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Methods for Monitoring and Measuring Cleanliness of Surfaces

Rajiv Kohli , in Developments in Surface Contamination and Cleaning: Detection, Characterization, and Analysis of Contaminants, 2012

4.2.4 Water-Break Test

The water-break test uses running water, allowing it to form a sheet across the surface [32,33]. Breaks in the water indicate the presence of hydrophobic residues. The water break test is a fairly crude test, which is suitable for detecting films of process oils and heavy fingerprints. The test is often used for parts washing and is generally not suitable for precision-cleaning applications. It does not readily detect nonhydrophobic contaminants.

A variation of the water-break test is the atomizer test that involves a gently sprayed water mist [34,35]. Any surfaces, where water repulsion occurs, indicate the presence of hydrophobic contamination. The atomizer test detects larger amounts of hydrophobic contaminants than the water-break test in which the kinetic energy of the flowing water may remove a hydrophobic residue. In contrast, the atomizer test enables the visualization of the atomized droplets of water being repelled by a hydrophobic contaminant.

In practice, the atomizer test is applied immediately prior to painting or coating. A mist of distilled water is atomized onto the surface. If the water droplets tend to coalesce into large lenses lasting for 25 s without flashing out, the surface is considered as having satisfactorily passed the water-break test. If the water gathers into droplets within 25 s (if the surface shows a "water break" within that time), the surface is considered to have failed the test. If the water forms a continuous film by flashing out suddenly over a large area, this is considered evidence of the presence of a contaminant on the surface, and the surface is deemed to have failed the test. Failure to support an unbroken water film will require additional cleaning of the part. Multiple cleaning procedures may be required to achieve the required water break-free surface.

An improved automated inspection system that includes an infrared (IR) camera has been developed for the wetted surface of the parts [36]. The IR camera offers greater contrast and is not subject to limitations of lighting angle and viewing orientation. Furthermore, by replacing manual inspection, the noncontact inspection system eliminates exposure of humans during manual inspection to hazardous chemicals used for the processing of parts.

There are several variables that must be taken into consideration when the water-break test is used for validation of surface cleanliness [32]. These include the presence of hydrophobic and hydrophilic contaminants; presence of abrasive particles and smearing of the surface from abrasion; smearing from organic compounds used in processing; contaminants in the water used for testing; water temperature; and angle of the test surface. This makes the water-break test acceptable for qualitative testing in industrial and commercial applications, but it is not very useful for cleanliness testing in the electronics or other high-precision industries, where surface cleanliness is critical to product performance.

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Surfaces: How to assess

John F. Watts , in Adhesive Bonding (Second Edition), 2021

3.3.2 The water break test

There are various forms of the water break test, which is essentially a method for assessing the surface cleanliness of metal substrates, for assessing the effectiveness of a cleaning process in the removal of any residual organic contamination resulting from protective greases or mechanical working lubricants. Such carbonaceous films will be hydrophobic (non-wetting) in nature, and the test involves withdrawing the metal panel under test from a container full to the brim with distilled water. On withdrawing a clean substrate, the water will drain uniformly over the surface. In the presence of residual contamination, the draining water film will break up into a discontinuous layer around the contaminated regions. Although this is a very subjective test, it is quick to carry out and lends itself to process control purposes. One form of the test is embodied in the relevant US standard [13].

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Adhesives and Sealants

John Comyn , in Handbook of Adhesives and Sealants, 2006

1.3.6 Practical Applications of Wetting

The relationship between work of adhesion and practical adhesion has been reviewed by Packham [78].

Whereas actual measurements of contact angles are usually made in science laboratories, the principles are exploited in the water-break test and in the use of liquids of different surface tensions to assess the printability of polyolefins.

The water-break test is a simple method to check that a metal surface is clean. A few drops of distilled water are placed on the surface, or alternatively the sample can be drawn from a bath of water. If the water does not break into droplets then the surface is free from contamination. Uniform wetting of the metal by water indicates that it will be similarly wetted by the adhesive.

A standard test (ASTM 1982) involves wiping a polyolefin surface with a series of liquids, starting with one of low surface tension and noting the time needed for the film to break into droplets. Liquids with increasing surface tension are used until one is found which will wet the surface for just 2 s. The surface tension of the plastic then equals that of the liquid. Twenty-two mixtures of formamide and 2-ethoxyethanol are used with surface tensions in the range 30–56 mN m^{− 1}.

Wetting is not a reciprocal property [79] in that if A spreads on B, B does not necessarily spread on A. An example of this is that a liquid epoxide resin will not spread on polyethylene, but if the resin is cured it will then be wetted by molten polyethylene. Surface tensions of epoxides are about 44 mN m^{− 1} and that of polyethylene is about 30 mN m^{− 1}. A solid can force liquids of lower, but not higher surface tension to wet it.

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Surface Treatment and Repair Bonding

Andrew N. Rider , ... James J. Mazza , in Aircraft Sustainment and Repair, 2018

7.1 Waterbreak Test

The aerospace industry routinely uses the tendency of clean water to 'bead' or 'break' as an indication of the presence of hydrophobic contaminant on an adherend during surface preparation. In practice, the waterbreak test relies on the skill and experience of the technician and is not necessarily reliable. Surface roughness has a significant influence on the outcome. Some contaminants have hydrophilic characteristics and, therefore, lead to a waterbreak free indication. Examples of hydrophilic contaminants commonly encountered are water-displacing fluids used in aircraft maintenance. As mentioned, new portable waterbreak test methods are now being applied in quality control applications [31,103].

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Optically Stimulated Electron Emission

Mantosh K. Chawla , in Developments in Surface Contamination and Cleaning, 2015

4.1.2.1.2 Application 2

The reliability of the carbon fiber-reinforced plastics (CFRPs) is governed by assuring the surface quality of CFRP material. Roughness of ready-to-bond CFRP surfaces affects, in a complex way, their wetting behavior as determined by water break test [9].

OSEE was investigated as an in-line technique to assess the surface quality prior to bonding. Sensitivity and accuracy of OSEE measurements were successful in distinguishing favorable surface states of CFRP adherents from surface states which were unfavorable for adhesive bonding. It has been shown that OSEE can be applied in the field and without electrically contacting the CFRP surface for sensing moisture, thermal degradation, or thin layers of contaminants such as release agent and hydraulic oil for aeronautical use.

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SURFACES, CHEMISTRY & APPLICATIONS

GUY D. DAVIS , JOHN D. VENABLES , in Adhesion Science and Engineering, 2002

5.1 Part inspection

The extremely fine, porous oxide morphologies illustrated in this chapter are a requisite for making high quality aerospace type bonds, but on the minus side they can be perfect traps for oils found on fingers (which should never be used to handle bondments) and on virtually all bond shop materials such as, nylon gloves, Kraft paper, silicone rubber, and Teflon [158]. Contaminant layers that fill the pores and cover the surface, as shown in Fig. 34, will prevent wetting by the adhesive and, consequently, prevent mechanical interlocking from forming thereby leading to low-strength bonds. Note that surface contact can also damage the oxide asperities, but the overall hardness of the oxides ensures that such damage may be limited to relatively small areas.

To inspect for contaminants, a water break test is frequently employed. Water, being a polar molecule, will wet a high-energy surface (contact angle near 180°), such as a clean metal oxide, but will 'bead-up' on a low-energy surface characteristic of most organic materials. If the water flows uniformly over the entire surface, the surface can be assumed to clean, but if it beads-up or does not wet an area, that area probably has an organic contaminant that will require the part be re-processed.

In some cases, it is not possible to use water break testing because water would be deleterious to the surface as for freshly cleaned steel. The steel solid rocket motor (SRM) cases of the Space Shuttle are examples where the actual bonding surface must be 100% inspected for cleanliness to verify that the preservative grease intentionally applied to prevent corrosion has been removed prior to priming and application of a rubber insulation layer. In this case, optically stimulated electron emission (OSEE) [159,160] is used because large areas can be inspected in reasonable times without the need for contacting the surface [161]. In OSEE, the surface is illuminated with an ultraviolet light, the low-energy photoelectrons emitted from the steel surface are collected and the photocurrent measured. If the surface is contaminated, the organic material will absorb the photoelectrons and a lower current will be measured. This is illustrated in Fig. 35 for two different contaminants. The silane contamination completely attenuates the substrate signal, but grease does not because it is slightly photoemitting itself and the OSEE signal begins to increase at high levels of coverage. Although this can present conflicting evidence with both high and low levels of contamination giving the same OSEE signal, high levels can be detected visually so that clean surfaces can be assured. OSEE works best on steel but not as well on aluminum because the oxide also attenuates the photoelectron signal from the substrate.

Other techniques to inspect bonding surfaces for contamination have also been proposed, including ultraviolet fluorescence [162]. Pulsed ultraviolet light incident on the surface excites fluorescence of organic contamination, which can be imaged using a low-light video camera. Because the metal surface does not fluoresce, organic contamination at levels as low as 1 μg/cm² can be detected and mapped quickly. Similar contamination maps can be obtained with infrared imaging [12,163], and here again emphasis is placed on contamination detection not on identification. This allows broad energy (wavelength) filters to be used, which speed up inspection but provides for little or no molecular identification through spectral analysis.

In addition to inspecting for possible contamination, it usually is also of interest to determine whether the chemical etching or anodization process has actually produced the desired oxide. For this purpose, anodization has somewhat of an advantage over etching (FPL, for example) because the thicker oxide developed by the former process can be readily detected optically by observation with crossed polarizers [54]. This method is useful for determining if the oxide is present but it does not reveal anything about the oxide morphology which must be examined by more sophisticated techniques. Thus, when bonding problems arise which are not resolved by less sophisticated techniques, it may be necessary to employ the use of analytical laboratories having high resolution HR-SEM and other sophisticated scientific apparatus. At such times, witness panels can play a crucial role in the investigation as discussed in the next section.

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Composite bond inspection

R.G. Dillingham , in Structural Integrity and Durability of Advanced Composites, 2015

25.9 Bond inspection tools

Bond inspection tools can be divided into two classes, those useful for monitoring quality during the bonding process and those useful for inspection of the complete bonded structure. In-process inspection is preferable from an economic standpoint, as quality issues are detected before completion of the structure, when correction becomes extremely expensive. However, certification typically requires some sort of inspection of the completed structure. Unfortunately, there are few inspection techniques available that provide useful information about bonded structure quality.

Tools for in-process inspection include those that monitor quality and consistency of surface preparation and those that provide information regarding the chemical structure of the adhesive and substrate.

Quality and consistency of surface preparation can be monitored through wetting measurements. Adhesive bonding is a wetting phenomenon, that is, it fundamentally rests on the interaction between a liquid adhesive and a solid surface. Wetting is a strong function of surface chemical composition. One way of assuring the quality and consistency of a surface prepared for adhesive bonding is through assuring that the surface has the desired wetting properties (Dillingham & Oakley, 2006). A common approach is through the water break test (ASTM F22-13, 2013), where the surface is flooded with water and uniform flow of the water over the surface is an indicator of the absence of hydrophobic contaminants. This test is not quantitative and is typically restricted to applications where a go/no go evaluation of cleanliness will suffice.

A quantitative and sensitive method for monitoring surface properties is through measurement of contact angle established between a probe liquid and the laminate surface. The contact angle is the angle formed by a tangent to a liquid drop and the plane of the laminate surface at the point of contact (Figure 25.4). Conceptually, when a liquid is brought into contact with a solid surface, the liquid molecules partition between continued interaction with other liquid molecules or establishing interaction with molecules in the surface. If interaction with the surface is energetically favored, the interfacial area is maximized and the liquid spreads, establishing a low contact angle. The contact angle established is defined by the Young–Dupré equation:

$θ = \cos^{- 1} (\frac{γ_{s} - γ_{sl}}{γ_{l}})$

which shows that high surface energies (γ _l) correspond to low contact angles.

One method for utilizing contact angles as a means of evaluating the suitability of a surface for bonding involves construction of wettability envelopes from contact angles obtained using multiple liquids of different surface tensions γ _l (Boerio, Roby, Dillingham, Bossi, & Crane, 2006; Smith & Kaelble, 1981). However, it is experimentally unwieldy to perform these measurements in manufacturing and repair environments. It has been demonstrated to be generally sufficient to measure the contact angle of a single liquid as a surrogate for the liquid adhesive; this approach has shown excellent predictive ability (Dillingham, Oakley, et al., 2012). A convenient way to obtain these wetting measurements in a challenging manufacturing or repair environment is through ballistic deposition of a water drop followed by determination of an average contact angle established by the drop perimeter with the surface (Dillingham, Oseas, Gilpin, & Ganance, 2012; Tracey & Flinn, 2012). This approach has shown excellent sensitivity to consistency of surface treatment of both metal and polymeric surfaces and is beginning to see significant acceptance as an in-process quality inspection method.

Surface and bulk chemical analysis. Fourier transform infrared spectroscopy has been in use for many decades as a sensitive technique for quantitative detection of functional groups in polymers. The recent advent of handheld instrumentation coupled with either attenuated total reflectance or diffuse reflectance techniques permits these measurements to be performed on composite structures; several investigators have been demonstrating the use of these techniques to identify the presence of undesirable peel ply residues, contaminants, and thermally degraded laminate surfaces (Tracey & Flinn, 2012).

Inspection of bonded composite structures consists of detection of voids or cracks and disbonds, or local proof testing of the interface.

Voids and cracks. The most popular ultrasonic methods use through transmission (Crane & Dillingham, 2008) or reflected (Forsyth, Yolken, & Matzkanin, 2006) ultrasound (typically 20–400 kHz). While the specifics of each of the ultrasonic techniques differ slightly, the basic configuration remains very similar to that shown in Figure 25.5. In the through transmission technique, transducers are coupled to the specimen surface, either through direct contact or through water as a coupling medium. The transducers are then scanned over the specimen. Physical discontinuities in the path of the sound waves represent discontinuities in the acoustic impedance of the part. These discontinuities are detected either by attenuation of the signal amplitude or reflection of the signal, which causes a change in the duration between excitation and reception. The signals are processed to create a map of the acoustic impedance of the interior of the structure.

The presence of disbonds or other void-type defects is not very important unless they are close to the edge of a joint where the stresses are significant (Crane & Dillingham, 2008). In fact, it is possible to completely eliminate the central region of this joint without affecting the structural performance of the joint (Forsyth et al., 2006; Schonhorn, Ryan, & Wang, 1971; Wang, Ryan, & Schonhorn, 1972). However, this is the region of the joint that is the most difficult to inspect with ultrasound due to diffraction of the relatively long-wavelength acoustic waves. So while bond quality as defined by absence of voids can be measured by ultrasonic techniques, these measurements are of limited usefulness in predicting bond strength.

Thermal imaging is another method for interrogating bonded structures that is sensitive to the discontinuities in thermal conductivity that correspond to voids or disbonds, but is not as confounded by the proximity of edges. These methods are characterized by the use of thermal measurements of a test object as it undergoes a response to a stimulus (Shepard, 2007). In pulsed thermography, the surface of a sample is heated with a brief (a few milliseconds) pulse of light from a xenon flash lamp array. An infrared camera monitors the time-dependent response of the sample surface temperature. Where the sample surface is close to a thermal discontinuity (such as a defect), the heat flow from the surface into the sample is obstructed. This causes a local temperature increase at the surface. The time required for these temperature deviations to occur is a function of the depth of the discontinuity, so it is possible to measure the depth below the surface of the defect.

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Repair of damaged aerospace composite structures

E. Archer , A. McIlhagger , in Polymer Composites in the Aerospace Industry, 2015

14.4 Typical repair procedure

An industry standard scarf repair first involves removal of topcoat and primer and then using a hand-held pneumatic router or grinder to machine the damaged portion of the laminate into a circular shape. The sides of the circular cutaway must then be machined to a taper at the required angle. Two approaches can be employed, either stepped or scarfed. The scarfed joint is theoretically superior (better stress distribution) but little difference is seen in practice. The joint face is sanded to a constant

taper ratio (different OEMs use different taper ratios but typically they are between 20:1 and 50:1 but could be as high as 100:1) [12]. Thickness of laminate and predominant nature of loading (tension or compression) impact the optimal taper ratio; however, OEMs usually simplify things and base all their repairs upon a single taper ratio. A final solvent wipe is performed followed by further inspection such as a water break test; the area is then dried before layup begins. Most repair material systems cure at temperatures above the boiling point of water, which can cause a disbond at the skin-to-core interface wherever trapped water resides. Depending on the resin system used, the repair will require curing at elevated temperatures. These could be as low as 50–60 °C for some room temperature wet layup resins to 180 °C for some prepreg systems. Some special repair prepreg systems have been developed with lower cure (120 °C) prepregs optimised for repair use such as Hexply^® M20. If the repair is to a sandwich construction, the inner skin and core are repaired in a first step. A non-structural backing may have to be applied to support the inner layup during cure.

For repairs to be made in situ (without removing the component), a vacuum bagging system is generally used. This is an integral part in the process as heating elements are used to cure the prepreg resin in the repair material. This process occurs under vacuum to optimise the resin distribution and to also provide a force to bond the patch to the surface of the parent component. A typical vacuum bag schematic is shown in MIL-HDBK 17-3F 3. The best vacuum bag schedule will vary from one repair layup to another, with different repair layups requiring different bleeder ply schedules. In the case of flat laminates, heat is usually applied by means of temperature-controlled electric heating blankets in conjunction with a vacuum bag arrangement. These blankets can also be manufactured to fit complex double curvature aircraft surfaces, or a temporary oven might be constructed. In order to substantiate the repair quality, a process control panel should be fabricated within the repair vacuum bag and cured simultaneously. When the repair is completed the process panel can be tested to verify the process results [1]. Clearly, this process is severely limited on a large scale as the accuracy and uniformity of the hand-machined scarf cavity and level of cosmetic finish is entirely dependent on the skills and experience of the operator. Another important limitation to consider is the pressure and temperature that can be achieved using a vacuum bagging system. To manufacture a patch to the equivalent quality of the original autoclaved part, it is imperative the patch is cured under autoclave conditions. To account for the loss in performance, extra plies are added to the repair scheme; this has the negative effect of prevent a 'flush' repair from being totally free from protrusion.

14.4.1 Bolted patch repair schemes

For thicker laminated sections, restoration of the design load carrying capacity can be provided with a bolted patch repair. This repair process is to remove the damage and create a hole with circular ends, remove any moisture, drill the locating fastener holes in the parent laminate, and attach the repair panel which could be of inner, flush and outer patch types. The patch panels and fasteners should be coated with a sealing compound and fitted wet. Bolted repairs can comprise an external or an internal patch that results in a single shear joint, or two patches, one on each side that leads to a double shear joint, see Figure 14.5. In both cases the load is transferred through the fasteners and the patch by shear forces, but in the case of the two-patch repair, transfer load eccentricity is minimised.

Bolted repair procedure consists of six distinct steps: (1) patch preparation and pilot drilling holes, (2) laying out hole pattern on the parent skin and pilot drilling skin holes, (3) the transfer of the holes in the skin to the patch if the patch covers some existing skin holes, (4) drilling/reaming of patch and skin, (5) patch and fastener installation and (6) sealing of the repair. For complex repairs, multi-row fastener patterns will be required to gradually introduce the load from the part being repaired into the repair patch. It is virtually impossible to distribute the load evenly between all the fasteners in a multiple row pattern, but careful design of patch geometry, fastener diameter and spacing can alleviate the high loads at the first fasteners. Such complex repairs are not usually identified in the approved repair manuals or procedures (SRM, TO or TM) and normally need engineering input for design. Care should be taken to prevent galvanic corrosion and as with composite joints, special titanium, monel or stainless fasteners are required.

External Patch with Backup Plate: This concept uses an external chamfered metal patch bolted to the panel being repaired. The bolts thread into nut plates mounted on metal backup plates that are on the side of the repaired panel. The backup plate can be split into two or more pieces and slipped through the opening.

External Patch with Blind Fasteners: This concept is similar to the previous one, except that the backup plates are not used. Blind fasteners are not as strong as bolts and nutplates, but if acceptable strength can be restored, this concept is easier to use.

Bolted Internal Doubler: This concept has been used as a standard repair for metal structures. Access to the backside is required to install the doubler. The doubler cannot be installed through the hole as a separate piece because the doubler has to be continuous to carry loads in all directions. Filler is used to provide a flush outer surface, and is not designed to carry loads.

14.4.2 Bonded versus bolted

Repair design criteria, part configuration and logistic requirements will dictate whether the repair should be bolted or bonded. Depending on the layup and material, composites can exhibit low bearing strength. Due to the bearing loads induced by mechanical fasteners, bolted repairs are not generally acceptable on thin laminates or sandwich structures. According to Baker and Jones [13], bolted repairs should not be used on laminates less than 8 mm thick. In reality, however, they are used on laminates greater than 3 mm thick. Adhesive bonded patched repairs are very attractive due to their high efficiency, more uniform stress distribution and good fatigue behaviour. Bonded repairs enables joining of thin sheets with minimal stress concentrations while providing an aerodynamically smooth repaired area with few irregularities. Also, bonded repairs can be more aesthetically pleasing with minimal weight increase and some control of joint and repair stiffness (Table 14.1).

Table 14.1. Repair design criteria, part configuration and logistic requirements will dictate whether the repair should be bolted or bonded (Ref: MIL-HDBK-17.3)

Condition	Bolting	Bonding
Lightly loaded, thin (<0.10 in. (2.5 mm))		×
Highly loaded, thick (>0.10 in. (2.5 mm))	×	×
High peeling stresses	×
Honeycomb structure		×
Dry and clean adherend surfaces	×	×
Wet and/or contaminated adherend surfaces	×
Sealing required	×	×
Disassembly required	×
Restore unnotched strength		×

However, the disadvantages of bonded repairs are that surface preparation is critical and sometimes difficult, and the processing and material storage are environmentally sensitive regarding temperature, time and humidity. Also, NDI is required, which increases the cost, skill, equipment need and time to complete the repair. Bonded repairs are sometimes severely size restricted and might require recurring inspections. Also, deep damages require removal of a lot of undamaged parent material and bonded repair must elevate the repair area to a uniform temperature, so care must be taken to alleviate heat sink.

Bolted repairs require similar skills and processes to repair of metal structure meaning materials, equipment and skills are more readily available, so there is an opportunity to save considerable time without the requirement to scarf the surface. Also, this brings a better confidence in process and the design. The disadvantage of bolted repairs is that it is difficult to design repairs for sandwich structures and for thin sheets due to high bearing stresses. If dissimilar materials are used there is the potential for thermal stresses, stiffness and strength mismatches and environmental deterioration. Bolted repairs generally incur an increase in weight and are therefore less structurally efficient and the aerodynamic smoothness more likely to be compromised even for 'flush' schemes. With composites, a greater edge distance is required than in metal structure and a drilled hole through complete thickness is required regardless of damage depth. Mechanically fastened repairs require care and accuracy in the drilling of holes and the alignment of parts during assembly. Fastener hole breakout is a characteristic problem, commonly solved by using a layer of woven fabric as the outermost ply for all laminates during initial component manufacture.

In summary, bonded repairs are the preferred approach for manufacturing repairs to both honeycomb sandwich and monolithic secondary structure. However, for the FAA and European Airworthiness and Safety Administration, the main reason for withholding certification of bonded repairs for primary structure is the lack of certainty over bond quality as it is not possible to assess strength and durability of bonded joints without destructive testing.

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Assessment of surface preparation for the bonding/adhesive technology

Anna Rudawska , in Surface Treatment in Bonding Technology, 2019

9.2 Assessment and degrees of surface treatment

The surface preparation assessment (Fig. 9.1) may include [31, 43, 58]:

●: surface preparation grades,
●: assessment of the degree of removing impurities from the surface, and
●: determination of surface roughness.

The aforementioned types of assessment concern the preparation of surfaces prior to application of paint coats; however, they are similarly applicable in the case of surface preparation for adhesive joining. International standards containing guidelines are detailed in the following groups of standards (Fig. 9.2):

●: ISO 8501—visual assessment of surface cleanliness,
●: ISO 8502—tests for the assessment of surface cleanliness, and
●: ISO 8503—Surface roughness characteristics of steel substrates.

Among these groups of standards, the following can be distinguished:

●: PN-EN ISO 8501-1. Preparation of steel substrates before application of paints and related products—Visual assessment of surface cleanliness—Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings,
●: PN-EN ISO 8501-2. Preparation of steel substrates before application of paints and related products—Visual assessment of surface cleanliness—Part 2: Preparation grades of previously coated steel substrates after localised removal of previous coatings,
●: PN-EN ISO 8501-3. Preparation of steel substrates before application of paints and related products—Visual assessment of surface cleanliness—Part 3: Preparation grades of welds, edges, and other areas with surface imperfections,
●: PN-EN ISO 8501-4. Preparation of steel substrates before application of paints and related products—Visual assessment of surface cleanliness—Part 4: Initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting,
●: PN-EN ISO 8502-2. Laboratory determination of chloride on cleaned surfaces,
●: PN-EN ISO 8502-3. Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method),
●: PN-EN ISO 8502-4. Guidance on the estimation of the probability of condensation prior to paint application,
●: PN-EN ISO 8502-5. Measurement of chloride on steel surfaces prepared for painting (ion detection tube method,
●: PN-EN ISO 8502-6. Extraction of soluble contaminants for analysis—The Bresle method,
●: PN-EN ISO 8503-1. Specifications and definitions for ISO surface profile comparators for the assessment of abrasive blast-cleaned surfaces,
●: PN-EN ISO 8503-2. Method for the grading of surface profile of abrasive blast-cleaned steel—Comparator procedure,
●: PN-EN ISO 8503-3. Method for the calibration of ISO surface profile comparators and for the determination of surface profile—Focusing microscope procedure,
●: PN-EN ISO 8503-4. Method for the calibration of ISO surface profile comparators and for the determination of surface profile—Stylus instrument procedure,
●: PN-EN ISO 8503-5 Replica tape method for the determination of the surface profile,
●: ASTM F22-02. Standard test method for hydrophobic surface films by the water-break test,
●: ASTM C813-90. Standard test method for hydrophobic contamination on glass by contact angle measurement,
●: ASTM D1193-06. Standard specification for reagent water,
●: ASTM D7334-08. Standard practice for surface wettability of coatings, substrates, and pigments by advancing contact angle measurement,
●: ASTM D2578-17. Standard test method for wetting tension of polyethylene and polypropylene films,
●: ISO 8296. Plastics film and sheeting determination of wetting tension,
●: ASTM D 5946-04. Standard test method for corona-treated polymer films using water contact angle measurements,
●: ASTM D 724-99. Standard test method for surface wettability of paper (Angle-of-contact method), and
●: Other standards.

9.2.1 Surface preparation grades according to PN-EN ISO 8501-1

Standards PN-EN ISO 8501-1 and PN-EN ISO 8501-2 describe the surface preparation grades in the visual assessment of rust grades and grades of steel substrate preparation for painting. The International Standard PN-EN ISO 8501-1 concerns uncoated surfaces, whereas PN-EN ISO 8501-2 previously painted steel substrates whose previous coatings have been locally removed. The methods and grades of surface preparation may well apply to bonding of substrates which are assembled for the first time, or rejoined (and therefore obtained after the disassembly), in case of which residual adhesives are found on the surface. The information on PN-EN ISO 8501-1 is presented below.

The said standard determines four rust grades of the surface:

●: A—steel surface covered with strongly adhering scale on almost 100% of surface, and little if any rust,
●: B—steel surface, which has begun to rust and from which the mill scale has begun to scale,
●: C—steel surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision, and
●: D—steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.

The surface is assessed visually in scattered light, eyesight corrected to normal vision, by comparing it to a specific photographic reference given in the standards.

The PN-EN ISO 8501-1 standard gives the degrees of surface preparation using the following methods (described in Chapters 4, 5, and 8 4 5 8 ):

●: abrasive blasting (the standard does not distinguish between dry and wet abrasive treatment methods),
●: cleaning methods with hand and power tools, and
●: flame treatment method.

Tables 9.1–9.3 show the surface preparation grades along with their description.

Table 9.1. Degrees of surface preparation according to PN-EN ISO 8501-1: abrasive blast cleaning

Grade	Description
Sa 1—Light blast-cleaning	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from poorly adhering mill scale, rust, paint coatings, and foreign matter Grades: B Sa 1, C Sa 1, D Sa 1
Sa 2—Thorough blast-cleaning	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from most of the mill scale, rust, paint coatings, and foreign matter. Any residual contamination should be firmly adhering Grades: B Sa 2, C Sa 2, D Sa 2
Sa 2½—Very thorough blast-cleaning	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from mill scale, rust, paint coatings, and foreign matter. Any remaining traces of contamination show only slight stains in the form of spots or stripes Grades: A Sa 2½, B Sa 2½, C Sa 2½, D Sa 2½
Sa 3—Blast cleaning to visually clean steel	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from mill scale, rust, paint coatings, and foreign matter. The surface should have a uniform metallic colour Grades: A Sa 3, B Sa 3, C Sa 3, D Sa 3

Source: Own work based on PN-EN ISO 8501-1.

Table 9.2. Surface preparation grades according to PN-EN ISO 8501-1: thorough hand and power tool cleaning

Grade	Description
St 1	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from poorly adhering mill scale, rust, paint coatings, and foreign matter Grades: B St 2, C St 2, D St 2
St 2	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from poorly adhering mill scale, rust, paint coatings, and foreign matter. The surface is cleaned better than for St 1 Grades: B St 2, C St 2, D St 2
St 3	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from poorly adhering mill scale, rust, paint coatings, and foreign matter. The surface is cleaner than in the case of St 2, metallic surface is locally visible Grades: B St 3, C St 3, D St 3

Source: Own work based on PN-EN ISO 8501-1.

Table 9.3. Surface preparation grades according to PN-EN ISO 8501-1: flame cleaning

Grade	Description
FI	When viewed without magnification, the surface is free from visible oil, grease, and dirt, and from poorly adhering mill scale, rust, paint coatings, and foreign matter. Any residual rust should be visible in the form of surface discolouring (shades) Grades: A FI, B FI, C FI, D FI

Source: Own work based on PN-EN ISO 8501-1.

The International Standard PN-EN ISO 8501-1 is supplemented with photographs of the initial surface condition and its condition following particular stages of surface preparation.

9.2.2 Surface preparation grades according to PN-EN ISO 8501-4

The International Standard PN-EN ISO 8501-4 details surface preparation grades in the case of high-pressure water jetting treatment. Currently, there exists no European Standard for surfaces preparation with wet abrasive methods. Should the need arise, the standards prepared by International 'Slurry blasting Standards' can be used instead [59]. High-pressure water jetting applied to steel substrates typically leads to the emergence of flash rust on the surface of cleaned elements, in which case the following are assessed:

●: initial surface condition,
●: surface appearance after cleaning, and
●: appearance of the surface with flash rust.

Surface preparation grades given by PN-EN ISO 8501-4 are presented in Table 9.4.

Table 9.4. Surface appearance after cleaning according to PN-EN ISO 8501-4

Grade	Description
Wa 1—Light high-pressure water jetting	When viewed without magnification, the surface is free from visible oil and grease, loose paint or rust, and other foreign matter
Wa 2—Thorough high-pressure water jetting	When viewed without magnification, the surface is from visible oil, grease and dirt and most of the rust, previous paint coatings and other foreign matter. Any residual contamination is randomly dispersed and can consist of firmly adherent coatings, firmly adherent foreign matter and stains of previously existent rust
Wa 2½—Very thorough high-pressure water jetting	When viewed without magnification, the surface is free from all visible rust, oil, grease, dirt, previous paint coatings and, except for slight traces, all other foreign matter. Discolouration of the surface can be present. The gray or brown/black discolouration observed on pitted and corroded steel cannot be removed by further water jetting

Source: Own work based on PN-EN ISO 8501-4.

Table 9.5 shows rust grades after surface preparation by means of high-pressure water jetting.

Table 9.5. Flash rust grades according to PN-EN ISO 8501-4

Grade	Description
L—Light flash rust	A surface which, when viewed without magnification, exhibits small quantities of a yellow/brown rust layer through which the steel substrate can be seen. The rust can be evenly distributed or present in patches, but it will be tightly adherent. It is not easily removed by gentle wiping with a cloth
M—Medium flash rust	A surface which, when viewed without magnification, exhibits a layer of yellow/brown rust that obscures the original steel surface. The rust layer can be evenly distributed or present in patches, but it will be reasonably well adherent. It will lightly mark a cloth that is gently wiped over the surface
H—Heavy flash rust	A surface which, when viewed without magnification, exhibits a layer of red-yellow/brown rust that obscures the original steel surface and is loosely adherent. The rust layer can be evenly distributed or present in patches and it will readily mark a cloth that is gently wiped over the surface

Source: Own work based on PN-EN ISO 8501-4.

The following initial surface conditions are distinguished:

●: DC A—a surface where the paint coating system has degraded to an extent similar to that illustrated by ISO 4628-3, grade Ri3,
●: DC B—a surface where the paint coating system has degraded to an extent similar to that illustrated by ISO 4628-3, grade Ri4,
●: DC C—a surface which has degraded to a major extent, as illustrated by ISO 4628-3, grade Ri5, or when completely degraded as illustrated by ISO 8501-1, rust grade C,
●: DP I—an iron oxide epoxy prefabrication (shop) primer surface that has degraded, and
●: DP Z—a zinc silicate prefabrication (shop) primer surface that has degraded.

9.2.3 Determination of water-soluble salts on cleaned surfaces, according to PN-EN ISO 8502-2

Ionic contaminants on the surface of substrates prepared for adhesive joining and bonding processes (paint or adhesive coating) may cause subcoating corrosion, blistering and joint delamination. When ionic residues are found on the surface following proper treatment methods, they need to be subjected to assessment. PN-EN ISO 8502-2 standard provides a description of several methods for removing ionic contaminants from the surface for quantitative analysis. Methods that can be used in laboratory and field conditions are:

●: the swabbing method,
●: the Bresle method, and
●: the conductometric method.

The most frequently used method for the determination of the total amount of all water-soluble ionic contaminants on the surface is the conductometric method. It consists of measuring the conductivity of the solution obtained from the surface by means of a conductometer.

9.2.4 Assessment of dust on steel surfaces according to PN-EN ISO 8502-3

The assessment of the level of dust contaminants on the surface is carried out in accordance with the PN-EN ISO 8502-3 standard, which consists of:

●: applying the 150 mm-long adhesive tape firmly onto the surface of a substrate by repeatedly pressing it with one's finger or a roller,
●: assessing the quantity of dust on the tape by referencing them with the pictures given in the standards. The assessment should be made against a contrasting background for better visibility of removed contaminants.

Concentration density standards (included in the PN-EN ISO 8502-3 standard) are shown in Fig. 9.3, and the classification of dust particle size is given in Table 9.6.

Table 9.6. Classification of dust size classes according to PN-EN ISO 8502-3

Class	Dust size
0	Particles not visible under × 10 magnification
1	Particles hardy visible under × 10 magnification (usually less than 50 μm in diameter)
2	Particles visible with normal vision (particle diameter between 50 and 100 μm)
3	Particles clearly visible with normal vision (up to 0.5 mm in diameter)
4	Particles between 0.5 and 2.5 mm in diameter

Source: Own work based on PN-EN ISO 8502-3.

9.2.5 Assessment of oil contamination on surfaces (according to ASTM F22 standard)

The term oil contamination embraces a number of compounds of various chemical structure, which is why it is impossible to simply label them based on their chemical structure. The previously referenced group of standards concerning surface contamination, PN-EN ISO 8502, fails to address the assessment of oil contamination. The only common feature of oil contaminants is their hydrophobic character, which is manifested by the fact that water on greasy surfaces occurs in drops. This property is the basis of the ASTM F22 standard, according to which oil contaminants are determined on vertical surfaces by spraying the surface with water and assessing after 10 s the behaviour of water: whether it forms drops or a continuous water film, as well as whether:

●: water covers 50% of the surface,
●: water covers 20%–45% of the surface, and
●: water covers up to 15% of the surface.

The visual test procedure constitutes either a preliminary assessment of the cleanliness of the treated surface or is implemented prior to the selection of a particular surface treatment method. With less accuracy, this method can also be used on horizontal surfaces.

In addition, the subsequent sections of this work shall present several methods, which may also be applied in workshop conditions that are used when assessing the surface preparation for adhesive joining and other adhesive processes (including the aforementioned paint coatings and working with steel substrates). The selection of particular methods depends on various factors, including interalia size and shape of the surface, or workshop conditions.

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