Stainless steel
The quality of the stainless steel used has a big impact on the corrosion resistance in a hot water vessel. NIBE has selected a ferritic steel grade, EN 1.4521. Among other things it contains 18% chromium and 2% molybdenum but hardly any nickel. As the steel is ferritic it is not sensitive to stress corrosion, which means it is resistant to high chloride content.
The high amount of chromium and especially the content of molybdenum improves the resistance to pitting corrosion. The extremely low nickel content is a health and environmental advantage as no nickel ions are dissolved that may otherwise be the case in the initial phase of operating the hot water vessel.
In addition to the selection of materials the different processing stages are important. The production process at NIBE is carried out in a way that prevents crevices. After welding a hot water vessel it is important that the surface that could be in contact with water has a homogenous composition. This is achieved at NIBE by using a unique pickling process passivating the surface by recreating a uniform oxide film over the whole inner surface of the hot water vessel.
In waters with extremely high chloride content pitting corrosion can occur, especially in combination with high hardness when deposits are formed. In this case natural oxygen in the water is prevented from reaching the steel surface and corrosion can appear under the deposit.
Copper
As a semi-noble metal copper is resistant to most types of consumption waters, among others most municipal waters where a minimum of the pH-value mostly is fulfilled. Hot water vessels with copper as corrosion protection are, like stainless steel vessels, also maintenance free as they do not require a protecting anode.
When NIBE use copper as corrosion protection the steel vessel is internally lined with a 0.4 – 0.7 mm thick copper sheet, which is welded together with argon as a protecting gas. The carbon steel vessel resists the water pressure while the fully covering copper sheet prevents corrosion.
Copper is a natural material and has also other advantages:
- Copper is an ideal recycling material and can be reclaimed repeated times. The copper used by NIBE is fully recycled with a utilization grade of 98%.
- Copper is the best material choice to minimize the risk of legionella.
- Antimicrobial effect keeps the water clean.
- Copper intake in small quantities is essential in principle, for all living organisms.
The recommendations of copper use are described in the standard DIN50 930-6 in the following way:
Copper can be used without restrictions for drinking water if
- the pH-value is 7.4 or higher or
- the pH-value is between 7.0 and 7.4 and the TOC-value does not exceed 1.5 mg/l. (TOC = total organic carbon)
At low pH-values (acid waters) or high chloride contents (for example brackish water) copper has a tendency to dissolve.
At low pH-values an aesthetic problem may occur as discoloration of porcelain and blonde haired people may get greenish hair when showering.
At high chloride contents pitting corrosion can occur but the risk is diminished with increasing calcium content (hardness).
Enamel
Enamel is a glassy material that is applied on the inside of the hot water vessel by heating. Having followed the development for a long time NIBE has switched over to direct-on-enamel. By starting from the best concept the market can offer and adding a substance that strengthens the glassy properties of the enamel the result is so good that the requirement for two-layer enamel is also fulfilled by a wide margin.
The glassy enamel coating is a so-called “hygienic surface“ and investigations have shown that enamel is not a breeding ground for bacteria.
The enamel gets its shape on the steel surface by a heating process at a temperature of 860°C. After this process microscopic pores can exist which means that the steel surface can be in contact with the water. To avoid corrosion there is always a protecting anode in an enamelled hot water vessel. The anode normally consists of a magnesium alloy. As magnesium is a less noble metal than iron the anode is “sacrificed“ instead of the steel which prevents corrosion in an electrochemical way and the pores in the enamel layer are covered with calcium- and magnesium compounds.
The largest amount of the anode consumption occurs as internal consumption at a speed depending mostly on the water quality. It is therefore important that the anode is regularly checked to evaluate its life length.