Trond Rogne

Erosion and corrosion properties of WC coatings and duplex stainless steel in sand-containing synthetic sea water

Abstract Two different thermally sprayed coatings have been investigated with respect to erosion and corrosion resistance and compared with a duplex stainless steel. The coatings consist of WC particles in a metal binder, one with a pure Co binder and one with a binder consisting of mainly Ni, Cr, Co and Fe. The materials have been exposed to flowing synthetic sea water containing sand. The test equipment consists of an inner rotating cylinder and an outer stationary cylinder. In between there is a 4 mm gap through which synthetic sea water flows. The cylinder walls are divided into ring-shaped specimens. Less sand attacks the inner cylinder than the outer owing to centrifugal forces. Weight loss rates were determined by weighing the specimens. In addition, corrosion rates were determined by electrochemical measurements. By cathodic protection of the specimens the rate of erosion was found. In that way, the relative importance of the corrosion and erosion rates has been studied. The results show that the corrosion properties of the coatings strongly affect the material loss rate. Coatings with a highly corroding matrix, such as Co, suffer from a strong synergistic effect where the corrosion gives enhanced erosion. This effect can be more than three times as high as the corrosion itself. It is also seen that the weight loss rates vary with exposure time, being larger from the start. This is an important issue to consider when testing these materials. Modelling of the flow conditions has been performed to determine sand particle energy and the angle of attack on the surface.

Combined erosion and corrosion of thermal sprayed WC and CrC coatings

Abstract This paper presents investigations of the combined effect of erosion and corrosion on one stainless steel and four different thermal sprayed coatings with carbide particles in a metal matrix. The materials were exposed to flowing synthetic sea water containing sand. Pure erosion, electrochemical corrosion and total material loss were determined at different sand concentrations. The effect of exposure time was also studied. The results show that erosion is the dominating process at high sand concentrations while corrosion is important at lower concentrations. The relative importance depends on the material. The corrosion resistance of the metal matrix has great influence on the total material loss rate. For the coating with lowest corrosion resistance there seems to be a synergistic effect between the mechanical and electrochemical processes.

Aspects of testing and selecting stainless steels for seawater applications

Abstract Relatively recent failures in seawater systems of highly alloyed (i.e., pitting resistance equivalent [PREN] > 40) stainless steels (SS) have highlighted the issue of corrosion testing and safe-use limits. Three aspects of testing and selection of SS for seawater application were reviewed: relevancy of the electrochemical test methods, the quality control system, and the importance of repassivation. A potentiostatic test for assessment of critical pitting (CPT) and crevice (CCT) temperatures was analyzed with respect to its ability to generate data relevant to chlorinated seawater systems. CPT seemed valid as safe-use limits, but the CCT determined were too high. A modified test approach was proposed. Generally, significant scatter in CCT and CPT values was registered, focusing on the importance of statistical experimental approach and data analyses. Recommendations were made on use of the ASTM G 48 test as a quality control method. The validity of critical pitting and crevice indices was analyze...

Critical Crevice Temperature for High-Alloyed Stainless Steels in Chlorinated Seawater Applications

Eleven high-alloyed stainless steels (SS) were tested for application in chlorinated seawater. Critical crevice temperatures (CCT) were determined using a potentiostatic test method. Results were evaluated in terms of the critical crevice index (CCI) value of the alloys and compared to results of duplicate specimens in other tests.

Testing of Stainless Steel Welds for Various Applications

Abstract In several investigations, potentiostatic, potentiodynamic, and standard ASTM-G48 test techniques have been used to evaluate the corrosion properties of stainless steel welds. Testing for various applications is discussed, and test methods are recommended. Tests should always be performed for the least resistant parts of a system, usually as-welded field welds and/or narrow crevices. The 6% FeCl3 test solution is often more severe than the real environment because of the higher potentials obtained. Therefore, excessively low, critical temperatures are measured. Testing according to ASTM-G48 seems to be relevant for highly alloyed stainless steels for exposure to oxidizing chloride solutions that raise the potential in the range of 400 to 700 mV SCE. In order to perform realistic testing relevant to practical applications, the maximum free potential obtained in real environments should be known. Potentiostatic tests, using the maximum potential for the actual environments, are recommended for test...

Hydrogen Absorption in Cathodically Polarized Titanium Alloys

Cathodic polarisation may cause hydrogen absorption and embrittlement of titanium alloys. There is no need to protect titanium in seawater, but polarisation is inevitable when titanium components are connected to steel that is cathodically protected. The risk of hydrogen embrittlement of risers and other heavily loaded components rouse a need to quantify the amount of hydrogen that titanium alloys may absorb as a function of alloy composition, polarisation potential, temperature, cold working and time. The test program included Ti-3Al-2.5V and Ti-6Al-4V alloys that are candidate materials for risers. The hydrogen uptake was measured over a 3 years period in natural seawater. The results show that titanium absorbs hydrogen when it is polarised to potentials less than −0.8V vs. Ag/AgCl in seawater, but the absorption rate decreased with time. The maximum hydrogen loading measured after three years exposure at −1.04 V vs. Ag/AgCl was 150 ppm. A calcareous deposit built up on all surfaces and limited the hydrogen evolution and thereby the hydrogen uptake. Ti-3Al-2.5V alloys picked up less hydrogen than Ti-6Al-4V alloys, and palladium or ruthenium enhanced hydrogen uptake to some extent. Welding or cold working did not influence hydrogen uptake of the alloys included in the tests. Temperature had little effect, and thermal gradients in the titanium materials had no measurable influence. The effect of hydrogen on the mechanical properties of the alloys was not studied in detail in these experiments, but most specimens were tensioned C-rings with permanent strain. Neither of these developed cracks or other signs of embrittlement during the tests.Copyright

Effect of Chloride and Partial Pressure of H2S on the SSC Susceptibility of Martensitic SS

Two weldable martensitic SS grades (1.5Mo and 2.5Mo) have been welded using matching consumables. Both seam welds and girth welds have been undertaken testing with respect to sensitivity towards sulphide stress cracking (SSC). The effect of chloride concentration, either simulating condensed water situation or simulated formation water situation, and H2 S partial pressure have been investigated. Based on these studies application limits are evaluated. Under condensed water conditions the critical partial pressure of H2 S increased 1–2 decades when the NaCl concentration was reduced from 1000 to 100 ppm. At 100 ppm of NaCl the critical partial pressure of H2 S was >100 mbar. The same situation was observed under formation water conditions. The data also show a large increase in the SSC resistance when the seam welds were grit blasted prior to pickling and passivation.Copyright