RESEARCH INTO THE CAUSAL EFFECTS AND DEVELOPMENT OF SOLUTIONS TO

PINHOLING OF POWDER COATED GALVANIZED STEEL

Alexander F. SPEAKMAN, Colin CHISHOLM, Mahmoud EL-SHARIF, Ray ANSELL

Glasgow Caledonian University, Glasgow, G4 0BA, Scotland, aspeak200@caledonian.ac.uk

Abstract

Powder Coating of galvanized steel substrates is used to add corrosion protection

and aesthetic quality to a manufactured product. The powder coating process is

widely utilised across a wide range of industries such as construction, automobile

and domestic appliance manufacture. The technique dates back to the 1950s and

was first established commercially in the 1970s, the current global market now

having a value of approximately $5.8bn per annum. Globally a range of surface

defects on the powder coated galvanized steel have been reported, often described

as pinholing and outgassing, which compromise corrosion protection as well as

aesthetic quality and are thus unacceptable in a finished product[1,2].

While many theories and mechanisms have been discussed and reported in articles

and journals it is clear that to date a real understanding of the mechanisms has still to

be established through systematic scientific investigation. The metallurgy of zinc

coated galvanized steel is well established and understood, however it is obvious

that the behaviour of the composite of galvanized zinc coated steel with the polymer

based coating needs further scientific research to understand the mechanisms

leading to unacceptable surface defects and how to conduct processing to avoid

such defects. The following work on pinholing of powder coated galvanized steel

provides a critical literature review combined with results from internal research

carried out in collaboration with Highland Colour Coaters Ltd who are an established

galvanizing and powder coating company with its facility in Scotland providing both

services. The multivariables discussed within this paper and previous work include

but are not limited to, steel thickness, application of primer, surface cleanliness of the

original substrate and the presence of white rust.

Keywords: Powder Coating, Galvanizing, Pinholing.

1. Introduction

The hot dip galvanizing (HDG) process is carried out on many steel components

such as sculptures, railings, children’s play parks, structural beams and automotive

frames, creating corrosion protection both in the form of barrier protection and

cathodic protection. HDG has been used as a process for over 150 years and is still

an important process for corrosion protection of steel. Powder coating is added to

galvanized steel for two main purposes; to improve aesthetic characteristics and to

provide greater corrosion protection through the addition of a second barrier coating,

which reduces the rate at which the zinc galvanized coating is consumed[3].

Pinholing and outgassing type surface defects have been reported on powder

coating since the introduction of powder coatings in the early 1970s, with these

surface defects being particularly prominent when the substrate is HDG steel[2].

These surface defects can result in a reduction in aesthetic quality and a reduction in

the corrosion performance of the duplex coating, and are clearly undesirable in the

final product, weakening performance in the working environments and causing

increased costs in the manufacturing process. Remedial work carried out on jobs

which display such surface defects can involve one or more of several options;

sanding down and applying a second powder coating to the galvanized steel,

stripping the component back to the original steel before carrying out the galvanizing

and powder coating again, or sanding down affected areas and using touch up liquid

spray. Touch up sprays are colour matched enamel paints in aerosol form which can

be used for repairing minor flaws in, or damage to powder coating. Using touch up

spray is not ideal however since the weathering performance of the spray is likely to

have a degree of dissimilarity to that of the powder coating. Applying a second

powder coating adds greater expense to the process, significantly affecting profit

margin associated with the job being manufactured. Even greater expense is

involved when stripping the component back to the original steel before recoating. In

addition to the cost implications of all three of these options, the three process

deviations all have a negative impact on the production flow within an industrial

process, significantly increasing the production time for the affected jobs, and also

having a negative production rate impact on jobs being processed in the facility at the

time that surface defects occur. In a large proportion of jobs at Highland Colour

Coaters Ltd (HCCL), where the industrial aspects of this study is based, a rapid

turnaround is expected by the customer. Thus the occurrence of outgassing and

pinholing defects can have a significant impact on profit margin for the coating

process and also on customer satisfaction with regards to meeting agreed deadlines

for the completion of work.

The authors examine and discuss previous work carried out with regards to

outgassing and pinhole defects and also report on trials and case studies researched

during the course of the project carried out at HCCL.

2. Previous Studies

There have been a few specific studies sourced into pinholing of powder coating on

metal substrates, some focussing on HDG as a substrate. Additional papers which

examine other aspects of powder coating also discuss surface defects, although not

to the same level of detail.

The earliest of these studies was a collaboration between Britannia Zinc and

Birmingham Powder Coaters[4]. This study by Haines & Bromley examined a number

of variables; varying substrate thickness, substrate type, nature of pretreatment types

and powder supplier. It was reported that trapped air in the powder was the primary

source of pinholing and retained water from pretreatment was a secondary source.

One of the main experimental findings reported was that steel of greater thickness

displayed more surface defects. Powder thickness was also reported as having an

impact where it was found that a threshold thickness correlated to the generation of

surface defects. When the powder level was above this threshold, surface defects

were not seen due to gas which could generate potential surface defects being

trapped within the powder coating. Another contributing factor discussed was

differing pretreatment technologies. Samples which had undergone phosphate and

chrome conversion coatings were compared to pieces which did not utilise any

conversion coating. The best coatings observed from this study were from samples

which had used chrome based conversion coatings.

The experimentation carried out within the Haines & Bromley study involved a

number of significant variables. The examination of metal type and gauge appeared

to be informative. There may be issues with the reported results as metals of all

thicknesses were given the same drying time in the experimentation. Thicker steel

pieces might be expected to need longer drying time than the thinner pieces thus

creating a set of conditions where the heavier pieces may be expected to display

surface defects due to having a relatively shorter drying time. The examination of

pretreatment types was less informative, with potentially too many variables being

examined at one time, and possibly too many conclusions being reached with

insufficient information. This work does however illustrate that varying of

pretreatments can have an impact on pinholing. Haines and Bromley’s research has

elements which have been well defined. It is worth noting that powder formulation

and pretreatment formulation has evolved since this study was carried out in 1992,

partly due to the European legislative drive to eliminate the use of TGIC in powders

and chromates in the pretreatment process. The use of TGIC as a crosslinking

agent is being reduced, particularly in European markets, due to being classified as a

category 2 mutagen. Hexavalent chromate is classified as carcinogenic and is

affected in Europe by REACH legislation which aims to reduce its use within industry.

A later study was carried out by the Norwegian Research Organisation SINTEF[2].

The authors Bjordal, et al draw different conclusions from the Haines & Bromley

study, suggesting that water is a likely source of pinholes as opposed to air due to the

significantly greater expansion experienced by water on heating to 180°C, the curing

temperature of the powders involved. Experimentation carried out included the

examination of moisture uptake by steels with changing humidity. These results

suggested that galvanized steel should be kept in dry environments prior to powder

coating. They also discussed the presence of white rust as a cause of pinholes.

White rust is a zinc corrosion product which can form on galvanized steel if it is

stored in humid areas with low oxygen levels. Bjordal et al recommend not exposing

galvanized steel to humid conditions prior to powder coating. If white rust is present

they suggest mechanically removing this as opposed to using acid due to the

negative impact that excessive acid can have on the zinc coating. The third major

aspect reported in this work discussed the nature of the steel substrate as being

important regarding the susceptibility of coatings to pinhole, with silicon steel in the

Sandelin range (0.04-0.12%) being the most susceptible to pinholes, followed by

steels of a higher silicon content. Low silicon steels which are more likely to have a

pure zinc surface are considered in this study to be less likely to have surface defects

on the powder coating. Work carried out by Tang[5] examines the silicon content

effect in greater detail and is discussed later within this paper. The work of Bjordal et

al also reports that a preheating cycle is beneficial for the reduction of surface

defects. It was further reported that the thickness of powder coating was not

significant regarding the development of pinholes on the surface of the powder

coating. Both findings contradict the results reported in the previously discussed

study. The discussion of a number of variables within this research and how these

variables interact is of great interest with regards to pinholing and outgassing and

helped to inform the authors’ studies and experimentation.

Work carried out by Pietschmann for the Research Institute for Precious Metals and

Metal Chemistry[6] examined various failure modes in powder coating on metal

substrates. Figure 1 shows outgassing on powder coating on a zinc coated substrate

taken from this work, figure 1(a) being a surface image and figure 1(b) being a cross

sectional image of the different layers within the coating. This defect was attributed

to air coming from the porous zinc layer. Other failure modes contributing to

pinholing and outgassing discussed within the work include surface contaminations

such as waxes, oils and corrosion. Moisture within the process is described as a

contributing factor, with the moisture from the work pieces or from the powder.

Figure 1 (a) Figure 1 (b)[3]

Whilst these studies are the only studies sourced which specifically looked at

outgassing and pinholing, there are other studies reported which examined aspects

which had some relevance to the surface defects on powder coating. A study into the

back corona effect and relative humidity[7] examined one particular effect which can

cause surface defects such as pinholing. The back corona effect occurs when the

electrostatic charge which is used to deposit the powder does not decay at a

sufficient rate. In severe scenarios, a polarity reversing of the powder charging can

occur, creating powder fusion which can result in pinholing and craters. It is noted

within this research that the presence of humidity within the powder can have a

reducing effect on the level of defect. A study into the effect of the anti-gassing

agent Benzoin[8] examined the generation of bubbles within the powder coating

during curing and how these bubbles disperse during the process. The authors

advanced the theory that the bubbles created do not rise through the coating, that

they reduce in size as the air in them permeate the coating and that pinholes are

caused by these bubbles collapsing as the coating cools.

An issue discussed within the SINTEF study[2] as well as by many industry specialists

is the effect that the silicon content in the steel can have on the nature of the

galvanized coating and on the subsequent development of surface defects. Whilst

not examining powder coating, a study into controlling the effect of silicon in

galvanizing[5] by Tang illustrates how significant an impact small changes in the

silicon content of the steel can have on the nature of the galvanized coating. With

low silicon steel (<0.04% Si), a standard galvanizing coating of 4 distinct zinc/iron

alloys are observed. When the steel contains a silicon content within the Sandelin

range (0.04-0.12%Si), these 4 alloys start to break down due to the silicon being

insoluble within one of the alloys. The impact of this is even more pronounced with

greater silicon content levels (>0.20%), where the galvanized coating produced is

abnormally thick and non-uniform. These coatings can also be brittle with poor

Coating

Pure Zn

Zn-Alloy layer

Defect

adhesion. The nature of the steel can have significant impact on the microstructure

of the galvanized coating and this can lead to galvanized substrates of a differing

nature being powder coated and this in turn can lead to the development of surface

defects due to the differing nature and behaviour of the galvanized coating during

treatments prior to and during the subsequent powder coating process.

3. Industry Specialists

Whilst there have been relatively few systematic studies into out-gassing and

pinholing there is a significant industrial knowledge base which has been developed

since the early 1990s. An article directly addressing outgassing from an industry

consultant[9] attributes pinholes and out-gassing to a number of factors, primarily, the

casting of the metal, gasses trapped within galvanized layers, surface contamination

and coating thickness. In addition to control of these factors, other

recommendations for surface defect prevention are preheating of parts being

produced to a temperature greater than the cure temperature, sealing of the surface

using a primer, using infrared as opposed to thermal curing and altering the

formulation of the powder itself. The same author in an later article[10] suggests

minimising the curing temperature of the powder.

The American Galvanizers Association examined outgassing[11], attributing pinholing

in the powder coating process to zinc and metal oxides present on the substrate prior

to powder coating. The report suggests that these oxides may potentially retain air or

moisture and that during the curing process this can be released as water vapour or

air and cause outgassing type blisters in the final coating. The report also discusses

water and air being trapped within porous regions and fracture areas in the

galvanized zinc coating causing similar issues during curing. To prevent these

surface defects the report suggests sweep blasting or chemical cleaning of the

surfaces to remove the zinc and metal oxides. The report also suggests using a

drying process to preheat the material to above the curing temperature and also

minimising the curing temperature itself can reduce the impact of trapped moisture or

air on the curing process.

Powder suppliers have also reported views on the origins of surface defects. Neat

Koat[12] discussed a number of contaminants which could have an influence, inclusive

of oils, rust, silicone and airborne contamination. Other issues discussed include the

effects of excessively thick powder and porous work pieces on defect development.

Suggested solutions to these causes included improving the pretreatment quality,

increasing the drying time, minimising powder coat thickness and checking the

substrates for porosity. Interpon[13] discuss similar contaminants to those reported

by Neat Koat and placed great importance on locating the sources of contamination.

4. Project Observations

4.1 Overview

It is apparent from the literature discussing outgassing and pinholing that there is no

one single cause which leads to outgassing and pinholing in powder coated products.

The project being reported has been conducted onsite at HCCL for over two years,

during which time 244 different defect occurrences have been identified. The

investigation into these surface defects allowed for examination of potential common

causality. Here we report and examine some of the key areas that have been

investigated and discuss some of the adjustments that have been carried out to

successfully reduce the occurrence of these surface defects within the overall

manufacturing process for powder coated galvanized steel products. Whilst single

variables have been examined individually, the authors believe that it is highly

probable that combinations of the variables investigated contribute to the

development of surface defects.

4.2 Contamination on Steel Prior to Galvanizing

The nature and surface condition of incoming steel at the HCCL Galvanizing plant

can be highly variable. The type, the geometry and the surface condition of the steel

can significantly vary across a range of jobs. It was observed during the early stages

of the project that there was a distinct propensity for pinholing on the surface of the

steel where it had been previously labelled with a steel marker. Figure 2(a) is an

example of a job displaying this defect, where the lettering which had been previously

stamped on the bare steel could be observed in the form of pinholes on the surface

of the powder coating. Figure 2(b) shows the results of a trial carried out using a

clay silicate based steel marker pen on triplicate samples where the pen was used on

bare steel prior to galvanizing. After powder coating there were significant pinholes

observed on the surface following the exact shape of the original marking. Figure

2(c) shows where this marker was applied to pieces that were already galvanized,

after powder coating there were noticeably fewer and smaller pinholes than that of

the samples where the marker had been applied to the bare steel. The presence of

this marker contamination provides a volatile contamination which can have a

negative impact during the curing of the powder. During the galvanizing process the

presence of the marker may also physically disrupt the formation of the zinc-iron

alloys, creating porous points on the galvanized coating which can be susceptible to

retaining moisture from the powder coating pretreatment immersion process which

could potentially volatilise during the powder curing process.

Figure 2 (a) Figure 1 (b) Figure 2 (c)

The presence of this marker contamination which can impact the final coating surface

is conclusive evidence of the sensitivity of the steel to surface contamination which

can lead to pinholing in the final powder coated product. The fact that this type of

contamination is not removed by the standard degreasing/ acid pickling prior to

galvanizing, suggests that many other contaminants may have a similar impact.

Experimental studies during the project with steel which was heavily contaminated

with an oil based substance prior to galvanizing appeared to produce a satisfactorily

galvanized coating. However when powder coated they showed a tendency to

outgas. Subsequent production runs of this same material underwent an exhaustive

in depth cleaning regime, and this resulted in powder coatings with no surface

pinholing defects.

The importance of cleaning the substrate prior to powder coating is documented in

industrial reports. Observations from experimental trials and case studies over the

course of the project suggest that the cleaning of the steel substrate prior to

galvanizing is highly significant in terms of minimising the subsequent development

of surface pinholing during the later stages of powder coating.

4.3 Primer

The majority of polymers used by HCCL over the course of the project are polyester

based. On some occasions an epoxy primer can be used as well as the polyester

coating. Epoxy powder coats provide a superior barrier coating, improving the

corrosion protection performance of the system in comparison to a single polyester

topcoat. However epoxy primers also require an additional polyester top coat due to

an epoxy powder coat being susceptible to UV degradation. It was observed during

the course of the project that a majority of coatings using epoxy primers and

polyester topcoats on top of galvanized steel displayed a significant degree of

outgassing as can be seen from figure 3(a). In collaboration with the powder

suppliers it was proposed to extend the primer curing time from a part cure to a full

cure, effectively doubling the time that epoxy primer was at curing temperature. This

resulted in a much better aesthetic coating with effective elimination of the pinholing

defect which had been previously prevalent during work of this nature. This

improvement of finish has been observed both through work being produced and

trials comparing the curing time of the epoxy primer. Figure 3(b) illustrates a trial

piece which has undergone a part cure while figure 3(c) shows a piece from the

same trial where the primer was given a full cure. It was observed that part curing

primer on bare steel does not provide the same aesthetic issues as that of part curing

primer on galvanized steel, and this may suggest that the impact of volatile

compounds retained through the galvanized coating are exaggerated if retained

within a part cured epoxy coating.

Figure 3 (a) Figure 3 (b) Figure 3 (c)

4.4 Acid Etch

The acid etch is used in the powder coating pretreatment process to etch the zinc

substrate prior to the addition of the conversion coating and also to remove any

oxides present. The standard time for acid etch at HCCL during the initial stage of

the project was 3 minutes. It was observed during the course of the project that after

routinely introducing fresh acid to the system for the etch there was a step change

increase in the number of surface defects being observed on powder coated work.

The time for the acid etch was reduced to 45s for routine work, with heavily white

rusted pieces being given a longer etch. The step changes that had been previously

associated with refreshing the acid are now no longer observed. The impact of

reducing the acid etch time is perceived to have had a positive impact on the

reduction of the surface defects.

4.5 Water Retention from Hollow Sections

A high percentage of work at HCCL is constructed using hollow sections of steel,

such as handrails and frames. It has been observed during the course of the project

that where handrails have a tendency to internally retain water through the

pretreatment immersion process that outgassing is more likely to be observed. This

retention of water can be caused by insufficient or blocked drainage holes on pieces,

or pieces that have been hung in such a manner that does not facilitate complete

drainage. Whilst the pieces then undergo a drying process, this standard process is

designed for removing surface water and has been shown to be inadequate for

significant amounts of pooled water. This pooled water can cause a number of

issues for the coating of the external surface. It will prevent the substrate from

reaching temperature, both in the drying stage and the curing stage. Further to this,

the pieces themselves will be retaining water which is likely to evaporate and interact

with the powder curing process. The presence of water is likely to cause significant

coating problems, both with regards to the increased presence of the pinholing defect

and also hinders the ability of the polymer coating to cure completely. Figure 4(a)

illustrates a frame from a greenhouse which had not been hung to maximise

drainage. The pieces demonstrated a high degree of outgassing after powder

coating as seen in figure 4(b). The pieces retaining water into the powder coating

curing process is extremely likely to have contributed significantly to this defect.

Ensuring that pieces being coated have effective drainage holes and that they are

hung to maximise drainage minimises the potential for this water retention and the

occurrence of defect caused by this.

Figure 4 (a) Figure 4 (b)

4.6 Process Controls

Whilst not a technical amendment to the powder coating process the implementation

of more precise quality assurance documentation has had a positive impact on the

containment of surface defect development. The frequency of process deviation has

been significantly reduced, typical deviations observed during the project are jobs

having extended waiting times at intermediate process steps during manufacture due

to either a weekend break or a change in job priorities. Production deviations such

as described were observed during studies of the overall processing of the powder

coated galvanized steel products and the frequency of such deviations has now been

reduced due to the introduction of more prescriptive quality assurance documentation

which also facilitates a better understanding of potential issues and their correction.

Early detection of process deviations due to increased documentation has facilitated

a more thorough investigation of defect occurrence. The improved documentation on

the addition of anti-gassing additives has reduced the occurrence of outgassing

defects, the additive not being used when required previously having resulted in

surface defects. Greater ownership of the improved process documentation for the

overall production process by production staff has also supported improvements in

minimisation of surface defects on the final products.

4.7 Fettling

Fettling is carried out using mechanical sanding and grinding apparatus to remove

any unevenness, sharp edges or surface roughness after the galvanizing process

and prior to the powder coating pretreatment process. Figure 5 shows an example of

a rotary grinder being used to fettle a panel. The function of fettling is an essential

process step to give a smooth galvanized surface prior to powder coating. This

results in a smoother finish quality to the final powder coated product and facilitates

safer handling, which can be critical if the piece is to be handled after installation,

such as handrails or a children’s play park installation. Any deviation in the fettling

process can lead to excess removal of the top surface layer of zinc and can cause

disruption of the zinc-iron compound layers below the outer zinc layer leading to an

overall reduction in the corrosion protection offered by the galvanized coating. From

metallographic studies conducted during the plant trials the authors believe that the

damage caused by deviations in the fettling process leads to fractures and

delamination within the zinc-iron compound layers which could create porous sites

within the galvanized layers leading to the retention of moisture and other chemicals

associated with the post galvanizing manufacture which could react during powder

curing thus producing gases which lead to pinholing. From study trials conducted

within the plant it was concluded that the fettling process step needed to be carefully

controlled to give a smooth galvanized surface for powder coating but equally fettling

needs to be minimised to avoid damage to the zinc-iron compound layers which in

turn could lead to reduced corrosion resistance and more importantly to rejection of

the final powder coated product due to unacceptable pinholing defects on the surface

of the polymer. Controlled fettling has now been achieved through the

implementation of better process control and the introduction of quality assured

documentation. Whilst fettling is necessary in preparing the galvanized surface, it

should be minimised for both corrosion performance and for reducing points of

weakness within the galvanized coating which could be sites of origin for surface

defects.

Figure 5

4.8 High Silicon Steels

The impact that an increasing silicon content within steel has on the formation of the

zinc iron alloys is well documented[5,14]. When the silicon content of the steel is

below the Sandelin range (<0.04% silicon), conventional zinc-iron galvanized alloys

occur. Within the Sandelin range (0.04-0.12%) these alloys do not form as uniformly

due to solubility issues and solid state diffusion between the silicon and the zinc-iron

alloy layers. This is further pronounced at higher silicon levels (>0.20%). When

steels with higher silicon contents are galvanized the result is a thicker non-uniform

galvanized coating. The addition of low levels of nickel (≈0.05%) to the galvanizing

bath reduces this effect when silicon levels are less than 0.20% but not at higher

silicon levels[5]. It seems likely that powder coating on high silicon steels is always

likely to be more challenging than on low silicon steels. Issues previously discussed

in this paper such as the impact of acid etch and the impact of fettling on the

occurrence of defects are likely to be more pronounced on high silicon steels due to

the brittle nature, thicker coating and reduced adhesion qualities of the galvanized

coating.

5. Discussion and Conclusions

The authors were able to confirm that the phenomena of pinholing and outgassing

are clearly caused by a number of process variables, either singly or in combination

regarding the formation of powder coated galvanized products and these results are

supported by similar results reported in the published literature. Variations in the

condition of the galvanized substrate have been shown to contribute to the

development of outgassing and subsequent pinholing defects and a number of other

process factors can influence the severity of defect development. Pretreatment

regimes, use of primers, handling of the material and the nature of the material can

all have a significant impact on the overall surface finish of the powder coating.

Cleanliness within the process is a necessity for a good coating system, and of

particular interest within this project was that the surface cleanliness prior to

galvanizing was as significant as cleanliness post galvanizing with regards to the

influence on pinholing and outgassing defects. Porosity within the galvanized coating

can be caused by a number of these factors, with this porosity leading to an increase

in the amount of volatile material carried into powder coating process from the

immersion process.

Over the course over a 30 month project the addressing of these issues as well as an

increase in process controls has seen a greater than 75% defect reduction within the

powder coating process at HCCL.

The main conclusions are as follows:

Contamination of the steel prior to galvanizing has a significant impact on

the zinc coating. Issues with this zinc coating can lead to outgassing and

pinholing on the final powder coating, due to the increased likelihood of

porosity and volatile material being carried through the powder curing

process.

The studies showed that the surface profile of the pinholing on the cured

powder coating directly correlates with the matching profile of forms of thin

film contamination on the steel which is not removed in subsequent

treatments prior to powder coating.

The use of epoxy primers on a galvanized steel substrate can cause

outgassing defects on the polyester top coat. By extending the primer cure

from a part cure to a full cure has a significant impact on defect reduction.

The acid etch process prior to the application of the conversion coating

within the powder coating pretreatment process should be carefully

controlled. An extended acid etch can cause issues with the zinc layer of

the galvanized steel, potentially increasing surface defect occurrence.

Water retention from hollow pieces within the process should be minimised

as it has the potential to cause problems within the coating curing process.

Correct hanging of the pieces to allow for drainage and correct positioning

of drainage holes reduces the presence of water being retained after the

drying oven.

Fettling is important prior to powder coating to smooth the galvanized

surface and should be carried out with care to minimise damage to the zinc

alloy layers.

The nature of the steel substrate can have a significant impact on the

galvanized coating formed, with high silicon steels being more susceptible

to surface defects when powder coated.

The appropriate process controls are important to minimise deviations

within the process and also improve the ability to investigate any surface

defects which do occur.

Acknowledgements

This paper was based on project work carried out by a Knowledge Transfer

Partnership between Glasgow Caledonian University and Highland Colour

Coaters Ltd.

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