Finish characterization and electropolished surface morphology made simple
Understanding finish characterization and electropolished surface morphology of 316L stainless steel components used in high purity gas and liquid chemical distribution systems
By Ron Hendry
Because of the ever increasing need for higher purity process gases, chemicals and water, the distribution systems for these fluids must be manufactured to not contribute any contaminants. Since construction materials are one of the primary considerations, this article will focus on 316L stainless steel and some of the possible detrimental effects that the manufacturing and electropolishing processes have on the cleanliness of the components used in an ultra high purity (UHP) system. Commonly used terms (see "Terms and Definitions") have been included to help the reader better understand the surface characterization issues as they are addressed in the various processes. Both manufacturing and electropolishing acceptance criteria will also be defined according to current industry consensus. In addition, some commonly accepted industry methods of testing and the recommended methods for evaluating each element of the acceptance criteria will be discussed.
Most of the following information has been compiled from other sources, i.e. SEMI standard F19-95 -- "Specification for the Finish of the Wetted Surfaces of Electropolished 316L Stainless Steel Components"; "ANSI/ASME Standard B46.1 -- Surface Texture, Surface Roughness, Waviness and Lay"; Parker SPCC Document S241-003 -- "Specification for Surface Finish Measurement"; and from Dr. Andrea C. Greene, senior scientist at Surface Science Laboratories and Dr. Tim Volin, manufacturing engineering manager at Parker Hannifin`s Integrated Connectors Division. Both Drs. Greene and Volin are members of SEMI Standards` Surface Analysis Task Force.
Surface roughness measurement by stylus type profilometry
One of the more common methods of surface roughness measurement is by stylus-type profilometry. ANSI B46.1 describes in detail the types, terms (many of which are contained herein), functional measurement criteria and operational aids. The following is a brief summary of some of the criteria for 316L stainless steel used in high purity components:
1. All measurements are made as roughness average (Ra) or an Rmax in microinches or micrometers.
2. The traverse of the profilometer for most applications range between 0.15 to 0.50 inches.
3. The measurement cut-off should be 0.030 inches. This cut-off length is critical both as a basis of universal comparison and accuracy because shorter cut-off lengths exclude the coarser irregularities and will give lower Ra values which are not really representative of the surface irregularities.
4. Measurements on any item`s wetted surfaces are usually taken at 3 random and diverse locations.
5. Any visually questionable area will be measured and used as part of the acceptance criteria.
6. Welds, screw slots, intersections, and non-metallic parts are not usually measured.
7. Any region of a component that, due to its geometry or size, cannot be measured using the stylus profilometer is usually evaluated using a calibrated surface finish comparitor or an acceptance/non-acceptance library.
Wetted surface acceptance/ non-acceptance criteria
Acceptance/non-acceptance criteria for flaws caused by material irregularities, machining processes or handling should be in accordance with the following criteria.
If a flaw is not defined here-in, then the surface condition will be considered acceptable. Other acceptable criteria include: any flaws that have been polished out to the degree that they can be considered waviness; waviness will be considered acceptable if it does not cause the Ra and Rmax measurements to exceed specifications; process machine marks that fall within the specified surface finish Ra acceptance measurements; and macroscopic pits and inclusions that are within the surface finish and meet the SEM criteria as defined in the "SEM" section (see "Terms and Definitions"). Flaws occurring within 0.050 inches of the surface edge that will be welded are considered acceptable if the flawed portion of the surface will be consumed in the weld.
Unacceptable criteria are dense considerations of micropitting in elongated patterns (called stringers) are not acceptable on 5, 7, and 10 Ra finishes. However, scratches caused by the machining process or inspection tools that meet the Ra acceptance criteria and then are electropolished can be considered acceptable. Other unacceptable flaws are dents in tubing or fittings.
Electropolished wetted surface acceptance/non-acceptance criteria
Acceptance/non-acceptance criteria for defects caused by electropolishing and final cleaning should be as follows:
Surface contaminants, such as water spots, rust, process residue, chemical staining, etc. are not acceptable. Other non-acceptable items include: electropolished (EP) surfaces with unintentional interruptions in the electropolish; haze on the wetted surface; and blistering.
Acceptable criteria include: orange peel, if the surface meets the SEM surface defect count and Ra surface finish criteria; and frosting is acceptable only if the Ra surface finish, SEM and electro polish criteria are met.
Criteria for electropolished surface finish evaluation
SEM/EDX. SEM pictures typically are provided at magnification levels of 200, 1,000, and 3,500. At least three to five pictures should be made of various representative defect areas at the 3,000 to 4,000 magnification level. Defects (flaws) not process lines (reference note below), as defined in Section 8 (Surface Finish Acceptance/Non-Acceptance Criteria) of SEMI Standard F19-95, counted at the 3,500 magnification level should not exceed 40. (Note: If a magnification level other than 3,500 is used the defect count should be proportioned accordingly.) Also note that since machining/finishing process lines are still visible down to approximately 3 Ra surface finish, these process lines do not mask real defects and the surface finish and electropolishing criteria has been met.
EDX shall be used to define an inclusion in the SEM picture where it cannot be otherwise identified.
ESCA. ESCA should be used for characterizing the surface composition. (It covers more area and is considered more representative of the typical surface.) Note: There may be some minor variation in instrumentation readings based upon the angle of detector to sample, known as the "take-off angle."
ESCA should also be used to determine surface Cr/FE and CrOx/FeOx ratios. ESCA is more quantitative for the CR/Fe ratio because in the AES process it is difficult to maintain consistency in results. This is because the form of Cr and Fe is continually changing and the sensitivity of a given element changes in the AES process with its chemical form. Typical Cr/Fe ratios for electropolished 316L steel are 1.5 or greater. ESCA is also the technique presently used for determining CrOx/FeOx ratios which are typically 2.0 or greater for a well passivated surface.
Elements expected to be found on the surface of electropolished 316L stainless steel are Fe, Cr, Ni, C, O, Mn and Mo and may include but not be limited to contaminants such as Ca, K, Zl, Na, Cl, S, P, N and Si, which are primarily due to machining processes, handling and water quality. Although acceptance levels are continually changing the present industry criteria are as follows:
1. The primary acceptable elements are Fe, Cr, Ni and O with possible traces of Mn and Mo.
2. Although carbon is not necessary, it will be there. Less than about 50 percent is considered acceptable. Carbon levels greater than 50 percent can mask other results.
3. S, P, N and Si should be less than 2 percent.
4. All other contaminants should be less than 1 percent.
Auger Electron Spectroscopy (AES). AES is best for depth profiling. AES Instrument geometries are much more critical for data comparison due to the use of different angles of the ion gun. This must be defined when data is presented and the specification of the energy should be defined also.
The depth of the surface FeOx layer should be preferably less than 2 to 3 angstroms (with zero being the ideal) since an electropolished surface generally is more fully chrome enriched. Note that some surface Fe in whatever form may be okay so as to maintain the integrity of the surface to the bulk material but should note be higher than the chrome.
The depth of the Cr enrichment should be greater than 15 angstroms and the total depth of oxide thickness should be greater than 20 angstroms.
Ronald G. Hendry is a graduate of California State Polytechnic University (San Luis Obispo, CA) with a degree in Aeronautical Engineering. He spent 13 years as an aerodynamist, senior wind tunnel designer and research test engineer in various aerospace programs. This provided the technical basis for his extensive work in characterizing the liquid/gas flow conditions in piping systems.
During the last 14 years, Hen dry`s work has focused on cryogenic and UHP gas piping systems, component design in his jobs as engineering manager, manufacturing engineering manager, quality assurance manager and director of UHP technology. Because of the semiconductor industry`s requirements for higher technologies, Hendry has become very involved in promoting the use of higher quality stainless steels, improved processing and better surface morphology of UHP piping system components.
Having actively participated for over 12 years in the SEMI Standards programs, Hendry is presently serving as co-chairman of the North American Region`s Facilities and Safety Division.
Terms and definitions
Acceptance/non-acceptable library: A library of components maintained by quality assurance for the purpose of making comparative evaluations.
Auger Electron Spectroscopy (AES): An alternative surface analysis that can also detect all elements with an atomic number greater than that of helium with the additional ability to analyze submicron-diameter features. It is not as quantitative as ESCA and cannot determine the chemical state of an element. The primary advantage of Auger is that when combined with etching, a chemical depth profile can be measured rapidly and it can image the distribution on the surface of spatial limitation resolution of 100 to 1,000 angstroms (depending on the equipment capability).
Blistering: A surface abnormality whose appearance pattern is like that of orange peel with randomly chipped or flake-off areas.
Carbon thickness: A measurement of surface organic material. Carbon thickness values typically range from 5 to 10 angstroms. Significantly contaminated surfaces can show surface carbon thickness of 20 angstroms or more.
Chromium enrichment layer thickness: The same as its maximum depth of enrichment unless a surface iron layer is present, in which case the chromium enrichment layer is calculated as the maximum depth of enrichment minus the thickness of the surface iron oxide layer.
Dent: A protrusion on the wetted surface that can be associated with an irregularity on the external surface.
Electron spectroscopy for chemical analysis (ESCA): also known as X-ray Photoelectron Spectroscopy (XPS) is a surface-sensitive technique capable of detecting all elements with an atomic number greater than that of helium. ESCA provides data on the outermost several atomic layers of a material, and has a sensitivity in the order of 0.5 atomic percent. A primary advantage of ESCA is that it can both determine and quantify the chemical state of the elements detected (i.e. metallic state or oxide state).
Flaws: Unintentional irregularities which occur at one place or at relatively infrequent or widely varying intervals on the surface. Flaws include such defects as cracks, blowholes, inclusions, pits, checks, ridges, scratches and other surface abnormalities as defined herein.
Frost/micropitting: A continuous surface abnormality whose appearance pattern is like that of a sparkly, very fine, sandy-textured surface.
Haze: The abnormal appearance of a localized diminishing in brightness or luster of a surface when compared to the adjacent surfaces.
Inclusion: A foreign material that is embedded in the metal and may be revealed on the surface.
Lay: The direction of the predominant surface pattern, ordinarily determined by the production method used.
Machine lines/process lines: Surface topography created from machining or honing/polishing lines will normally run parallel, but bisecting (perpendicular) lines may occur when the honing stone mandrel is removed from the work piece.
Maximum Cr/Fe ratio: The maximum ratio of chromium to iron and the depth at which it occurs are the most direct measures of the chromium enrichment in a materials oxide layer. Typical ratios are about 1.5 or greater for well-electropolished 316L stainless steel. The depth at which the maximum Cr/Fe ratio is found varies but is usually about one-half the oxide thickness.
Maximum depth of enrichment: The chromium enrichment layer comprises all depths at which the chromium concentration is greater than the iron concentration. For well electropolished 316L stainless steel, the maximum depth of this layer is typically 20 to 25 angstroms.
Nominal surface: The interned surface contour, the shape and extent of which is shown and dimensioned on a drawing or descriptive definition.
Orange peel: A surface abnormality whose irregular surface appearance pattern is like that of an orange peel.
Oxide thickness: The depth at which the oxide signal has fallen to half the maximum peak height. Typical values for well-electropolished 316L stainless steel range from 20 to 50 angstroms.
Pit or stringer (multiple pits): A small cavity or series of small cavities on the metal surface.
Roughness: Consists of the finer irregularities of the surface texture, usually including those irregularities that result from the manufacturing process. These are considered to include traverse feed marks and other irregularities within the limits of the roughness sampling length.
Scanning Electron Microscopy (SEM): Utilizes an electron beam to produce images over a very broad magnification range of 10X to 105X. The technique is somewhat limited by the conductivity of the material but works very well with 316L stainless steel. Typical magnification levels for surface defect evaluation are from 100 to 4,000.
Scratch: An elongated mar in the surface, not associated with the predominant surface texture pattern, that is visible to the unaided eye.
Subsurface carbon enrichment: A rise in the carbon signal at depths from 15 to 20 angstroms. This indicates that organic material is buried in cracks, crevices, pits or smeared material. Subsurface carbon is most commonly found in materials having rough morphology generally associated with machining processes.
Surface contaminants: Water spots, rust, process residue, chemical staining, etc.
Surface iron oxide layer: Surface iron oxide layer present when the 316L stainless steel`s iron composition signal is higher than its chromium signal at the surface.
Surface texture: The repetitive or random deviations from the nominal surface which form the three-dimensional topography of the surface. Surface texture includes roughness, waviness, lay and flaws.
Waviness: The more widely spaced component of surface texture. Unless otherwise noted, waviness includes all irregularities whose spacing is greater than the roughness sampling length. Waviness may result from such factors as machine or work deflection, vibrations, chatter, heat treatment or warping strains. Roughness may be considered as superimposed on a "wavy" surface.
Wetted surface: The surface(s) of any valve or component that will be exposed to a fluid (liquid or gas) when in service.