Coal Preparation » General
The main objective of this study was to develop a cost effective methodology to ensure that only high quality coating systems are approved for use at the Coal Handling and Preparation Plants (CHPPs) structural components. This goal is to be achieved through development of methods that allow reliable assessment and selection of potential coating systems as well as allowing initial pre-qualification testing of candidate coating systems in order to provide a benchmark for coatings performance that could be then upgraded as better coatings are developed. The report outlines the tests that are undertaken and the test data that is submitted by the manufacturers for coating evaluation purposes. It also provides guidance on the performance acceptance criteria and the significance of each criterion in the coatings ranking process.
This document has been written following an investigation that included the following main steps:
- Review of laboratory and field testing methods for key physical properties of coatings developed by the recognized standard institutes, paint manufacturers and by end users.
- Development of a protocol of the accelerated laboratory and field tests relevant to corrosive environment present at the CHPPs .
- Establishment of coating performance acceptance criteria
- Laboratory and field testing program of coating systems submitted by the coating manufactures
- Development of coating selection methodology
- Ranking the selected coating systems according to expected performance in different sections of typical CHPPs.
A number of internationally recognized coating pre-qualification standards have been reviewed for relevance of deployed testing methods to operating environment present within the CHPPs (i.e. fitness for purpose) and correlation of these tests with the results of field exposure tests and actual operating experience. The selected pre-qualification standard was the National Association of Corrosion Engineers (NACE) standard TM 304. Whilst this standard was developed specifically for offshore platforms, the standard's committee stated that the recommended test program and coating performance acceptance criteria would also be applicable to many industrial operations. The test protocol deployed in this standard was found to be the most rigorous laboratory testing program of all standards reviewed.
Six coating manufactures were approached to participate in coatings pre-qualification program all expressed interest in the study. However, following the presentation of testing protocol and criteria on which the coatings performance would be evaluated, two of the companies declined to sign-off the indemnity and input agreements. The companies that agreed to participate in the coating performance evaluation program were:
- International Protective Coatings (AKZO Nobel),
- Ameron Coatings,
- Wattyl Protective and Marine Coatings and
- Dulux Coatings.
The project monitors also requested to include in Thermal Spray Metalised (TSM) coatings in the test program due to the positive feedback on the performance of these coating systems at two of the Hunter Valley CHPPs. In total, fifteen coating types were submitted for pre-qualification testing under three coating systems categories. These were:
- Two/three coat new construction system (7 systems)
- Two coat maintenance system (4 systems)
- One coat maintenance system (4 systems)
In addition to laboratory testing program about 130 test panels were scheduled to undergo field exposure tests at two CHPPs over a period of 3 years. The selected sites were BMA Peak Down mine in Bowen Basin with process water being mainly alkaline and Austar mine in Hunter Valley with mainly acidic water. Sixty five test (scribed and unscribed) test panels were installed at five different test locations at each of the mine sites. The selected test locations covered the broad range of operating/corrosive conditions present within each mine site. The test panels were inspected for their conditions after 10 months of exposure.
The main findings arising from laboratory and field exposure tests are as follows:
- No single coating passed all recommended laboratory testing acceptance criteria. The pass rate was:
- Rust creepage (uncontaminated surface) 8 out of 15
- Rust creepage (water contaminated surface) 3 out of 8
- Rust creepage (Chloride contaminated surface) 0 out 8
- Adhesion (uncontaminated surface) 8 out 15
- Adhesion (water contaminated surface) 5 out 8
- Adhesion (Chloride contaminated surface) 2 out 8
- Thermal cycling 14 out 15
- Edge retention 1 out 15
- Impact resistance 3 out 15
- Flexibility 5 out 15
- Abrasion resistance 7 out 15
- Substrate contamination has major impact on the development of coating blistering and surface rust both of which are responsible for poor long term performance of the coating system. Most of the maintenance coating systems with either water or chloride contaminated substrate developed blistering or surface rust in laboratory rust creepage and seawater immersion tests. The one-coat systems and coatings with thinner dry film thickness were most affected by substrate contamination. These test results clearly demonstrate the utmost importance of good surface/substrate preparation for the coating system to achieve its best performance.
- In addition to blistering and surface rust formation, substrate contamination also has a detrimental impact on adhesion of the coating system to the substrate (particularly for those contaminated by chloride solution). However, the majority of coatings achieved the pass criteria of >3.4MPa adhesion strength. Coating systems supplied by International and Ameron were found to provide best tolerance of water contaminated substrates. The bonding between the metal substrate and between the coats for these coating systems exceeded the bond between the coating and the dolly in many of the pull-off adhesion tests.
- Coating adhesion appears to have only a minor effect on rust creepage resistance of coatings. The main drivers of rust creepage/undercutting resistance are atmospheric factors including UV and frequency of wet/dry cycling. It was found that coatings which shown significant rust creepage in laboratory cyclic corrosion tests, performed relatively well in site environments of fairly constant exposure conditions (i.e. non UV exposure). It is recommended that coatings with rust creepage values outside the acceptance criteria should preferably not be used in areas where they will be exposed to conditions involving combination of UV with frequent wet/dry cycling.
- No significant difference in corrosion performance was observed for coating systems utilizing different primers (zinc rich epoxy, zinc phosphate epoxy or epoxy).
- Most coating systems have poor edge retention and would require significant edge profiling and stripe coating prior to application of the main coat. Some of these coatings did not even manage to retain 10% of the recommended coating thickness at the edges. The one coat maintenance coating systems had lowest edge retention ratios. The one pass grinding (or two pass grinding leading to about 2mm edge radius) followed by abrasive blasting surface treatment and a stripe coat is considered to be most recommendable practice to be exercised by the CHPPs to achieve sufficient coating edge coverage prior to application of topcoats. Coatings with poor edge retention should preferably not be used where good surface profiles may not be achieved such as sharp edges left after welding, un-chamfered edges, inadequately profiled corrosion pits etc.
- Majority of epoxy coating systems have poor flexibility and will crack as soon as they start to flex. It is recommended that these types of coatings should not be used over bolted joint connections and structural component that are likely to be exposed to bending due applied heavy load.
- All Dulux coating systems were found to develop cracking during thermal cycling test. This test provides insight into the strain tolerance of the coatings particularly along edges and corners (i.e. from vibration, wet/dry cycles, thermal variations, internal stresses etc.). From the durability perspective, the edges and corners are the weakest areas of any applied coating system and poor strain tolerance can lead to premature cracking of the coating in these areas. Since edge corrosion is the main driver of corrosion observed at the CHPPs the use of Dulux Durebuild STE HS glassflake epoxy systems is questionable.
- Only two epoxy based coating systems (International one and two coat maintenance systems) were found to have sufficient toughness to withstand large stress forces imposed through impact. Most of other epoxy based systems are relatively brittle and would be easily damaged particularly at the edges.
- Some epoxy coatings used as sealer coats may not be compatible with the thermal spray metallised coatings and could lead to coating blistering and delamination. The results of field testing have also revealed that the performance of the epoxy sealer coats may also be affected by the pH levels of the process water. It is strongly recommended that any sites considering the use of TSM coatings should carry out field exposure tests to confirm the compatibility between the TSM coating and the sealer coat.
- Inspection of the zinc based TSM coatings at one of the Hunter Valley mine has shown blistering of the coating in immersion type conditions and rusting mainly around lacing to column welded and bolted type connections. Many painted sections lost the topcoat and there was only thin film of zinc remaining on structural components exposed to high water pressure washing.
- The preferred TSM coating for use in industrial environments is 85% zinc/15% aluminium alloy. This coating material provides better corrosion protection in low pH and high chloride environments than zinc based TSM coating systems. Due to poor edge retention of the TSM coatings it is also very important to select an appropriate sealer coat and to apply thicker film at sharp edges and areas subject to water impingement and abrasion.
- Early results from field exposure show relatively good correlation with laboratory test results for most of the coatings. The exceptions are the TSM coating systems and International and Wattyl two coat maintenance systems.
- The laboratory tests adopted by NACE TM0304 standard have been able to identify coating systems with likely poor performance.
The means of quantifying the results of laboratory and field testing are numerous. This study presents a simple system that provides the relative performance ranking of candidate coating systems. The system is based on a scheme in which a number of points or percent are awarded for particular performance characteristics. A universal scoring system could be based on a 100 point/percent that are distributed among different criteria (i.e. specific properties of the candidate coating system). Criteria can come in different levels of significance/weighting including:
- Prerequisites (i.e. must have). Some examples of required characteristics are non carcinogenic, compatibility with substrate, can be recoated etc.
- Obligatory (say 60% weighting). Points are allocated for extent of blistering, surface rust, adhesion and rust creepage.
- Desirable (say 40% weighting). Points are allocated for such characteristics as flexibility, edge retention, abrasion resistance impact resistance, surface preparation and coating application quality etc.
Coatings with total score greater then 80% are considered to have the potential to provide good long term performance. The question of how long a particular coating will last is not one which can be quantified using this system. In fact there are currently no accelerated test methods that can reliably predict the service life of a coating in a particular environment. However, ongoing field exposure tests should give very good insight into the likelihood of long term performance of the fifteen coating systems in relatively wide range of CHPPs operating environments.
The above described coating selection methodology was used to undertake preliminary ranking of the coating systems potential performance in different sections and operating environments of a typical CHHP. Coating systems showing best overall performance are:
(a) New construction systems:
- Wattyl three coat system - Galvit EP100 zinc epoxy primer + Sigmacover HS MIO epoxy mastic + Sigmacover DTM800 HS/HB
- Ameron two coat systems - Amercoat 307 zinc rich primer + Amerlock 2K HS epoxy mastic
(b) Two coat maintenance systems
- Wattyl Sigma EP universal primer + Sigmacover DTM800 topcoat.
(c) One coat maintenance system
- International Interzone 954.
The uniformly good results for these top performers in immersion and atmospheric exposures is encouraging because most CHPPs applications for these materials involve structural components that are likely to be intermittently immersed in water and exposed to UV.
It is recommended that all mine sites insist on coating manufactures to provide test data according to laboratory tests outlined in NACE TM0304 standard. This will facilitate the poor performing coating systems to be readily identified/screened out and better performing products can be compared against performance benchmarks established in this report. The recommended test data to be submitted by coating manufacturers are:
- Rust creepage resistance (uncontaminated substrate - essential; uncontaminated substrate - upon agreement between involved parties)
- Sea water immersion resistance (uncontaminated substrate - essential; uncontaminated substrate - upon agreement between involved parties). Information to be provided include are results of adhesion tests and observations on blistering or surface rusting.
- Edge retention
- Impact resistance
- Thermal cycling resistance
- Abrasion resistance
It is emphasized that the use of accelerated testing techniques as sole basis for coating prequalification may pose some interpretation problems. Most important of these is the realism of these tests for a given environment. For this reason, coating systems identified to offer good performance from initial pre-qualification ranking should undergo field exposure tests to verify their performance under actual operating environment. The test panels should preferably be scribed to accelerate the corrosion process. Site exposure of a year or longer (preferably in severely corrosive environments present at the sites) would be extremely useful in identifying differences among the higher-performing products.
Improved selection of coating performance testing protocols along with proper interpretation of results and the methodology of how to quantify performance variables in a rating system based on individual test results should allow a site engineer or the coating consultants to provide enough information on how the coatings may perform in service. However, the ultimate criteria for pass-fail of coatings must be defined by the end user based on the criticality of service and probability of failure.
The final ranking of coating systems submitted for evaluation in this study will be provided at the end of field exposure testing which is expected to be completed in September of 2009. However, interim updated report on the condition of the coatings at the mine sites and any changes to current ranking of the coatings will be provided to ACARP in September 2008.