How to Maintain a Stable Solution Chemistry when Simulating CO₂ Corrosion in a Small Volume Laboratory System
In laboratory experiments, corrosion of mild steel specimen in glass cells or autoclaves with a relatively small internal volume (of the order of 1 liter or less), will usually lead to a change in solution chemistry, (i.e., increase in ferrous ion concentration and solution pH) which will affect the corrosion product formation and ultimately the corrosion rate. However, in much larger field systems that are being simulated in the laboratory, such as for example oil and gas mild steel pipelines, the solution chemistry at any specific location does not change significantly over the same time period, since it is governed by the flow coming from further upstream. Therefore, it is very important to be able to maintain a stable solution chemistry in small scale laboratory experiments, in order to get a better simulation of corrosion seen in the field. In this work, a stable solution chemistry system was developed using ion exchange resins. H-form and K-form exchange resins were successfully tested in long term experiments aiming to keep pH and ferrous ion concentration reasonably stable. The results show that pH can be controlled within 0.02 pH units and ferrous ion concentration within 3 ppm. The results of electrochemical measurements and surface analysis show that there is a significant difference in both corrosion rate and corrosion product layer formation when a stable solution chemistry system is used.
Localized Corrosion of Mild Steel in Marginally Sour Environments
Localized corrosion has been a challenge for the integrity of mild steel pipelines, specifically at operating conditions where a trace amount of H2S is present alongside CO2 at lower temperatures. The presence of H2S will lead to the formation of a protective iron sulfide layer that will decrease the general corrosion rate; however, a trace amount of H2S may only form a partially protective mackinawite layer that could result in localized corrosion. In the current study, mild steel specimens (API 5L X65) were exposed to a 1 wt% NaCl solution sparged at 0.096 MPa pCO2 and 15 ×10−6 MPa or less pH2S (≤150 ppm H2S/CO2). At pH 5.0 and 30°C the bulk solution was undersaturated with respect to iron sulfide—mackinawite and iron carbonate. At these marginally sour conditions, a H2S/CO2 threshold of approximately 100 ppm was deduced, below which localized corrosion happened. No localized corrosion occurred for the same environmental conditions when H2S/CO2 ratio was above 100 ppm or when there was no H2S present.
Hygroscopic Expansion of some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.