Corrosion testing continued water

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Panels were prepared using an airless spray in accordance with the manufacturer s instructions. The manufacturer s specification recommended the product for applications involving immersion in well fluids up to 150 C. The product was tested at 150 °C in well fluids supplied by Amerada Hess from the Scott Field (North Sea). Permeation and corrosion occurred both in the gas and water phases of the test panel after only 3 days continuous service. Further testing in water only confirmed rapid breakdown at 100 C after 1 week s immersion. 

The corrosion test methods may be any method that can provide corrosion data in the low water environments. This is generally coupon testing, but may include electrochemical tests that are applicable to the high resistivity crude oil continuous environments at low water levels, e.g., electrochemical impedance spectroscopy and electrochemical noise. 

Procedures for monitoring corrosion of pipelines and related surface facilities using retrievable specimens are described in NACE Standard Practice for Preparation and Installation of Corrosion Coupons and Interpretation of Test Data in Oilfield Operations (RP0775). One of the main concerns in the use of such specimens is the problem of location, since corrosion is generally associated with a separate water phase. In some cases, special dropout pots are used to trap a water phase to provide a severe location for specimens. Electric resistance probes can be used in place of specimens. In locations where there is a continuous water phase and fouUng is not a problem, polarization type probes can be used [74,75]. 

Tests at Miami, Florida have exposed identical specimens by continuous total immersion and by intermittent immersion during high tide. The continuously immersed specimens tended to develop fewer but deeper sites of corrosion. Tests on pilings and pipe exposed above and below water have shown that increased pitting sometimes occurs at the waterline and in the splash zone. This most likely is the result of increased oxygen content near the surface, plus concentration effects due to partial drying in the splash zone. 

Alternate Immersion in 3.5% NoCl. Exposure to 3.5% sodium chloride or to substitute ocean water (ASTM D 1141) by alternate immosion (ASTM G 44) (see Table 1) is a widely used procedure for testing smooth specimens of aluminum alloys. Aeration of the specimens, achieved by the alternate immersion, enhances the corrosion potential (Ref 26) and produces more rapid SCC of most aluminum alloys than continuous immersion. The ASTM G 44 standard practice consists of a 1 h cycle that includes a 10 min soak in the aqueous solution followed by a SO min period out of solution in air at 27 °C (80 °F) and 45% relative humidity, during which time the specimens are air dried, lliis 1 h cycle is repeated continuously for the total number of days recommended for the particular alloy being tested. IVpically, aluminum alloys are exposed from 10 to 90 days, depending on the resistance of the alloy to corrosion by salt water. This test ntethod is widely used for testing most types of aluminum alloys with all types of smooth specimens. 

A comprehensive table of corrosion rates in sea water has been compiled by LaQue . This appears to show no obvious dependence of corrosion rates on the geographical location of the testing site, and few of the rates depart widely from an average of 0-11 mm/y. It is suggested that a figure of 0-13 mm/y may be taken as a reasonable estimate of the expected rate of corrosion of steel or iron continuously immersed in sea water under natural conditions, in any part of the world. 

Fig. 3.50 Corrosion rate versus temperature in de-aerated sea-water continuous test exposure...

A similar method of test was used at the International Nickel Company s Corrosion Laboratory at North Carolina. The specimen discs are mounted on insulated vertical spindles and submerged in sea-water, which is supplied continuously to the tank in which the specimens are immersed. The maximum peripheral speed of the spinning disc is about 760cms , and the characteristic pattern of attack is shown in Fig. 19.3a. Studies of variation of depth of attack with velocity indicate that at low velocities (up to about 450 cm s ) alloys such as Admiralty brass, Cu-lONi and cupro-nickel alloys containing iron maintain their protective film with a consequent small and similar depth of attack for the diflferent alloys. At higher velocities the rate increases due to breakdown of the film. 

Various membrane materials are to be compared for corrosion resistance in hydrochloric acid. Membrane samples are ultrasonically cleaned with Freon for 5 minutes and dried at 200°C for 2 hours followed by similar steps of ultrasonic cleaning with demineralized water and drying. The conditioned membrane samples are then immersed in 35% HG solution, making sure that no air bubbles are trapped in pores. The acid exposure at the test temperature (e.g. 25°C) continues for a given period (e.g. one week). The tested samples are ultrasonically washed with demineralized water for 5 minutes and dried at 200°C for 2 hours. The weights of the cleaned membrane samples before and after the acid exposure are compared to assess the relative corrosion resistance of various membrane materials. 

A polished steel spindle is immersed into 300 mL of fuel maintained at 100°F (37.8°C). The fuel is continually stirred during the entire test period. Upon immersion for 30 minutes, 30 mL of distilled water are added to the fuel. After remaining in the fuel for an additional 3lh hours, the spindle is removed and the surface is rated for percent corrosion. 

Protective films on metals also can be destroyed and corrosion accelerated by the impingement of a high-velocity stream of sea water onto the metallic surface. The inlet ends of condenser tubes, are frequently attacked (Figure 3,a). Jet tests have been devised for ranking the susceptibility of metals to such impingement attack. A corrosion cell is formed between the bare surface directly under the jet (anode) and the adjacent filmed surface (cathode). These jet tests give more dependable information, if natural sea water is continuously supplied to the equipment. 

The treatment of bilge water and emulsions resembles that of the treatment of oil field brines and produced water. Chen et al. [25], using ferric chloride and other chemicals to enhance the performance of Membralox 0.2, 0.5 and 0.8 pm membranes, describe permeate fluxes between 1400 and 34001/m h. Without pretreatment however severe fouling occurred as well as break-through of oil. Zaidi et al. [26] report about the continuation of this work. They quote fluxes between 800 and 12001/m h, but also mention substantial lower fluxes in long term pilot tests using 0.8 pm membranes. In addition they indicate a drop in permeate flux caused by conditions of low pH, the presence of sea water, corrosion inhibitor, oil slugs or flow variations. 

Cold water at 2°C temperature is poured into the aluminum tube, taken away after three hours and the sample is then examined. Three samples are tested at once. In the case that none of them shows any sign of corrosion the testing is continued. 

Most known procedures of this group of methods are called oxidation and sulfuration tests. In the former case a metal test specimen wrapped in a VCI film material is placed in a desiccator (about 10 1 in capacity). The internal atmosphere reaches 100% RH using 20 cm of water. The desiccator is blocked up and is placed in a 50° C constant-temperature tank or in normal-temperature room to promote the growth of rust. In sulfuration test the desiccators about 2.5 1 capacity are used. After having adjusted the inside atmosphere to reach 93% RH using 10 cm of a saturated solution of Na2S04, a test metallic strip wrapped in inhibited film material is placed inside. The tests are continued until a corrosive phenomenon is observed. 

Water permeability test. Depending on the sea water level, the mortar used for corrosion protection can become wet or completely dry. Continuous fluctuations in moisture content increase the intrusion of damaging substances, which might attack the mortar as well as the steel tower. 

The close examination of experience presented to the Technical Committee led to the conclusion that plant safety is not conqrromised for design basis accidents. Continued efforts to validate the predictive methods against experimental data are worthwhile. Protective coatings for fuel and graphite components which provide high corrosion resistance should continue to be developed and tested as these potentially could provide assurance of safety even for very extreme and hypothetical water or air ingress accident conditions. 

Data on the corrosion of steel reinforcement embedded in concrete made from alinite cement are not uniform. In most investi tions an intensified corrosion of steel was observed under such conditions however, in a recently published study (Kostogloudis et al, 1998) no significant corrosion was observed in test specimens that were stored continuously or in interrapted fashion under water for up to 12 months. 

Some boilers are equipped with an embrittlement detector by means of which the chemical treatment of a water can be evaluated continuously in terms of its potential ability to induce stress-corrosion cracking (Fig. 18.3) [13]. A specimen of plastically deformed boiler steel is stressed by setting a screw adjustment of this screw regulates a slight leak of hot boiler water in the region where the specimen is subject to maximum tensile stress and where boiler water evaporates. A boiler water is considered to have no embrittling tendency if specimens do not crack within successive 30-, 60-, and 90-day tests. Observation of the detector is... 

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