Stainless Steel Analysis
Stainless steel is defined as a steel alloy with a minimum of 10.5 weight percent chromium. Although stainless steel does not generally corrode as easily as ordinary steel, different grades are necessary for different environments. For example, several manufacturing groups (including semiconductor and pharmaceutical) have stringent requirements for stainless steel surfaces.
The corrosion-resistant properties of stainless steel are due to the formation of a passive chromium oxide layer. Although this layer forms naturally when the material is exposed to the atmosphere, additional chemical treatments (passivation) can improve the relative amount of chromium at the surface.
The surface chemistry changes due to passivation occur within the outer ~50Å of the surface. In order to measure these changes, surface analytical tools such as x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are necessary. One of the stainless steel measurements that is made using Auger and XPS is the near-surface chromium/iron (Cr/Fe) ratio.
When stainless steel is analyzed with XPS or Auger:
- A survey scan determines the overall elemental composition of the surface from atomic numbers 3-92. This yields the surface Cr/Fe ratio.
- If the analysis is conducted with XPS, the major elements detected (usually carbon, oxygen, silicon, chromium, iron, nickel, and molybdenum) are analyzed in high energy resolution mode to determine their binding energies and then to make some inferences about the compounds present on the surface. The ratio of chromium oxide to chromium metal, and iron oxide to iron metal, is obtained. The distribution of organic states is measured.
- A third step in the analysis includes a depth profile to determine the concentration of the primary elements as a function of depth. This also yields the Cr/Fe ratio as a function of depth.
- A final survey scan is obtained. This is useful for internal calibration.
Step 1. Stainless steel XPS survey spectrum
- Survey scan - Passive layer elemental composition and surface Cr/Fe ratio.
- High C and O are typical on metals exposed to the air. Other elements are present due to processing and to handling.
Step 2. High resolution XPS
- Relative amounts of metals versus oxides for chromium, iron, and nickel,
- Distribution of organic carbon states
The right iron spectrum was obtained from a sample that had undergone experimental passivation, while the left iron spectrum was from a standard passivation treatment. XPS demonstrated there is more metallic iron present on the surface of the experimentally passivated material, resulting in subsequent rust formation.
Step 3. Depth Profile
- Concentration of C, O, Cr, Fe, and Ni as a function of depth (150Å) and calculated oxide thickness.
- For the polished sample (as shown on the graphs), the iron concentration is always higher than the chromium concentration. For the passivated sample, the concentration of chromium is greather than the concentration of iron in the near-surface region.
Step 4. After-profiling composition of all elements (for quantitative comparison).
Effect Of Chromium/Iron Ratio
A sample of corrugated 304L panel was exposed to a chloride-containing environment. The chromium/iron ratio and the sample appearance are shown. A lower Cr/Fe ratio results in more corrosion.
Cr/Fe = 2.1
Specific procedures have been developed by Material Interface to record the surface chemistry of these samples. The analysis can also be conducted according to specifications in standards published by Sematech and SEMI.
Sematech, 2706 Montopolis Drive, Austin, TX 78741, Specifications 90120403B-STD and 91060573B-STD
SEMI Global Headquarters, 3081 Zanker Road, San Jose, CA 95134, Specifications F30-0306 (XPS), F72-1102 (Auger), F73-1102 (SEM).