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Developments in coatings for high-temperature corrosion protection: this paper describes development of new products in high-temperature corrosion protection, the difficulties encountered in the development of a non-zinc high temperature coating for use up to 400[degrees]C, and an alternative approach to thermal insulation with in-built corrosion and damage resistance.

The right coatings system correctly applied on structural steel and tankage in process plants can confidently be expected to give good corrosion protection for periods between 10 and 20 years. However on hot areas both atmospheric, with temperatures up to 500[degrees]C, and protection under insulation for high temperature pipes and vessels this is often not the case. Many coatings used in these areas are too intolerant for site application and general use in this industry, leading to expensive premature failure.

This paper describes development of new products in these areas and also the difficulties encountered in the development of a non zinc high temperature coating for use up to 400[degrees]C. An alternative approach to thermal insulation with in-built corrosion and damage resistance is also covered. The development of internal test methods necessary to undertake this work is also described.

Background

Use of the correct high performance coating specification with the required standards of surface preparation and application currently gives systems which will last in excess of 10 years, and even more than 20 years, before significant maintenance is required, depending upon the corrosivity of the atmospheric environment. This is the situation for ambient atmospheric exposed steel. Unfortunately it is not the case for hot process areas such as vessels, piping and stacks, where failure and breakdown are all too common.

It is worth considering what factors lead to eventual coating breakdown under the two situations described above. Firstly, considering the atmospheric coatings, this has been the main area of coatings study over the past years, and there is now general agreement that breakdown is due to the synergistic effect of a number of climatic conditions. Coatings degrade by an accumulation of the effects of wetting and drying, some diurnal temperature cycling and UV degradation of the polymer. This is well recognized in that the use of ASTM B117 Salt Fog as a method of selecting coating systems is now treated with severe skepticism, and it is being replaced by a variety of wet and dry cyclic tests combined with UV exposure, primarily ASTM D5894.

What happens when a typical organic coating is subjected to high temperatures? Eventually as the temperature rises, "charring" or "browning" of the coating occurs as the organic polymer degrades, eventually leaving a mixture of ash and pigments on the surface with no film integrity and no protective value.

However, even before these stages are reached, significant changes are likely to have occurred in the coating. In crosslinked coatings the effect of high temperatures is generally to further drive the crosslinking reaction, increasing the crosslink density, generally improving most aspects of performance of the film but potentially causing increased brittleness at ambient temperatures. With the old chlorine based polymers, widely used in chlorinated rubber and vinyl coatings, as the temperature approached 100[degrees]C the thermally unstable polymer would 'unzip' and hydrogen chloride gas formed. The other immediate effect of heat on a coating film is to drive out any low molecular weight or volatile components, often included as diluents, flexibilizers or plasticizers, or extending resins.

Typical materials here can be plasticizers such as di isodecyl phthalate, diluents such as benzyl alcohol, low molecular weight unreacted resin or curing agent, and liquid hydrocarbons used as extending resins. Cycling between elevated temperature and ambient temperature places further stress on the coating, especially when considering the differences in coefficients of thermal expansion. It also needs to be realized that with many coatings taking them above their Tg will cause an increase in permeability with the possible consequence of increased corrosion.

Historically, the approach adopted for high temperature areas has been to move to inorganic coatings, typically based on silica where the -Si-O-type bonds are very thermally stable. Unfortunately, coatings based on these types of polymer tend to be inferior to organic coatings in many other aspects, such as adhesion, flexibility toughness, etc. Up to around 250[degrees]C it is possible to modify the organic resins (as is normal in high durability polysiloxanes) to impart improved film properties to the glass-like silicone or siloxane coating.

Often these inorganic films have only been suitable for application in thin films of 0.5-1.0 rail, thus giving limited corrosion resistance (especially on blasted steel) and relatively poor mechanical properties. Silicone coatings which fall into this category do not crosslink until around 20[degrees]C, part of the reaction is to produce water vapor which must leave the film.

When the film is too thick blistering and flaking occurs because of the inability of this reaction product to escape. When considering coatings on high temperature uninsulated surfaces it is worth listing the various stresses on the coating, which include those mentioned earlier plus others:

* UV;

* Wetness;

--shutdown or low temperature part of cycle

--thermal shock from rain at high temperatures); and

* Thermal cycling, expansion/contraction.

High Temperature Exposure Surfaces

As well as the silicones mentioned earlier, there have been other approaches to corrosion resistance in these circumstances (i.e. exposed uninsulated high temperature steel), e.g.
Zinc Bust/ Inorganic coating
Graphite/ Heat (corrosion
 resistance but no
Bodied Oil film integrity)

Zinc Ambient Corrosion
Silicate Cure resistance to
 400[degrees]C


There has been concern with zinc silicate, with the belief that the zinc would melt at 400[degrees]C, and this was thus the maximum temperature of use (as with galvanizing). In fact, this does not happen, the zinc particles remain intact because of being embedded in the silicate matrix, and more rapid oxidation occurs. (Note, zinc dust subjected to same conditions melted.)

The zinc silicate can be overcoated by silicone aluminum, but over-application of this type of material frequently leads to loss of adhesion from the zinc silicate and flaking as the temperature is increased.

In this case, unlike atmospheric coatings, there are no sensible test methods, those mentioned in ASTM (e.g. D2485) are more or less inadequate and so it has been necessary to develop in-house methods involving thermal cycling, thermal shock and corrosion resistance.

One beneficial circumstance for high temperature uninsulated atmospheric coatings is that they do not stay wet. When operating at high temperatures they are only wet for very limited periods of time during part of the cycle or shut down. In these circumstances zinc silicate by itself can give good protection for long periods of time.

Insulated Surfaces

With insulated surfaces, cycling and thermal shock situations are "damped' and are less drastic than those previously described. However, this benefit to the coating is far outweighed by the fact that extremely corrosive environments can be achieved with wet insulation and maintained for long periods of time, to give either a hot wet or hot humid environment. The presence of chloride can make the environment even more aggressive, as can any soluble materials in the insulation, taking the pH of the hot water to slightly acid or alkaline condition, extremely aggressive for any zinc which may be present due to its amphoteric nature.

Initially, zinc silicates appeared to be the solution for piping and vessels to be insulated, it allowed them to be prepared and coated off site and had ample robustness and corrosion resistance to allow coated objects to remain uninsulated during erection, without being damaged. However, the amphoteric properties of zinc meant that in the conditions described earlier the coating had an extremely limited life. There has also been a suggestion with zinc silicate in these hot, wet conditions that there can be a reversal in polarity so that the steel protects the zinc and pitting corrosion occurs. This is based on a well documented phenomena regarding zinc metal and steel in chloride solution. It is difficult to find any literature references to this being the situation with zinc silicate and being proven experimentally. Certainly, attempts made to do this at International have not been successful.

In practice, this area of 60-80[degrees]C under insulation has been found to be the most aggressive because of both the longer period of wetness and the acceleration of zinc, and other, corrosion due to temperature. (1,2)

These failures of zinc silicate, whatever their cause, has led to considerable reluctance of many engineers to use zinc silicate under insulation and the current NACE recommendation is not to do soft Some individuals, however, find that the flexibility and workability of zinc silicate coated objects outweighs these potential problems, and by adopting various methods of sealing off the zinc they attempt to harness the anti-corrosive properties without suffering from the in-use problems mentioned. It should be stressed that thin film silicone coatings do not have sufficient corrosion resistance on blasted steel for use in these circumstances, and it is best to use thicker barrier coats over zinc silicates, such as zinc free inorganic silicate.

Current Position

There are a number of drivers for the current product types used in this area, mainly performance and a desire to simplify the process by using a 'universal' system.

In many instances the bulk of hot steel does not operate above 200[degrees]C, and often not above 150[degrees]C. These types of temperatures allow the possibility of coating all steel, insulated and uninsulated, with an organic coating, typically this is a specially formulated epoxy phenolic which will resist dry heat up to 230[degrees]C (450[degrees]F), when applied in two coats to give 200-250 microns d.f.t. Corrosion resistance is achieved from both ambient cure films and from coatings which have been heated. This type of densely crosslinked coating will resist the hot, wet conditions which can be found under insulation.

Often areas down as low as 60[degrees]C are insulated for personal protection, greatly increasing the potential for the problem of corrosion under insulation, and although most high performance epoxy systems will work okay in hot, wet conditions at 60[degrees]C, this tends to be a maximum.

Where a universal primer is needed for a wider temperature range, zinc silicates are often still used, the practicalities of paint application and construction outweighing any concerns regarding the performance of zinc primers under insulation. The topcoat is selected depending on the area of use, the topcoat being based on silicone or siloxane resins, silicate or a silicone/acrylic blend for lower temperatures. With the exception of the zinc free silicate type, these are often too thin to give good long term protection, and the conventional silicones also suffer from sensitivity to over-application.

In the absence of any well recognized or meaningful methods of coatings evaluation under these conditions, it has been necessary to consider the development of new test methods to evaluate the performance of both existing systems and developmental materials.

These have needed to consider both uninsulated and insulated situations. The basis of these tests is as follows:

Coating evaluation tests

Cyclic heating/anti-corrosive tests: Panels of applied system are heated in a furnace to target temperature at a ramp rate of 20[degrees]C/minute. This target temperature is maintained for eight hours and then allowed to cool naturally to ambient (~16 hours).

Two heat/cool cycles are performed before exposure to various anti-corrosive tests (accelerated and natural weathering).

Thermal cycling based on ASTM D2485: Coated panel is exposed to thermal cycling/quench, each cycle is incrementally higher up to the target temperature (400[degrees]C). The cycled panel is then subjected to anti-corrosive testing (prohesion/water immersion 40[degrees]C).

Cyclic heating|wet dry cycling (pipe test): A coated pipe is insulated and placed on a hot plate where a measurable thermal gradient (typically 450[degrees]C 60[degrees]C) is created. The pipe is heated for eight hours with 16 hours natural cooling, the calcium silicate insulation is soaked before and after the heating cycle--30 cycles are performed.

Atlas Cell Evaluation--Modified SMT 52H: Immersion in hot water (500 hours at 95[degrees]C) is carried out on coating system. Systems are evaluated for breakdown and also any changes in adhesion.

Thermal cycling cold/hot: Panel is cycled from -200[degrees]C to 200[degrees]C+, depending upon type of coating to be tested.

Intention here is to evaluate coating performance under insulation on refrigerated pipework with occasional steam purge.

Current Development

Work is now directed at developing an inorganic coating which will extend the temperature range where corrosion resistance can be achieved by non-zinc coatings, in both insulated and non-insulated environments, above that which is currently achievable with the epoxy phenolics. The main technological barrier to cross here is to achieve good anti-corrosion properties before any high temperature cycles are present.

Examples of the type of performance which has been achieved to date are shown in the photographs below. In all instances the coating is an aluminum pigmented inorganic system applied at 1 x 125 microns d.f.t., direct to grit blasted steel.

Anti-Corrosive Thermal Insulation Systems

An alternative approach to corrosion protection under thermal insulation has been considered, i.e. to produce an insulating epoxy based syntactic foam which will act as both a thick anti-corrosive coating and a thermal insulator. A further benefit of this type of approach is that the system is much more mechanically robust than conventional insulation systems, resisting impact damage, e.g. operators stepping on insulated pipes.

On page 34 is a graph showing thickness-determination by calculation and by actual temperature measurement against thickness on a coated vessel containing hot oil.

Although the thermal efficiency may not be as high as for conventional systems, such as foam glass, the benefits of off site or spray application, resistance to mechanical damage and corrosive environments and potential lifetime use without replacement outweigh the greater thickness.

The maximum steel temperature for this system is 150[degrees]C, which allows most of the hot areas on a typical site to be insulated. This temperature has been determined by coating a steel pipe and running hot oil through it then examining adhesion etc. Further tests on adhesion at 150[degrees]C, 180[degrees]C, 200[degrees]C, have shown no significant adhesion drop until 180[degrees]C.

Properties for this insulation coating are shown in the appendix (page 40) and help to demonstrate the robust, water resistant nature of the material.

Summary

A number of typical product types used to protect high temperature steel have been reviewed, along with assessments of them by a range of test methods. The performance of a new experimental material has been described, showing significant advantages over current approaches.

Finally, an alternative approach to protection at the corrosively aggressive bottom end of the high temperature range has been presented, utilizing an epoxy syntactic foam which functions as both an anti-corrosive and thermal insulation.
APPENDIX: EPOXY SYNTACTIC FOAM

Physical Properties
Density = 0.6g/cc

PROPERTY VALUE SOURCE TEST METHOD

Specific Heat 1.25j/g- International DSC
 [degrees]C Coatings

Thermal Conductivity
 (at 2[degrees]C) 0.118W/m = SGS ASTM C177
 [degrees]C
 (at 60[degrees]C) 0.12W/m - SGS ASTM C177
 [degrees]C
 (at 100[degrees]C) 0.126W/m - SGS ASTM C177
 [degrees]C

Exposure to Water at 0.245W/m - FMC ASTM C177
3000 p.s.i. And 93 [degrees]C
[degrees]C

Thermal Expansion
 (at 2[degrees]C) 33X10-6/ Touchstone ASTM E228
 [degrees]C
 (at 60[degrees]C) 49X10-6/ Touchstone ASTM E228
 [degrees]C
 (at 100[degrees]C) 58X10-6/ Touchstone ASTM E228
 [degrees]C

Abrasion 72mg/1000g Touchstone ASTM 4060

Catholic Disbondment None Touchstone ASTM G8
 Observed

Creep-Uniaxial 0.019in/in at 4000 p.s.i. (1000 hours)
Compression

Shear Modulus 0.026 m.p.s.i. International Prism Method
 Coatings

Hardness 60 Shore D International ASTM D2240
 Coatings

Hardness after 1176 50 Shore D Geoscience ASTMD2240
Hours at 93[degrees]
C and 3000 p.s.i.


References

(1) R. Nixon, Corrosion Under Insulation, p14-17, Corrosion Management, May/June 2002.

(2) T. M. Mallen, "A Refinery Approach to Corrosion Under Insulation," p18-21, Corrosion Management, May/June 2002.

(3) NACE Publication 6H189 (Latest Revision), "A State of the Art Report on Protective Coatings for Carbon Steel and Austenitic Stainless Steel Surfaces under Thermal Insulation and Cementitious Fireproofing," (Houston TX, NACE).

Cost, Performance Matter Most in Corrosion Battle

Even as industry focuses on finding less toxic materials, performance and cost factors remain paramount.

Rust never sleeps, and neither does any type of corrosion for that matter. The annual direct cost of corrosion in the U.S. is estimated to be $276 billion a year, according to Nace International, The Corrosion Society.

Meeting current environmental regulations and price are common issues faced by all coatings manufacturers, but for makers of coatings formulated specifically to combat the toughest types of corrosion, performance is paramount. It often becomes a tightrope walk.

"Environmental issues are probably the primary, factors driving our industry and seem to be getting more attention daily," said Walt Conti, technical specialist at Buckman Laboratories. "Coatings that have low VOCs and are safer to use are the goal of this concern. However, coatings must continue to be reasonably priced and meet the performance needs of the customer," Conti added.

"Corrosion inhibitors with hexavalent chrome, such as zinc chromate or strontium chromate are restricted in use," said Conti. He added that while VOCs have been an issue for a number of years with certain liquid corrosion inhibiters, they continue to be of concern to regulatory bodies pushing for greener coatings and raw materials.

Among the newest entries from Buckman that help companies move toward green materials is Butrol 465M, an oxyaminophosphate salt of magnesium designed to give performance equal to zinc and/or strontium chromate. Conti said Butrol 465M was designed for high-end coatings in the aerospace industry for use over aluminum because it has shown excellent performance in stopping filaform corrosion, and it has shown outstanding performance in conventional and high solids, solvent-based resins.

Tony Gichuhi, R&D scientist with Halox, also noted the focus on reducing VOC and HAPS solvents, and the evolution toward water-based technologies. But he was quick to note that there is no getting away from cost.

"The paint and coating market is cost-driven. Customers are unwilling to pay premium prices unless the products can demonstrate remarkable performance against the often cheaper toxic-based corrosion inhibitors."

Gichuhi continued, "The present trend in coatings is toward thinner films, fast cure (UV or EB cure), water-based, high solids, VOC and HAPS-free systems that can meet the demanding requirements established by conventional solvent-borne technology for e.g. weather resistance, chemical resistance, and mechanical strength. High performance coatings--such as aerospace coatings--are more difficult to formulate with non-toxic corrosion inhibitors because of the strict requirements that so far only toxic corrosion inhibitors can meet."

According to Gichuhi, Halox's product roster offers corrosion inhibitors that can improve the performance of waterborne systems to match those of traditional solventborne coatings, and these products "can overcome some of the major problems associated with newer technologies such as high solids, powder coatings where poor adhesion is often a hurdle since they demand that very clean surfaces be used." The firm's Coil-X products are specific as strontium chromate alternatives for the coil coating market.

"Performance is the final driving factor in corrosion science," added Conti. "How well an inhibitor protects the metal substrate from destruction by corrosion will remain an important factor. No matter how inexpensive the inhibitor is, it still must work."

STAEHLE TO DELIVER PLENARY LECTURE AT NACEXPO CORROSION/2004 IN NEW ORLEANS

If you're into corrosion, or at least formulating coatings that combat it, new Orleans is the place to be come March 28. NACE International, The Corrosion Society, will host its annual conference--NACExpo/Corrosion/2004--March 28--April 1 at the Ernest N. Morial Convention Center in New Orleans. The annual event includes an exhibition and technical symposia.

The plenary lecture (8:00 a.m, March 29) will be delivered by Roger W. Staehle, adjunct professor of the department of Chemical Engineering and Materials Science at the University of Minnesota and an industrial consultant. A former dean of the Institute of Technology at the university, Staehle was also a professor of Metallurgical Engineering at The Ohio State University And directed the Fontana Corrosion Center. His research interests include predicting the corrosion performance of engineering equipment, stress corrosion cracking, passivity and corrosion in aqueous environments. His consulting includes work for major international industries and governments in the area of predicting corrosion performance, corrosion and the prevention and analysis of failures. He has edited or co- edited 23 volumes relating to corrosion and has published 160 technical papers. He is a former Editor of Corrosion and Advances in Corrosion Science and Technology.

For more information on attending or exhibiting contact NACE International. 1440 South Creek Drive, Houston, TX 77084-4906; (281) 228-6242; Fax: (281) 228-6342; Web site: www.nace.org.

Michael J. Mitchell INTERNATIONAL PROTECTIVE COATINGS, AKZO NOBEL, ENGLAND
COPYRIGHT 2004 Rodman Publications, Inc.
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Publication:Coatings World
Date:Feb 1, 2004
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