Corrosion of metals, effects of UV light: 2 problems, just 1 solution with ArenStrong!
Could a same coating be useful to prevent corrosion of metals and UV light color degradation at once? The answer is YES and we explain you how!
Almost all metals are susceptible to be corroded when they are exposed to the environment. This environment can be the atmospheric air or an aqueous solution like the ocean water. Metal corrosion is a very common problem in industry with costly effects such as loss of production, equipment failure, accidents, etc. Dealing with corrosion has had a cost equivalent to 3.4% of GDP (Global Gross Domestic Product) or US$ 2.5 trillion in 2013 according to theNACE institute.
On the other hand, weather, specifically ultraviolet light, can also impart damage to coatings reducing their service life. All molecules are excited by absorbing radiant energy, which produce molecular structure changes promoting coating degradation.
When using a pipe work for offshore activities you will want your metal to be protected for a long period of time from the harsh environment of the oil & gas industry. A similar request is found when architects want to use metallic claddings in a maritime environment. In both outdoor conditions, corrosion and UV light coating degradation are two factors that make a combined effect leading to an aggressive and rapid coating deterioration.
ArenStrong provides a solution to protect your metals from those two combined environmental effects at the same time. With ArenStrong we produce prepainted stainless steel, aluminium, zinc and galvanized steel coils capable of withstanding the most severe weather conditions.
What is ArenStrong?
ArenStrong is a Polyvinylidene fluoride (PVDF) coating system composed by 2 layers but it can be composed up to 4 layers:
1. Up to 15 µm primer
2. Up to 20 µm UV-barrier PVDF
3. Up to 20 µm coloured PVDF
4. Up to 15 µm transparent PVDF varnish
PVDF is a very inert polymer with an outstanding corrosion resistance and known chemical and physical properties. Moreover, PVDF is also an easy available material already proved in the Liquefied Natural Gas (LNG) field in projects like Ichthys, Woodside, Gorgon and Yamal. ArenStrong gives an increased corrosion protection to stainless steel cladding, and, thanks to its molecular structure it gives further advantages to the coating service life.
How does ArenStrong work to prevent metals from UV light color degradation?
Ultraviolet light belongs to the electromagnetic spectrum. It is also called visible light because it is the radiant energy that the human eye can absorb. All forms of electromagnetic radiation are characterized by their wavelength (λ) and their frequency (ν) (number of waves per second).
Figure 1. Electromagnetic radiation. Molecules are excited when absorbing radiation. The shorter the wavelength the more energetic the radiation. When it is energetic enough, radiation can break chemical bonds of molecules.
When molecules absorb radiant energy they are excited (an electron goes from the ground state to a higher energy state). The smaller the wavelength the higher the radiation energy and when it is energetic enough, radiation can break chemical bonds.
Visible light has a sufficiently short wavelength to break covalent bonds of organic molecules. Double carbon bonds (-C=C-) are less chemically stable than single carbon bonds (-C-C-), so double carbon bonds are broken more easily than the single ones.
Paint coatings are basically composed by a resin (binder, an organic molecule), a solvent and a pigment that could also be an organic compound. If the pigment is made of a double carbon bond chain, ultraviolet light will break these chemical bonds and photochemical reactions will introduce molecular changes producing color and coating degradation (both pigment and resin will be damaged) in a short period of time.
PVDF is a polymer composed by a single carbon bond (-C-) chain. Furthermore, the C-H bonds are next to 4 C-F bonds. The result is a material completely resistant to the effects of UV light as well as resistant to corrosion in an outstanding way.
Figure 2. Molecular structure of Polyvinylidene fluoride (PVDF). It is composed by a single carbon bond (-c-c-) chain and every C-H bond is next to 4 C-F bonds. This makes PVDF molecule completely photochemically and electrochemically stable.
How metals are corroded?
Corrosion is an electrochemical process and it always occurs when the metal is in contact with an electrolyte, which is a solution containing ions. A simple example of an electrolyte is water that contains oxygen, H+ (Protons) and OH- (Hydroxide ions). In case of the atmospheric air, there will be an electrolyte when the metal is in contact with the critical relative humidity value of 60%.
Corrosion is a surface phenomenon and is produced when metals react with ions in the electrolyte to form the metal oxide or hydroxide. The most common metal widely used in industry and prone to corrosion is steel. This material is an alloy of mainly iron (Fe) and carbon (C). Iron react with OH- to form Fe(OH)2 that continues to oxidize if water is present to form [Fe2O3.xH2O]. This last compound, commonly called rust, is the typical red deposition that we observe on the steel or iron surfaces.
Current approaches to prevent metals from corrosion
Steel galvanization: this is the most typical technique to protect steel from environment degradation. It consists of a zinc coating deposited on the steel surface. Zinc is much less susceptible to corrosion than steel.
Making steel stainless: it is an alloy of iron and at least 10.5% of chromium plus other elements such as carbon, silicon, manganese, nickel and molybdenum. These two last elements improve protection from corrosion and give formability to stainless steel. Chromium is the key element as it forms a passivation layer when combined with oxygen to produce chromium oxide. Stainless steel can be austenite or ferrite.
Duplex and super duplex stainless steel: duplex stainless steel is a stainless steel with a chemical composition made of an equal mixture of ferrite and austenite. Super duplex stainless steel is a duplex grade with a higher content of molybdenum and chromium. These kinds of stainless steel provide a higher corrosion resistance than traditional stainless steel.
Even if duplex and super duplex stainless steel are excellent options for corrosion protection of metals, especially in the oil & gas industry, their metallurgy is much more complex than for traditional stainless steel (austenite and ferrite) and therefore, their production at mills is more complicated. Additionally, corrosion tests for these materials in specific environments are not always available, so tests in-site are needed.
Traditional stainless steel is easy available but the passivation layer of chromium is not capable to resist in highly corrosive environment such as those found in the oil & gas industry (stainless steel is susceptible to pitting, crevice and intergranular corrosion).
1. Clive H. Hare (1992). The degradation of coatings by ultraviolet light and electromagnetic radiation. Journal of protective coatings & linings. (LU-8029).
2. McCafferty, E. (2010). Introduction to corrosion science. New York, NY: Springer New York.