Fellow’s Corner

Fellow’s Corner

In this series of features from ICorr Fellows who have made significant contributions to the field of corrosion protection, Malcolm Morris discusses the use of Thermal Sprayed Metal Coatings.

An Introduction to Duplex Thermal Sprayed Aluminium (TSA)
Thermal metal spraying is a process by which a steel substrate is protected from corrosion by coating with atomised particles of softened or molten metal. The most commonly applied metals used for this process are aluminium (TSA), or zinc (TSZ), however any metal which is ductile and able to be produced in a wire form can be thermally sprayed, (e.g. tungsten, molybdenum, lead, and even silver or gold), plus combined alloys of zinc and aluminium.

The most common methods of application are:
Gas Applied: A process in which a coil of metal wire is fed to the tip of an application gun, where the metal is melted by a gas flame, and the molten metal is simultaneously atomised by compressed air jets and directed onto the substrate.

Arc Applied: Similar to gas applied – Arc spraying is performed by feeding two electrically conducting metal wires towards each other. An electric arc is produced at the point just before the wires meet. The arc melts the metal wires, and a high-pressure air line is used to atomise the molten metal into fine droplets which are sprayed onto the previously cleaned and prepared steel surface. Arc application gives faster productivity compared to gas application.
Thermally sprayed surfaces have historically been supplemented with protective coatings, in order to increase the level of protection, and also as a cosmetic coat to mask the appearance of the corrosion products of the metal spray, which appear as aesthetically poor, white salt deposits. Paint systems range from a relatively thin single coat (designed to fill and seal the porous voids in the film, which naturally occur in the application of the metal spray), to multi-coat high build systems which are intended to significantly extend the service life of the structure.

A common feature of all thermal spray coatings is their lenticular or lamellar grain structure. Due to the nature of the application, the molten metal cools and solidifies instantaneously upon impact with the substrate, forming a porous matrix of flattened metal particles, interspersed with inclusions of metal oxide and unmelted metal. This porosity is sealed by normal ageing, through formation of salts of the component metals (oxides, hydroxides, carbonates, etc.), or as stated above by sealing, and or, painting.
The correct surface preparation is essential, involving cleaning followed by grit blasting to preferably white metal, Sa3 (BS EN ISO 8501-1), with a surface profile of at least 75µm. This provides the chemically and physically active surface needed for good bonding of the sprayed metal.
Both zinc and aluminium are anodic to steel. Zinc affords greater electrolytic activity than aluminium, providing good galvanic protection to steel exposed in the atmosphere. Breaks in the coating are protected by the galvanic couple, i.e. the zinc corrodes, rather than the structural steel, and its’ corrosion products will self-seal the breaks in the coating.

Aluminium is less active than zinc. Aluminium’s protection of the substrate relies more on a thin oxide passivating film at the surface, however because aluminium is less active than zinc, it cannot protect breaks in the coating.
It has been reported in the literature that the maintenance-free service life of an unprotected aluminium alloy metal spray coating on a typical bridge was predicted to be 25-40 years. Using a sealer and topcoat over the metalised layer is predicted to extend the service life 15-20 years [1]. Also, testing at SINTEF, Norway, in natural seawater, gave free corrosion rates of 2 to 3 microns per year after 11 months of exposure for both Al and Al 5% Mg. This would imply a service life of 60 years for a 200-micron Thermal Sprayed Aluminium Coating [2].

TSA with a single coat of sealer has a track record stretching over several decades, however the disadvantage of this system is the dull, rough appearance of the metal spray. Since the sealer is only applied as a thin coat, it is not possible to enhance the surface appearance, and the corrosion products of the metal spray may become obvious within a short period.

This led to the application of a high-performance paint specification on top of the metal spray in order to enhance its durability, and these so-called duplex systems have been used extensively, based on both TSA and TSZ processes. TSA based systems were specified in the Norwegian Offshore sector, with TSZ used widely on structures such as road bridges.

TSZ systems have performed very well over extended periods (up to 30 years), however TSA duplex systems began to display signs of premature breakdown of the coating system, coupled with extensive degradation of the metal spray coating. Examples of such breakdown included Hutton TLP, Sleipner Raiser Platform, Troll A platform, and at the Troll plant at Kollsnes.

Thermal metal spray coatings have a long service track record in the petrochemical and marine sectors listing advantages such as:

  • Limited maintenance
  • Low service life cost
  • Superior adhesion
  • Resistant to mechanical damage
  • Self-healing
  • No drying or curing time
  • Wide range of operating temperatures: -45°C to 538°C

Research was carried out with a series of panel exposures using both gas and arc applied TSA, plus TSZ. These metal sprayed substrates were sealed with a selection of different sealers, and then overcoated with a medium build epoxy MIO/polyurethane specification, and also a high build glass flake epoxy. Panels were immersed in salt water and fresh water, plus natural weathering in both industrial and marine environments.

The results of this series were quite startling. After 24 months saltwater immersion severe degradation of all the paint systems over TSA was observed; with extensive degradation of the underlying metal spray. The large quantities of white aluminium corrosion products effectively blew off the paint system. No differences were observed between any of the different sealers, or between gas or arc applied TSA. All systems applied over TSZ were satisfactory on immersion up to 12 years salt water immersion. All fresh water and atmospherically exposed panels also performed well up to 12 years.

From this testing, it was evident that application of high build coating systems over TSA gives very poor performance when subjected to high chloride salt environments, such as hot salt spray or salt water immersion. The speed of the failure is variable, possibly depending upon the efficiency of the sealer coat, however the end failure effect is the same in all cases. Performance of the same specifications on natural weathering in an industrial C3 environment and fresh water immersion is generally good. Thermally sprayed zinc substrates perform well in all environments.

The definitive document on the theory of degradation of coating systems over TSA was published in 2004 by Torstein Rossland of Statoil [3]. The paper describes a number of system failures in the North Sea oil industry, and suggests the following mechanism for coating failure.

When the duplex coating is in galvanic contact with bare steel, galvanic corrosion of the thermally sprayed aluminium is initiated. Cathodic oxygen reduction takes place at the bare steel, while anodic dissolution of the thermally sprayed metal takes place under the organic coating.

In chloride containing environments, such as in marine atmospheres, chloride ions migrate under the organic coating to maintain the charge balance. Aluminium chloride is then formed.

This is highly unstable in the presence of water, reacting to form hydrochloric acid. The electrolyte under the organic coating will therefore be acidic. Cathodic hydrogen evolution will start under the organic coating, which will increase the corrosion rate of the TSA. The TSA will not be passive in the low pH electrolyte. Hence, the thermally sprayed aluminium corrodes actively and rapidly. The organic coating holds the aggressive electrolyte at the surface. TSA with only a thin sealer will not suffer from this type of degradation since the aggressive electrolyte will migrate out of the sealer.

The recommended treatment for pure aluminium metal spray in a high chloride environment would be to apply a thin sealer coat to fill the porosity in freshly applied TSA in order to prevent corrosion through the pores before the surface can self-seal naturally by weathering, and then protect the substrate from corrosion as intended.
It should be noted that thermally sprayed zinc, or aluminium alloys, do not suffer from the same problems when overcoated by high build coating systems in high chloride environments.

References
1) Journal of Protective Coatings & Linings, May, 1995.
2) Materials Performance, April, 1995. Authors: Karl P. Fischer, William H. Thomason, Trevor Rosbrook, Jay Murali).
3) Rapid degradation of painted TSA. Author: Torstein Rossland, (Statoil, Bergen, Norway ).

Corrosion Under Insulation

Corrosion Under Insulation

Corrosion under a layer of insulation is a hidden threat that is very hard to detect. Additionally, it can lead to extensive damage that can burden companies with high reparation costs.

Join our successful Corrosion Under Insulation Masterclass led by Dr. Clare Watt and Dr. Steve Paterson and gain insight into the latest developments!

Since the event starts next week, make sure to save your spot now.

Key topics to be discussed include:

  • Understanding key characteristics of corrosion under insulation (CUI)
  • Examples of critical CUI failures
  • Elements of reliable CUI management
  • Risk-based prioritisation and prediction methods
  • Design considerations (insulation systems and coatings)
  • Direct and indirect detection and monitoring technology
  • An overview of published guidance
  • Latest innovation status and projects

For more information, please visit the event website.or download the event brochure to see the detailed program

Ask the Expert

Ask the Expert

The question in this issue features solvent free epoxy coatings. Readers are reminded to send in their technical questions for possible inclusion in this column in future.

Question:
I wish to use a solvent free epoxy coating to lower the environmental impact of my maintenance painting project, however when I look at the products available there appears to be a wide range of volatile organic compound (VOC) content in such coatings. Why does a solvent free epoxy coating contain VOC , and what is the difference between a solvent free epoxy, and a 100% solids epoxy coating? PF

 

Answer:
A 100% solids epoxy coating contains no solvents, no VOC and retains the same level of thickness from the time they are applied to the time they dry. There is currently no clear universal definition of a Solvent Free Epoxy Coating. The generic name indicates that the epoxy coating should be “solvent free”, but when looking at these coatings in the market today, we see that that is not always the case. To be able to spray apply such a solvent free coating through single feed airless spray equipment, the viscosity needs to be low enough to get a good spray pattern without the need for thinning with solvents. A very common formulation approach for solvent free epoxies is to use low viscosity liquid Bisphenol A or Bisphenol A/F epoxies that have been modified with reactive diluents. The epoxy binder is cured with low viscosity polyamine or polyamide curing agents that can be supplied with 30% benzyl alcohol as a solvent to further lower viscosity, and which is compatible with the epoxy, and aids the cure. In many cases the viscosity needs further reduction to reach optimal application properties, so 10 weight % or more benzyl alcohol solvent can be added as a non-reactive epoxy resin diluent.

Without any scientific proof (that we are aware of) benzyl alcohol has been claimed to remain as a solid in epoxy coatings, but benzyl alcohol is an aromatic alcohol with a boiling point temperature of 205 C and should strictly speaking (in our opinion) be classed as a solvent, and contribute to the coating’s VOC [1] content. Some argue that due to benzyl alcohol’s high boiling point temperature most of it will not evaporate or diffuse out of the coating film. There is however a reason why over the last two decades we have seen solvent free epoxy coatings for potable water tanks move towards benzyl alcohol free formulations as it has been observed that over time the solvent diffuses out of the coating film giving taste and smell to the potable water [2,3]. The significantly lower practically determined volume solids, compared to the values calculated treating benzyl alcohol as a non-volatile, is a strong indication that it is in fact volatile and should be classified as a solvent.
The benzyl alcohol contents in the epoxy base and curing agent part of marine solvent free epoxy tank coatings taken from the coating manufacturers material safety data sheets, are shown in table 1.

Table 1. Benzyl alcohol (BA) content in some typical marine solvent free epoxy cargo and potable water tank coatings.

Table 1. Benzyl alcohol (BA) content in some typical marine solvent free epoxy cargo and potable water tank coatings.

From the table it is clear that not all paint manufacturers count benzyl alcohol as a solvent that increases VOC and lowers volume solids of the coating, so we get the unfortunate situation that solvent-free coatings that should have very close to 100% volume solids and VOC 0 g/l can vary anywhere between 95 – 100% solids, and 0 – 180 g/l in VOC. Considering that ultra-high solids epoxy coatings can have up to 97% VS and down to 50 g/l in VOC, a universal common definition of solvent-free epoxy needs to be made so that users can clearly distinguish between solvent free, ultra-high solids epoxy coatings, and 100% solids epoxy. As the main advantages of solvent free epoxy coatings are very low or no VOC emissions, the possibility to apply thick coating films with little or no film shrinkage and lower film formation stress.
Based on work we did more than a decade ago, we would like to propose the following universal definition for a solvent free epoxy coating to avoid confusion by users. Hopefully, this will start a discussion among relevant stakeholders to agree on a common clear and universal definition.

Michael Aamodt, Alan Guy and Raouf Kattan, Safinah Group

 

“A ‘solvent free epoxy’ coating can be defined as an epoxy paint where all of the non-reactive components of the formulation have an initial boiling point greater than 250 C at an atmospheric pressure of 101.3kPa. Benzyl Alcohol added to epoxy coatings should be counted as a volatile since it is not reactive and falls within the definition of a solvent according to EU Paint Directive 2004/42/CE. Benzyl alcohol will also lower the practical volume solids compared to that calculated and often stated on data sheets, when assuming it is non-volatile. At the same time its inclusion increases the volatile organic compound (VOC) content of the coating.”

References
1. “Directive 2004/42/CE of the European Parliament and of the Council of 21 April 2004 on the limitation of emissions of volatile organic compounds due to the use of organic solvents in certain paints and varnishes and vehicle refinishing products and amending Directive 1999/13/EC”. Official Journal L 143, 30/04/2004 P. 0087 – 0096.
2. J. Romero et al. “Characterization of Paint Samples Used in Drinking Water Reservoirs: Identification of Endocrine Disruptor Compounds”. Journal of Chromatographic Science, Vol. 40, April 2002.
3. Aamodt, M., “Eco friendly coatings for potable water tanks offshore”, Offshore Marine Technology, 1st Quarter 2010, 20-21.

Nippon Paint Marine launches completely new nano antifouling technology

Nippon Paint Marine launches completely new nano antifouling technology

FASTAR is a self-polishing antifouling paint that incorporates a unique nano-domain resin structure designed to minimise the effect that seawater temperatures, vessel speeds, and other external factors have on coating performance. By precisely controlling the release of biocides, Nippon Paint Marine has been able to deliver a high-performing AF system with lower polishing rates, as in general, antifouling performance becomes less reliable when its polishing rate is low.

The resin’s nano-domain structure which derives from the micro-domain structure used in Aquaterras, a biocide-free self-polishing hull coating, allows antifouling components to be accurately delivered and released in a much more precise, controlled and effective way.

Due to the reduced film thickness needed, shipowners applying the new product will benefit from shortened in-dock application and drying times, resulting in further cost savings. The required total minimum drying time at drydock for a large container vessel, for instance, is reduced by a maximum 37%, compared to the length of time needed for other typical coating systems, depending on the actual ambient temperature during application, continued the company.

FASTAR, is available in four main versions – FASTAR I & II and FASTAR XI & XII, of which the latter two products incorporate Nippon Paint’s hydro-gel technology. In addition, Nippon Paint Marine stated that the addition of the hydro-gel gives FASTAR XI & XII the same 8% reduction in fuel consumption and CO2 emissions as the company’s A-LF-Sea series, which has been applied to more than 3,300 vessels since market introduction.

New water-based, micro-corrosion inhibiting topcoat

New water-based, micro-corrosion inhibiting topcoat

Cortec® Corporation has launched EcoShield® VpCI®-380 topcoat for the industrial market. According to the company, EcoShield® VpCI®-380 is a fast drying, water-based fluoropolymer modified acrylic one-coat system that can be applied direct to metal and provides corrosion protection and weatherability on metals in harsh outdoor environments.

As a water-based fluoropolymer modified acrylic one-coat system it has many special advantages for protection in challenging environments, such as excellent UV resistance colour and gloss retention and resistance to cracking or chipping upon prolonged exposure to sunlight. Its good hardness, moisture resistance, and fast-dry properties make it an excellent choice for air-dry/force-dry industrial finished. It also has good corrosion protection, having passed more than 1,000 hours in ASTM B117 salt spray conditions and ASTM D1748 humidity conditions at 100-112.5 µm dft, concluded the company.