Index to NASA Tech Briefs, January - June 1966

Index to NASA technological innovations for January-June 196

Improving the Corrosion Performance of Organically Coated Steel Using a Sol–Gel Overcoat

Organically coated steels are widely used in applications in which they are subjected to the natural environment and therefore require excellent corrosion resistance. Organic clearcoats are typically employed as a barrier that improves the overall corrosion resistance; however, they are typically derived from fossil fuel-based feedstock. A more sustainable alternative could be possible using sol–gel coatings. The application of a simple tetraethoxysilane (TEOS)-based sol–gel was applied to polyurethane-coated steels using a spray coater. The concentration of TEOS was altered to produce coatings containing either 2.5% or 10%. The 10% TEOS resulted in dense, homogeneous coatings that offered a significant improvement in corrosion resistance compared to an uncoated substrate. Whereas the 2.5% TEOS coatings were inhomogeneous and porous, which indicated a limitation of concentration required to produce a uniform coating. The successful demonstration of using a simple TEOS-based coating to improve the corrosion resistance of organically coated steel highlights the potential for further investigation into the use of sol–gels for these applications

Development of sensors and non-destructive techniques to determine the performance of coatings in construction

The primary objective of this work was to examine and develop techniques for monitoring the degradation of Organically Coated Steel (OCS) in-situ. This included the detection of changes associated with the weathering to both the organic coating and metallic substrate. Initially, a review of current promising techniques was carried out however many were found to be unsuitable for this application and the adaptation of current techniques and the development of new techniques was considered. A brief concept investigation, based on initial testing and considerations, was used to determine a number of sensing techniques to examine. These included embedded, Resonant Frequency Identification (RFID), Magnetic Flux Leakage (MFL) and dielectric sensing. Each of these techniques were assessed for the application, prototyped, and tested against a range of samples to determine the accuracy and sensitivity of degradation detection provided. A range of poorly and highly durable coated samples were used in conjunction with accelerated weathering testing for this aim. Track based electronic printed sensors were presented as both a cut edge corrosion tracking and coating capacitance measurement method. While suffering somewhat from electrical paint compatibility issues both concepts showed merit in initial trials however the capacitive sensor ultimately proved insufficiently responsive to coating changes. The embedded, progressive failure-based, cut edge corrosion sensor was produced and tested in modern coating systems with moderate success. Novel applications of RFID and MLF techniques were considered and proved capable of detecting large changes in substrate condition due to significant corrosion. However, there was a lack of sufficient sensitivity when considering early-stage corrosion of durable modern OCS products. Finally, it was shown that a chipless antenna could be designed and optimised for novelly monitoring the changes to the dielectric properties of a paint layer due to degradation. However, ultimately this test, due to equipment requirements, lent itself more to lab testing than in-situ. Due to some of these limitations a different approach was considered in which the environmental factors influencing degradation were examined with the aim of relating these to performance across a building. It was observed that a combination of high humidity and the build-up of aggressive natural deposits contributed to high degradation rates in sheltered regions, such as building eaves, where microclimates were created. The build-up of deposits and their effect was presented as a key degradation accelerant during in-use service. A unique numerical simulation approach was developed to predict the natural washing, via rain impact and characteristics of the building analysed. This approach showed promise for determining areas unlikely to be naturally washed, and therefore subjected to a degradation accelerating, build-up of deposits. Given these understandings coated wetness sensors were considered as a realistic live-monitoring device capable of determining deposit build up and ultimately OCS lifetime

Atmospheric corrosion of painted galvanized and 55% Al-Zn steel sheets: results of 12 years of exposure

Zinc or 55%Al-Zn alloy-coated steel sheets, either bare or covered by different painting systems, have been exposed for 12 years to the action of the urban atmosphere at the CIDEPINT station located in La Plata (34° 50′ South, 57° 53′, West), province of Buenos Aires, Argentina. The samples exposed surface was evaluated through periodical visual inspections, standardized adhesion tests, and electrochemical impedance measurements. The ambient variables monitored were average annual rains and temperatures, time of wetness, sulphur and chloride concentration, relative humidity, and speed and direction of the winds. It was found that in this atmosphere, the corrosion resistance of the bare 55% Al-Zn/steel sheets was higher than of the galvanized steel, and the polyurethane painting system was more protective than the alkyd and epoxy ones, which degraded after 6-7 years of exposure.Facultad de Ingeniería (FI

Adherence measurements and corrosion resistance in primer/hot-dip galvanized steel systems

This paper focuses on the adherence during ageing of a primer (made of polyester resins crosslinked with melamine) applied onto hot-dip galvanized (HDG) steel for coil coating application and its influence on corrosion protection. A chromium-free surface treatment, composed of fluorotitanic acid, phosphoric acid, manganese phosphate, and vinylphenol was applied on the HDG steel to obtain high corrosion resistance and high adherence of a polyester and melamine primer. The influence of the manganese phosphate on the corrosion and adherence was investigated. To measure the adherence between the metal and the primer, a three-point flexure test was set up. The adherence was then linked with corrosion resistance during ageing, using electrochemical impedance spectroscopy

The development of surface treatments on galvanised steel products for the automotive industry utilising a chemcoater

This thesis investigates the development of coatings to be applied via a two-roll roller coating system, a Chemcoater. Novel coatings were investigated to enhance a substrate’s performance and add overall value to the hot-dipped galvanised steel product, by adding fuctionalities such as improved corrosion resistance, enhanced surface passivation and increased surface lubricity. Historically, hexavalent chromate was a common component of coatings for hot-dipped galvanised steel to provide corrosion protection during material storage and transportation. However, REACH law within the European Union prohibited its use due to its intrinsic toxicity and detrimental environmental effects. This legislation accelerated industry and researchers’ attention toward developing alternative, more environmentally friendly processes. Initial investigations included the analysis of current coatings, PrimeCoat and PLT. Both coatings were shown to have similar rheological characteristics and similar thermal stabilities with PLT performing well as a dry film lubricant. A common laboratory polishing unit was adapted to create a novel Pin-on-Disk tribometer, providing an effective and reliable method for measuring the dynamic coefficient of friction of coated substrates. The adapted unit was designed, built, and commissioned according to ASTM G99-17, and it was able to produce repeatable results for measuring the coefficient of friction. An additional investigation showed that sulphate-based treatments could reduce the coefficient of friction of zinc-coated steel substrates. Tests indicated that a neutral ammonium sulphate solution improved lubricity by 15-10%. Ammonium and sodium sulphate treatments were deemed suitable for coil coating, improving the capacity to reduce oil volume and overall production costs. A systematic study was conducted to improve the processability of a one-step hydrophobic coating. The best-performing coating had a contact angle of 146° with a reduced processing time of ~67%, using IPA as the solvent, 2% wt. 1 M NaOH (aq), 0.2 M stearic acid and micro-SiO2 (0.2–0.3 μm). This method significantly reduced processing time and fewer health and safety risks, avoiding the use of toxic chemicals. On further analysis, the coating was shown to be relatively formable and durable, with no significant reduction in hydrophobic performance after 30% strain was induced. Several corrosion analysis techniques showed increased performance in comparison to uncoated hot-dipped galvanised steel samples. These results suggested that the coating could be a viable option for some industrial applications and showed a potentially positive alternative to banned hexavalent chromate options. The chemical constituents of the newly proposed coatings have significantly less toxicity and pose a reduced negative effect on the wider environment and its users

Investigation into the role of primer, pre-treatments and coating microstructure in preventing cut edge corrosion of organically coated steels.

Investigations were carried out to assess the role of primer, pretreatments and coating microstructure in preventing cutedge corrosion of chrome free organically coated steels. Zinc runoff was monitored from a range of organically coated steels with a large cutedge length exposed over 18 months at Swansea University roof top site. The zinc in the runoff leaches from the zinc-aluminium alloy coating of the substrate. The paint systems' corrosion performance was assessed by monitoring the levels of zinc in the runoff. Consequently the levels of zinc reflected the effectiveness of the applied paint system against corrosion. Runoff was high in initial months with zinc levels reducing with time due to the build up of corrosion products that hindered the progress of corrosion. An accelerated laboratory test using a distilled water electrolyte was developed that predict long-term external weathering runoff from panels of a range of organically coated steels. The corrosion mechanisms of a variety of organically coated Galvalloy steel have been examined using the scanning vibrating electrode technique (SVET) in 0.1%NaCI. The corrosion behaviour of a coating is related to the zinc-aluminium alloy coating structure and combination of pretreatment and primer. The SVET has been used to assess total zinc loss and the corrosion rate for a comparative measure of organically coating system performance. A correlation has been developed from SVET 24hour experiment data to accelerated weathering data and external weather data that can aid more accurately predicting the in service life of the product. Also considered were the effects of electrolyte conductivity on the morphology of corrosion on pure zinc. A mathematical model has been developed to predict corrosion pit population. Altered microstructure of solidifying zinc aluminium alloy melt via ultrasonication was investigated. Ultrasound irradiation significantly altered the final microstructure. The influence of morphed microstructure upon the corrosion behaviour was explored using the SVET in 0.1%NaCI. The ultrasound manipulated microstructure had generally a positive effect on the corrosion behaviour

The corrosion of Zn-4.8%Al sacrificial coatings used for the protection of steel

This work set out to elucidate the microstructural corrosion mechanisms of sacrificial corrosion coating, Galvalloy® (Zn-4.8wt.%Al), which is used extensively in the strip steel industry. Corrosion of Galvalloy® occurs at the surface, where only Galvalloy® is exposed, and the cut edge, where Galvalloy® and steel are coupled, which increases the corrosion rate of Galvalloy®. Chapter 3 demonstrates the time-lapse microscopy (TLM) technique being used to analyse, in-situ, the microstructural mechanism of surface and cut-edge corrosion of Galvalloy® immersed in pH 7 1 wt.% NaCl. Rotating disk electrode (RDE) and potentiodynamic polarisation (PD) tests were performed on the individual phases of Galvalloy® to identify their anodic and cathodic activity in 1 wt.% NaCl. TLM showed that surface corrosion initiates and propagates through the binary eutectic Zn-Al phase, whereas cut-edge corrosion initiates within the primary zinc dendrite phase and proceeding through the entire microstructure. The electrochemical data validated this as the RDE showed that the Al containing phases could not support cathodic activity as well as the primary zinc phases and PD showed that the zinc phases are more susceptible to anodic dissolution when polarised. Chapter 4 investigated, using TLM and PD, the corrosion rate and mechanism of the Galvalloy® surface across pH 3, 7, 10 and 13 in 1 wt. % NaCl. At pH 3 and 13, D showed a maxima of corrosion rate was seen and TLM illustrated no precipitation of corrosion product. PD showed pH 7 having the lowest icorr, however, the precipitated corrosion product formed at a smaller radius relative to active anodes during TLM experiments of pH 10 compared to pH 7. Chapter 5 utilized ZRA and TLM to investigate the rate and mechanism of the corrosion of Galvalloy® next to a steel ‘cut-edge’ across pH 3, 7, 10 and 13 in 1 wt. % NaCl. The corrosion rate of Galvalloy® was greater compared to the surface corrosion, due to the polarisation imposed by the connection to the steel substrate and the same corrosion rate to pH trend in Chapter 4 was see. At pH 7, 10 and 13, corrosion initiation occurs in the zinc dendrites, whereas at pH 3 the corrosion is generalised. Chapter 6 investigated the effect of increasing steel to Galvalloy® on the corrosion rate of Galvalloy® at pH 7 in 1 wt.% NaCl using ZRA and TLM. ZRA demonstrated a linear trend, whereas TLM showed a non-linear trend which is suggested to be due to the increased ease of precipitation in the experimental set-up. Chapter 7 is an example of a real-world corrosion problem involving organically coated Galvalloy®. 2 µL of HCl, FeCl2, NaCl and Acetic acid (CH3OOH) were administered to a scribed region of PVB coated Galvalloy® and exposed to a high relative-humidity environment for a month to induce under-film corrosion in order to compare the results to deduce which salt was responsible for the real-world corrosion. NaCl was the salt that posed the greatest similarity and cathodic delamination is the postulated corrosion mechanism

Atlas 5013 tank corrosion test

The type and cause of corrosion in spot welded joints were determined by X-ray and chemical analysis. Fatigue and static tests showed the degree of degradation of mechanical properties. The corrosion inhibiting effectiveness of WD-40 compound and required renewal period by exposing typical joint specimens were examined