surfaces.
As mentioned previously, there have been many problems with zinc silicate bad systems when ud under insulation which can potentially become wet, and there have been other and different problems with high temperature areas, i.e. greater than around 200ºC (392ºF) , where protective topcoats on the zinc have in many cas failed through lack of adhesion or blistering.
apple序列号
It is necessary to consider this high temperature atmospheric scenario further, the zinc silicate is basically prent to give corrosion resistance during construction, and whilst the plant is not operating at high temperature. The aluminium silicone aler which is normally ud is prent to prevent oxidation of the zinc particles in the zinc silicate, which occurs more quickly at elevated temperatures and is thought would reduce corrosion protection, destroying galvanic contact. This is correct but, in fact, the formation of the oxide, and other salts, gives a more effective barrier which although not having cathodic protective properties has superior barrier properties. There are many reported instances of untopcoated zinc silicate protecting for many years at temperatures above the melting point of zinc, presumably becau of this effect. Generally the aluminium silicone is prent for aesthetics and to protect zinc from alkaline or acid conditions.五月丁香花开网
There is a considerable lack of understanding of the performance parameters of the aluminium silicone bad coatings. Basically they are designed to be applied at dry film thickness of 15 micr
ons (0.6 mils), (not the often specified 25 microns (1 mils)), and require stoving at around 200ºC (392ºF) to give a cured film with optimum film properties. Application at dry film thickness of greater than 15 microns (0.6 mils) can lead to blistering and adhesion loss, primarily caud by the water vapour generated by the curing mechanism (Figure 8). It is always much safer to u one of the new ambient curing (moisture curing) systems which are more tolerant in film thickness, do not require heat curing, and allow application of multi-coat systems without heating between coats.
The problem with this type of material is that although it gives sufficiently thick (40-60 microns, 1.6-2.4 mils) films in multi coats to prevent corrosion of zinc silicate primed steel, films without this primer do not have good corrosion resistance, mainly due to too low a film thickness. This potentially caus problems on insulated high temperature steel (>250ºC, >482ºF) where becau of concerns regarding wet conditions zinc silicate may not be ud, but 60 microns (2.4 mils) of aluminium polysiloxane certainly does not give the required hot water resistance.
Recent developments have focusd on a number of areas to try to improve the coatings industry solutions to the problem areas.
The have concentrated on alleviating the limitations previously identified in the current portfolio of materials available., i.e.
•Organic systems, e.g. epoxy phenolic, maximum operating temperature 230ºC (446ºF).
•Zinc silicates – well documented.
•Aluminium silicones etc – insufficient thickness to give good barrier properties.
•High build polysiloxanes – limitations in repeated temperature cycling.
Test methods have had to be developed to evaluate coating performance in a number of situations, i.e.•Cyclic insulated high temperature piping with intermittent wetting.
•High temperature exposure followed by quenching in water.
•High temperature exposure followed by accelerated corrosion testing or natural weathering.•Natural weathering followed by high temperature exposure.
•Accelerated testing followed by high temperature exposure.泰坦尼克号经典台词
Examples of developmental results are shown in Figures 9 to 14. Excellent corrosion resistance has been obtained after heating, and although accelerated corrosion performance of ambient cured syst
ems may give poor performance, in a C5M environment appears satisfactory for 6 months to date. No defects are en after subquent high temperature cycling.
Thus to summari, a system has been developed to replace conventional insulation at temperatures up to 150ºC (302ºF), and significant progress has been made in the development of a “universal” high temperature anti-corrosive system for temperatures up to 400ºC (752ºF) and which is zinc free.
FIGURES
Figure 1
Apparatus for circulating hot oil at up to 160ºC (320ºF) around insulated test pipe.
Figure 2
干姜水Test run at 121ºC (250ºF) then ction cut for examination, temperature incread to 160ºC (320ºF), then area cut out for examination. Corrosion can be obrved on 250 area which was not
protected during 320 test.
Figure 3 Demonstrates u of epoxy foam for repair
Figure 4
Epoxy syntactic foam test specimens
after 6000 hours ASTM B117 salt spray
Figure 5
Epoxy syntactic foam after 6000 hours
ASTM B117 salt spray.
Surface expod after removal by chilling.
Figure 6
Epoxy syntactic foam insulation
6000 hours ASTM B117 salt spray
