EN 17243-2020 pdf download.Cathodic protection of internal surfaces of metallic tanks, structures, equipment, and piping containing seawater.
defined in EN 12496) or + 0,25 V when measured with respect to a zinc electrode made with galvanic anode alloy types Z1, Z3 or Z4 as defined in EN 12496.
When the water temperature or surface temperature of steel is higher than 60 °C, the criterion shall be — 0,90 V wrt Ag/AgCI/sea water. Between 40 °C and 60 °C the protection potential shall be interpolated between -0,80 and -0,90 V wrt Ag/AgCI/sea water.
For tanks and structures containing seawater or brackish where the electrolyte is not frequently renewed (e.g. less than once a month) the criterion for anaerobic conditions, i.e. — 0,90 Vwrt Ag/AgCI/sea water, shall be adopted regardless of temperature.
6.3 Stainless steels and nickel alloys
In chloride-containing aerated environments such as natural seawater and brackish waters, stainless steels are known to resist uniform corrosion and the possibility for crevice corrosion and pitting remains the principal concern. To ensure the protection of such alloys in these environments, the protection potentials given in EN 12473 apply, i.e. —0,30 V wrt Ag/AgCl/sea water for stainless steels with PREN 40 and —0,50 V wrt Ag/AgCI/sea water for stainless steels with PREN <40.
However, less conservative criteria can be used provided that these are justified and documented. In this case, the selected protection criteria shall be more negative than the critical crevice potential determined for a combination of a particular alloy (PREN. microstructure, etc.), crevice parameters (geometry, surface finish, type of gaskets, sealing pressure of flanges, etc.), and operating environmental conditions (composition, temperature, velocity, etc.).
The determination of the critical crevice potential shall be carried out on the basis of demonstrated service feedback and/or on laboratory tests relevant to the service conditions. In the absence of a more relevant method for a given practical situation, the methods given in the References 161171181 should be used.
In the case of galvanic couples between parts in stainless steel or nickel alloy and parts in carbon and low alloy steel, the protection potential criterion of the carbon and low alloy steel shall be more negative than – 0,80 V wrt Ag/AgCl/sea water.
In chlorinated seawater, the recommended protection potentials criteria for stainless steels and nickel alloys are the same as those in natural seawater, but the protection current densities can be less (see subclause 7.2.7).
6.4 Cracking risks induced by over polarization
Where there is a risk of hydrogen embrittlement or HSC of high strength steels or other metals which may be adversely affected by cathodic protection to excessively negative values, a less negative potential limit shall be defined and applied. If there is insufficient information for a given material, this specific negative potential limit shall be determined relative to the metallurgical and mechanical conditions by testing. Refer to EN 12473 for more details.
For carbon and low alloy steels, a potential negative limit of — 1,10 V wrt Ag/AgCI/sea water is recommended in order to minimize cathodic disbondment. Other potential negative limits shall be applied to prevent HSC of vulnerable metal compositions (refer to EN 12473).
For stainless steels and nickel alloys, ferritic and martensitic microstructures can suffer from hydrogen embrittlement when the potentials are too negative. Potentials more negative than the particular limit values should be avoided. EN 12473 gives information on this risk and also provides recommendations for qualification of the materials.
Titanium and its alloys are prone to titanium hydride formation in cathodic protection applications. This hydride can lead to cracking when stresses reach a critical level.
Heat exchanger tube Inlets are areas of high mechanical stress due to construction methods. When heat exchanger tubes are swaged into the tube sheets the tubes may be subject to high residual stress.
Titanium grade 2 tubes should not be subject to a potential more negative than — 0,75 V wrt Ag/AgCI/sea
water [9], some more conservative figures being found in the literature, see References [10][11J, e.g. — 0,70 V 112] and even —0,65 V 1131.
This conservative approach can be replaced by other criteria provided that these are justified and documented by an assessment to ensure that the risk is acceptable. Such an assessment shall consider all load contributions causing stress and strain. In any case, potential of the titanium shall not be more negative than – 1,00 V wrt Ag/AgCI/seawater as specified in EN 12499 and documented in the literature, see Reference [9].
For other applications where stresses and strains are lower, e.g. tubular plates, a negative limit of— 1,05 V wrt Ag/AgCI/seawater is sufficiently conservative, see Reference 1141.EN 17243-2020 pdf download.
EN 17243-2020 pdf download
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