Electropolished and Mechanically Polished Tubing, Part 2
Read "Electropolished and Mechanically Polished Tubing, Part 1"
By John Tverberg, Vice President of Technology, <%=company%>
Introduction
Type 316L stainless steel is the material of choice for most high purity water and water for injection systems in the pharmaceutical industry. Two surface finish conditions are used for these systems: electropolishing and mechanical polishing. Occasionally discoloration occurs in mechanically polished surfaces, especially during hot, humid weather. Discoloration is readily removed by immersion in hot nitric acid followed by a water rinse. Once the tube is acid-treated it does not discolor again providing the treatment takes place at an elevated temperature for a sufficient period.
This study was performed to determine what changes occur in the surface of mechanically polished tubing at several nitric acid passivation times.
Methods
Reagent grade nitric acid was diluted with deionized water to 20 volume percent and heated to a constant 136° F. Five samples of mechanically polished tubing were immersed in this solution, one each for 1, 5, 15, 30, and 60 minutes, respectively. One additional sample was analyzed in the "as polished" condition.
After treatment, rinsing, and drying, samples were evaluated using x-ray photoelectron spectroscopy (XPS). There were no visual differences among the six samples and all had identical surface luster. In addition, we sputtered the as-received sample and the 30 and 60 minute passivated samples with ionized argon until we reached the maximum depth of oxygen penetration or until there were no further changes in composition. Sputtering was done to determine elemental composition and oxidation state as a function of depth.
Results
Chromium and oxygen concentrations reach a maximum after 30 minutes of passivation, at which point iron has its lowest value. When all our data were analyzed we found that the highest Cr/Fe ratio occurs after 30 minutes of passivation. For some unexplained reason this ratio decreases at both the 15 and 60-minute passivation times.
Examination of specific binding energy peaks for each element indicates that both oxide and zero-valent metal are present. For iron, both the oxide and elemental metal are present in significant quantities, especially at passivation times under 30 minutes.
These data indicate that the iron oxide abruptly decreases after one minute and continues to drift downward until the chromium oxide reaches a near saturation point somewhere between 15 and 30 minutes. After 30 minutes both ratios increase, although the rate of increase is greater for chromium oxide than for iron oxide. This indicates that the surface is becoming more passive with longer exposure to hot nitric acid.
Discussion
The mechanism for passivation appears to be related to the progressive oxidation of chromium as the first step. Once the free chromium is essentially consumed, iron begins to form its oxide. The atmosphere-formed iron oxide, which dominated in the as-received material, rapidly dissolves in the hot nitric acid and metallic iron remains the dominant species. This holds for up to 30 minutes, at which point the oxide finally exceeds the metallic iron.
In other words: True passivation does not appear to occur until the metallic elements are totally converted to the oxide. For mechanically polished material this requires more than 60 minutes of passivation in hot nitric acid.
We are often asked, "Is passivation of the completed water for injection system necessary, especially if nitric acid passivated tubing is used for the construction? The answer is "Yes." Even if the tube surface is adequately passivated it is in the best interests of the end-user to passivate. Orbital welding results in an area within which the alloy chemistry is not balanced for optimum corrosion resistance. Typical weld chemistries have Cr/Fe ratios of 0.11:1 and high manganese. Passivation is necessary to remove the manganese and to boost the Cr/Fe ratio above 1. Unless this is done there will be an area where accelerated corrosion can occur, and in those environments containing electrolytes, galvanic corrosion may also take place.
Conclusions
- Dramatic changes occur in the surface chemistry of mechanically polished Type 316L stainless steel during passivation. Iron decreases, as do silicon, nickel, and molybdenum. Oxygen and chromium both increase. The Cr/Fe ratio increases with passivation time.
- Passivation appears to be controlled by the oxidation of metallic chromium to the trivalent oxide. Iron does not begin to form appreciable trivalent oxide until the metal is satiated in chromium
- Even after 60 minutes of passivation in hot nitric acid, a definite metallic iron peak still remains, indicating that further passivation could occur.