Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|Temperature plays an important role in determining the corrosion resistance of a material. Make sure that any process transient temperatures are within the allowable limits of the material being considered for the application.
Remember this picture? It was in the Corrosion - Case Study No. 94: Consequences of a Leaking Corrosive Gas or Liquid article. The flange is 304 stainless steel. The granular appearance within the red circled region, and along the interior, are signs of aggressive attack by the liquid medium going through the pipe. The liquid was, and still is, sodium chlorate. More information came to light after the article was written. It seems that the temperature had been held at over 200F. for a period of 72 hrs. during the winter weather. The chemical was new to the plant and to avoid it setting up and becoming a solid the temperature was kept high. In this case it was kept too high.
|Having found out that the temperature had been kept above 200F. led to closer monitoring of the tank that held the sodium chlorate. It was constructed out of welded 316L material. Plain 316L stainless steel coupons were placed inside the tank, along with some weld coupons. The coupon to the left is a bend-test coupon. You are looking at the outside bend radius where there is a weld. The weld is not visible because it was ground flush and had good fusion (i.e. no tears).
The idea behind the testing was to evaluate welds under stress in the sodium chlorate. The coupons were lowered inside the tank and left inside for two months. The interval was chosen to be identical to the interval in which the flange in the first picture was exposed.
|After the two month period the coupons were removed for examination. The bend-test coupon in the lower left picture showed no signs of attack. If you look closely you can see the same scratches at the bend as in the previous picture. The underside of the coupon is shown in the lower right picture.
|A more realistic test for the welds was simulated. The weld to the lower left is a SMAW, or "stick" weld. The weld to the lower right is a TIG weld. Both coupons showed no signs of attack. The coupons were placed back into the tank and will be monitored every two months for a year. The general hysteria that was generated when the first corroded flange was discovered had subsided, and for good reason. The panic subsided because the corrosion mechanism was now understood. The culprit was temperature.
|After the discovery of the corroded stainless steel flange, the temperature was regulated at 140F. A temperature controller, which had not been installed, was now controlling the temperature. The literature shows that 316 stainless steel has a temperature limit of 200F in sodium chlorate. Having learned the hard way, the literature is correct.
Temperature can affect the ability of a material to develop and/or maintain its protective oxide layer. That oxide layer makes the material passive. Passivity is a condition where the ability of the metal to react with the environment has been significantly reduced or eliminated. The ability of a metal to react is called potential; the greater the potential, the greater the ability to react given the right circumstances. The rate at which a metal reacts is called current density; the greater the current density, the greater the metal loss. The relationship between potential and current density can be plotted on a graph. The resulting figure is called a polarization curve. This is where it gets crazy. The polarization curve isn't linear. It can have all sorts of shapes based upon the environment. One minute the metal can be passive, and the next minute it can be reactive, all based upon where the metal is operating on its polarization curve. Temperature messes with where a metal is located on its polarization curve.
Suffice it to say that when the literature puts a temperature limit on a material ... BELIEVE IT !!!!
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