The chemical properties of seawater make it an ideal medium for accelerated corrosion of metallic parts such as pipelines. The chemical reactions that cause corrosion of pipes in sea water is predominately electrochemical in nature.
It is estimated that globally circa 30% of all metal waste is a product of corrosion and marine environments are undoubtedly a significant contributor to this number.
Failure to prevent corrosion of pipelines can and does lead to disastrous consequences environmentally speaking and it is crucial to understand the nature of pipeline corrosion in seawater environments.
Seawater corrosion is a combination effect of oxidation and reduction (sometimes called redox reaction).
The oxidation part is summarised as the metal atoms (M) lose their negatively charged electrons meaning the metal adopts an overall positive charge (M+). This takes place at the anode (the site at which the metal is corroded) and is written as:
M – ne- = Mn+
The Redox Reaction part is summarised as the oxygen (O2) and water (H2O) combine with the electrons lost by the metal to form negatively charged hydroxide ions (OH-). This happens at the cathode.
O2+ 2H2O +4e- = 4OH-
A further reaction then occurs where insoluble metal hydroxide (corrosion products) is formed. The actual corrosion formed is wholly dependent on several factors, including metal type, the manner of exposure, and the seawater properties.
Common Types of Corrosion
As the name suggests this is corrosion that’s evenly distributed over the meal surface. Anodic and Cathodic sites are present throughout the surface and can move location to another.
This is a type of localised corrosion where small holes form in the surface. Pitting is often associated with a weak barrier protection of the surface. Pitting usually starts with a small hole and then grows into one of several patterns.
Similar to Pitting except that the electrolyte (seawater) is largely stagnant. This would typically take place in areas such as bolt threads, gaskets and sealing points on the pipes.
Where pipes are fabricated from differing materials (flanges, bolts, welds…) can be a cause for electrochemical reactions between the two types of metal used in the presence of an electrolyte (seawater). One metal acts as the anode and the other the cathode. This reaction is driven the by the highly conductive seawater.
Salinity & Conductivity
Salinity – the amount of salt in water, measured as higher than in freshwater, acts as the conductive media for electricity and so is an ideal electrode for the electrochemically driven corrosion process. Furthermore, the salt in the water can react with passive films causing these to breakdown.
Dissolved oxygen is directly linked to rates of corrosion. It acts to destroy the protective hydrogen films present in the metal leaving it open to corrosion. Basically the higher the level of dissolved oxygen the faster the rate of corrosion.
The lower the pH the more acidic the seawater is and so the higher the concentration of hydrogen ions in the seawater. Therefore, low pH (acidic) seawater accelerates corrosion because of the abundance of hydrogen ions for the electrochemical reaction.
How to Protect Pipes in Seawater
An assessment of the physical environment will help to determine which materials might be the most appropriate for use. The selection process should weigh corrosion resistance, mechanical strength and cost, as well as trying to avoid the use of dissimilar materials together.
Surface coatings when applied correctly can provide protection against corrosion or at the very least, slow the process down. The coating acts as a barrier between the seawater and the metal surface, meaning it is crucially important that the correct coating materials are specified and that the applicator is competent in the application process.
Cathodic Protection (CP) is a system where the pipe is connected to another metal to force the pipe to become a cathode in the electrochemical cell. This metal attached to the pipe becomes the anode, which is designed to be a sacrificial metal anode in preference to the pipe.
Northpoint’s Anti-corrosion Coatings
Northpoint has been an applicator of corrosion protection coatings since the late 1970’s and boasts a wealth of experience to draw on. Northpoint presently has 3 products DWI Reg. 31 (4)(a) approved for use as a coating service:
- Resicoat R4 – DWI 56.4.659 – an FBE coating
- Jotaguard VA 5001 – DWI 56.4.1304 – an FBE coating
- Eurokote 468 Thixo – DWI 56.4.1209 – an Epoxy coating
Through research and development, Northpoint has pioneered the application of Fusion Bonded Epoxy (FBE) coatings in combination with Liquid Epoxy coatings such as Eurokote 468 Thixo as a solution aimed at reducing drag and turbulence of long stretches of pipelines. Ultimately the coating reduces the amount of energy needed to pump water over extended distances and the steel pipework is protected from corrosive attack, at the same time offering a design life of circa 50 years in many instances.
A more recent innovation is the application of a UV protective layer over the external FBE coating. Epoxy coatings generally (including FBE) will fade when in direct sunlight. Whilst this won’t materially affect the performance of the FBE or Epoxy coating in contact with water, it can become unsightly over time. Northpoint has developed a unique coating system that combats this process meaning that the large capital investment made remains looking like new for years to come.
Northpoint also holds WRAS Approval on the Jotaguard VA 5001 product, for internal pipework found in buildings.
Northpoint has the in-house capabilities and experience to provide coating systems that meet the WIMES Standard. Coatings under this standard will be quoted as: “Water Industry Mechanical and Electrical Specification 4.01 – Paints and Polymeric Coatings for Corrosion Protection”.
This standard provides an outline for how the application of coatings is to be managed. It goes on to provide a number of tables that describe coating systems that meet differing parts of ISO 12944 C3 – C5M/C5I requirements and Imm 1 – 4 requirements.
In all cases, Northpoint’s coating inspectors act as independents to the process and are qualified NACE II Coating Inspectors and supervised by a NACE III Coating Inspector who will cross-check and pass-off their work and, where contracted, provide inspection services as per the customer requirements.