| Surface chemistry evaluation techniques commonly employed for determining the chemical structure of UHP systems component wetted surfaces are (alphabetically arranged for your convenience):
- Auger Electron Spectroscopy (AES) - An analytical technique that analyzes only the top few atomic layers. In this technique, a beam of electrons is focused on the sample as in the scanning electron microscope. One of the ways in which the electrons can interact with the atoms in the sample causes low energy electrons to be ejected, in a process first identified by Pierre Auger, a French scientist for whom the process is named. The Auger electrons produced are of very low energy; therefore, only those produced in a few atomic layers immediately adjacent to the surface can escape and be detected. The energies of the Auger electrons are characteristic of the elements from which they are emitted, and the atomic concentrations of the elements detected may be determined quantitatively from the intensity of their peaks by applying appropriate sensitivity factor corrections. Auger electron spectroscopy is particularly sensitive to low atomic number elements such as carbon and oxygen.
An Auger depth profile, showing the changes in elemental concentrations below the initial surface, may be obtained by removal of the surface atoms layer by layer with simultaneous Auger analysis. The removal of the surface atoms is performed by Argon ion beam sputtering, which may be calibrated to determine its rate so that the sputtering time can be converted to a depth below the initial surface.
- Electron Spectroscopy for Chemical Analysis (ESCA), sometimes called X-Ray Photo Electron Spectroscopy (XPS) A beam of monoenergetic X-rays is directed onto the surface to be analyzed, causing photoelectrons to be ejected from the surface. The energies of these ejected photoelectrons is characteristic of the elements from which they are emitted. As these photoelectrons are of low energy, only those produced in the vicinity of the surface may escape and be detected by the analyzer. Thus, ESCA analyzes only the top 2 to 10 atomic layers in the surface. The atomic concentrations of the elements detected may be determined quantitatively from the intensity of their peaks by applying appropriate sensitivity factor corrections. The chemical state in which the elements are present, such as metallic states or oxide states, causes very small shifts in the energy of the photoelectrons emitted from these elements. By using high resolution ESCA techniques, it is possible to quantify the relative amounts of the elements that are present in the elemental versus the compound states.
- Energy Dispersive X-ray (EDX) Spectroscopy Analysis This is useful for detecting and analyzing inclusions in the metal that may be revealed on the surface or regions of very thick contamination. EDX is not a technique for the analysis of the chemistry of the passive layer on a stainless steel surface, as the depth from which the x-ray signal emanates is much greater than the approximately 50 Angstroms thickness of the passive layer.
- Solvent Extraction Test Methods by Fourier Transform Infrared Spectroscopy - These methods depend upon the selection of an appropriate solvent for the surface contaminant to be detected. For hydrocarbon contaminants, a freshly distilled high vapor pressure solvent such as freon or hexane is generally used. After exposure of the surface to be checked, the solvent is evaporated to leave the extracted contaminants as a residue on an appropriate substrate, and the residue is analyzed by Fourier Transform Infrared Spectroscopy (FTIR). FTIR is an analytical technique that identifies chemical groups based on their characteristic frequencies of absorption of infrared radiation. This analytical technique is qualitative, and only order of magnitude estimations of the quantity of the contaminants may be made. The technique is very operator-sensitive and may be subject to error. Any finding of contaminants should be corroborated by complementary analytical techniques.
Solvent Extraction and Analysis by Chromatography - An analytical technique that depends on adsorption or ionic change in chromatography column. For identification of ionic residues on surfaces, the ionic residues are generally extracted using deionized water. The deionized water with the extracted ionic contaminants is passed through a column containing ionic exchange resins. Identification of the ionic species present depends upon measurement of the retention times in the column. The technique is capable of quantifying the concentration of the species in the deionized water solvent.
A clean, properly electropolished stainless steel surface will have the following typical specifications:
- No visible damage to surface, contamination or residual machining marks.
- Minimal pitting.
- Surface will be bright, without "hazy" or "milky" appearance.
Scanning Electron Microscopy
- Average defect count <10; maximum <40.
- No contamination.
- No micropitting.
- Ra per specification. Roughness average (Ra) is the arithmetic average of the absolute values of the measured profile height deviations taken within the sampling length and measured from the graphical centerline. Ra is expressed in micrometers, or millionths of a meter. Stylus-type profilometers are designed to respond only to irregularity spacing less than a given value, called the cutoff. In other words, all irregularities having a spacing less than the value of the cutoff are included in a measurement.
It is important to reduce the area of the surfaces exposed to vacuum in stainless steel chambers, thus reducing outgassing. The use of electropolished cold rolled sheet without sanding or other mechanical treatments, achieves the smallest surface area and improves the vacuum performance. Mechanical sanding of cold rolled sheet always produces a greater surface area. Other benefits of a smooth surface are that it is easier to clean and it pumps down faster.
Solvent Extraction Analysis
- No elements detected other than those permitted in material composition specifications.
Auger Electron Spectroscopy
- Oxide Layer Thickness: >20 Angstroms
- Maximum Cr/Fe Ratio: >1.5
- Iorn Oxide Layer: <5, preferably 0 Angstroms
- Carbon Layer Thickness: <8 Angstroms
- Initial Carbon Concentration: <30 Atomic Percent
- Minimal levels of other contaminants.
Electron Spectroscopy for Chemical Analysis
- Total Cr/Fe Ratio: >1.5
- TOTAL Crox/FeOx: >2.0
Surface Condition Problems and Causes
1) Contamination spots, water spots.
- Contaminated water.
- Incomplete cleaning.
- Improper handling.
2) Visible Pitting
- Improper mechanical polishing (AFM process).
- Improper electropolishing.
- High inclusion content stainless steel.
3) "Dull" Surface
- "Fine scale" surface roughness due to: (a) improper preparation of surface (polishing); or (b) improper electropolishing process.
4) "Hazy" Surface
- Microscopic pitting due to: (a) improper mechanical polishing (AFM process); or (b) improper preparation for electropolishing.
- Surface contamination.
5) High Surface Carbon Levels
- Contamination of cleaning/electropolishing fluids.
Source: "Surfaces in UHP Process Tubing Systems, Physical and Chemical Properties," Parker-Hannifin, June, 1994.