Corrosion and Thermal Insulation in Hot areas A New Approach Mike Mitchell International Protective Coatings Stoneygate Lane, Felling, Gateshead, Tyne & Wear, England Fax No: + 44 (0)191 438 1709 E-mail: mike.j.mitchell@uk.akzonobel.com ABSTRACT In many process areas atmospheric corrosion problems are often most severe in areas where high temperature surfaces are subject to cyclic conditions and also where these areas are insulated. Solutions are proposed to prevent this corrosion by the use of non-zinc coatings. Also, a route to inherent corrosion protection, along with thermal insulation is presented. This also gives damage resistance, ease of application, adhesion and personnel protection. Further derivatives of this technology, which will also be discussed, allow thermal insulation of substructures, 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 losses and to improve the efficiency of the process. Typically, insulation used has been based on Rockwool, Foam Glass or Calcium Silicate. These 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 seal 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 seals 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 serious on onshore sites, especially in dry areas, if there is any water present ideal corrosion conditions do exist, pitting can occur and, consequently, premature failure of the pipe. Offshore the situation is obviously much worse, where sea water can be used for hosing down and also possibly as part of a fire control deluge system which is often regularly tested.
As well as these 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 these 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). These 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 ceases 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 then 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, seemed to be the solution to protection in all high temperature areas. However, in the wet situations under insulation previously described, failures started to be observed sometimes leading to actual perforation of the pipe. There are various theories to why this severe 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 severe 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 present which can be slightly acidic or alkaline (this can increase due to evaporation), and due to the amphoteric nature of the zinc any move from neutral ph will cause an increase in solubility. Due to these problems a divergence of approaches of corrosion engineers has occurred. Many follow the NACE recommendation that zinc silicate should not be used 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 case, the corrosion problem can potentially be alleviated by sealing off the zinc silicate with a suitable primer and thus obtaining all the benefits. 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 proposed which, although initially more expensive, will not require the normal maintenance and is designed to eliminate the corrosion problems. Because the material can be either installed by spray direct to the substrate on large areas, or using pre-cast half shells on piping, sealed and glued to the surface, water ingress and hence corrosion has been eliminated. This has been achieved by utilising two phase and three phase epoxy syntactic foams. At first sight it may be thought that epoxy will not have sufficient temperature resistance, however, research shows that on most plants more than 90% of the hot steel is operating below 120ºC (248ºF), and thus this material can be used in the majority of circumstances. Long term tests (30 months) have been undertaken with a cast two phase 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 these areas repaired. Repeating these tests on pipe sections, coated with syntactic foam and placed in ASTM B117 Salt Spray for 6000 hours 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 46 Density 0.6 Thermal Conductivity ASTM C177 0.118 W/M-C Specific Heat DSC 1.25J/CM-C Tensile Strength @ 25ºC (77ºF) ASTM D638 3270 p.s.i. 60ºC (140ºF) ASTM D638 740 p.s.i. 100ºC (212ºF) ASTM D638 220 p.s.i. Tensile Strain @ 25ºC (77ºF) ASTM D638 0.8% Tensile Modulus @ 25ºC (77ºF) ASTM D638 0.337M p.s.i. 60ºC (140ºF) ASTM D638 0.056M p.s.i. 100ºC (212ºF) ASTM D638 0.018M p.s.i. Compressive Strength @ 25ºC (77ºF) ASTM D695 8145 p.s.i. 60ºC (140ºF) ASTM D695 2467 p.s.i. 100ºC (212ºF) ASTM D695 1389 p.s.i. Compressive Strain @ 25ºC (77ºF) ASTM D695 5.20% 60ºC (140ºF) ASTM D695 9.20% 100ºC (212ºF) ASTM D695 7.30% Compressive Modulus @ 25ºC (77ºF) ASTM D695 0.157M p.s.i. 60ºC (140ºF) ASTM D695 0.027M p.s.i. 100ºC (212ºF) ASTM D695 0.019M p.s.i. Hardness ASTM D-2240 60 Shore D Water Absorbtion @ 3000 p.s.i. BS903 1% (5% NaCl)
Depending upon the method of application, it is possible to vary the specific gravity as the foam moves from a two phase material based basically on insulating beads and cured epoxy resin with glass spheres, to a three phase material also including air (or other gases) dispersed through the matrix. The impact of this enables the specific gravity to be varied between 0.6 (two phase) and 0.4 (three phase), with a corresponding increase 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, based on epoxy phenolic resin systems. This is one possible approach to the prevention of corrosion of hot surfaces, especially those which are insulated, but realistically is unlikely to achieve universal acceptance in the short to medium term, consequently, a more conventional approach has also been considered which can obviously be utilised on uninsulated steel, as well as insulated surfaces. As mentioned previously, there have been many problems with zinc silicate based systems when used 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 cases failed through lack of adhesion or blistering. It is necessary to consider this high temperature atmospheric scenario further, the zinc silicate is basically present to give corrosion resistance during construction, and whilst the plant is not operating at high temperature. The aluminium silicone sealer which is normally used is present 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 because of this effect. Generally the aluminium silicone is present for aesthetics and to protect zinc from alkaline or acid conditions. There is a considerable lack of understanding of the performance parameters of these aluminium silicone based coatings. Basically they are designed to be applied at dry film thicknesses of 15 microns (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 thicknesses of greater than 15 microns (0.6 mils) can lead to blistering and adhesion loss, primarily caused by the water vapour generated by the curing mechanism (Figure 8). It is always much safer to use 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 causes problems on insulated high temperature steel (>250ºC, >482ºF) where because of concerns regarding wet conditions zinc silicate may not be used, but 60 microns (2.4 mils) of aluminium polysiloxane certainly does not give the required hot water resistance. Recent developments have focussed on a number of areas to try to improve the coatings industry solutions to these problem areas. These 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. 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 systems may give poor performance, in a C5M environment appears satisfactory for 6 months to date. No defects are seen after subsequent high temperature cycling. Thus to summarise, 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 Test run at 121ºC (250ºF) then section cut for examination, temperature increased to 160ºC (320ºF), then area cut out for examination. Corrosion can be observed on 250 area which was not protected during 320 test. Figure 3 Demonstrates use 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 exposed after removal by chiselling. Figure 6 Epoxy syntactic foam insulation 6000 hours ASTM B117 salt spray
Figure 7 Epoxy syntactic foam insulation 6000 hours ASTM B117 salt spray Figure 8 Detachment of topcoat from zinc silicate primer after heating to 400ºC (752ºF) Figure 9 5000 hours external exposure test panels Single coat system : 100-125µm (4-5 mils) d.f.t. Figure 10 5000 hours hot salt spray Single coat systems : 100-125µm (4-5 mils) d.f.t. Heated to 400ºC (752ºF) for 8 hours, cooled (2 cycles) Figure 11 Cyclic Testing Apparatus (180ºC 450ºC, 356ºF - 842ºF)
Figure 12 3 x 25µm (1 mils) Silicone Aluminium 42 days cycling Cycle : Wet, Heat to 450ºC (842ºF) for 8 hours, wet, ambient for 16 hours, wet. Note corrosion spots. Figure 13 1 x 125µm (5 mils) Experimental 42 days cycling Cycle : Wet, Heat to 450ºC (842ºF) for 8 hours, wet, ambient for 16 hours, wet. Free of corrosion.