© ISO 2016 Zinc diffusion coatings on ferrous products — Sherardizing — Specification Revêtements par diffusion de zinc sur les produits ferreux — Shérardisation — Spécification INTERNATIONAL STANDARD[.]
INTERNATIONAL STANDARD ISO 17668 First edition 2016-02-01 Zinc diffusion coatings on ferrous products — Sherardizing — Specification Revêtements par diffusion de zinc sur les produits ferreux — Shérardisation — Spécification Reference number ISO 17668:2016(E) © ISO 2016 ISO 17668:2016(E) COPYRIGHT PROTECTED DOCUMENT © ISO 2016, Published in Switzerland All rights reserved Unless otherwise specified, no part o f this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission Permission can be requested from either ISO at the address below or ISO’s member body in the country o f the requester ISO copyright o ffice Ch de Blandonnet • CP 401 CH-1214 Vernier, Geneva, Switzerland Tel +41 22 749 01 11 Fax +41 22 749 09 47 copyright@iso.org www.iso.org ii © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Page Contents iv Introduction v Scope Normative references Terms and definitions General requirements Surface condition base material f Acceptance inspection and sampling Coating properties 6.1 Appearance 6.2 Thickness 6.2.1 6.2.2 Test methods 6.2.3 Reference areas Magnetic method or electro-magnetic method 6.2.5 6.2.6 Thickness requirements 6.3 Acceptance criteria Additional clearances for threaded components Ceritificate o f compliance Annex A (normative) Information to be supplied by the purchaser to the sherardizer Annex B (informative) Determination of thickness Annex C (informative) General information Annex D (informative) Corrosion resistance of sherardized layers Bibliography Foreword 4 I n o rmatio n to b e s up p lied by the p urchas er 4 General 4 6.2 Gravimetric metho d 6.4 © ISO 2016 – All rights reserved iii ISO 17668:2016(E) Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies) The work o f preparing International Standards is normally carried out through ISO technical committees Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters o f electrotechnical standardization The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part In particular the different approval criteria needed for the di fferent types o f ISO documents should be noted This document was dra fted in accordance with the editorial rules of the ISO/IEC Directives, Part (see www.iso.org/directives) Attention is drawn to the possibility that some o f the elements o f this document may be the subject o f patent rights ISO shall not be held responsible for identi fying any or all such patent rights Details o f any patent rights identified during the development o f the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents) Any trade name used in this document is in formation given for the convenience o f users and does not constitute an endorsement For an explanation on the meaning o f ISO specific terms and expressions related to formity assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers to Trade (TBT) see the following URL: Foreword - Supplementary in formation The committee responsible for this document is ISO/TC 107, Metallic and other inorganic coatings, Subcommittee SC 4, Hot dip coatings (galvanized, etc.) iv © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Introduction Sherardizing is a thermal diffusion coating process in which ferrous articles are heated in the presence of a sherardizing mixture consisting of zinc dust with or without an inert material The process is commonly per formed in closed, slowly rotating or fixed containers at temperatures ranging from around 300 °C to 500 °C The normal processing temperature is below the melting point o f zinc (419 °C) During the process, zinc reacts with the sur face to form inter-metallic layers on ferrous articles A coating thickness of 10 µm to 75 µm (and higher if required) can be achieved The coating thickness is accurately controlled by the amount o f zinc dust, processing time and temperature The coating closely follows the contours of the base material and uniform coating thicknesses are produced on articles, including those of irregular shape After sherardizing, the container load is cooled down A screening process separates the sherardized articles from the unused sherardizing mixture The articles, with the zinc-iron inter-metallic layers, are eventually post-treated (by phosphating, chromating or another suitable passivation process) resulting in a clean and passivated surface It is common to use articles coated with zinc-iron inter-metallic layers as a primer or base-coat for duplex-systems For additional information about the sherardizing process and the application possibilities of sherardized articles, see Reference [12] and Reference [13] Sherardizing (thermal diffusion coating) is also known as the following: — di ffusion zinc plating (Germany); — thermal di ffusion coating (Russia); — thermal di ffusion galvanizing (Ukraine); — vapour galvanizing (UK); — zinc di ffusion coating (USA); — zinc inter-metallic coating (Russia); — zinc thermo diffusion galvanizing (Israel) In China, Europe and the USA, the common name for the thermal diffusion coating process is sherardizing © ISO 2016 – All rights reserved v INTERNATIONAL STANDARD ISO 17668:2016(E) Zinc diffusion coatings on ferrous products — Sherardizing — Specification Scope T h i s I nternationa l Sta nda rd s p e c i fie s m i ni mum th ickne s s re qu i rements layers appl ie d to ferrou s corrosion and wear pro duc ts b y the s herard i z i ng pro ce s s for T h i s I nternationa l Stand ard e s no t s p e c i fy a ny re qui rements for s i x cla s s e s o f z i nc d i ffu s ion the pu rp o s e o f pro te c tion agai n s t for the s u r face cond ition (fi n i s h or roughness) of the basis material before sherardizing Post-treatments (conversion coatings), after-treatments or organic over-coatings (Duplex) of sherardized articles are not in the scope of this International Standard NOTE For general information about post-treatments, see Annex C and Annex D T h i s I nternationa l Standard e s no t apply to sherard i z e d pro duc ts (e g fas teners , tub e s) for wh ich s p e ci fic s tand ard s e xi s t and wh ich m ight i nclude add itiona l re qui rements or re qui rements wh ich are different from those of this International Standard NO TE I nd ividu a l p ro duc t s ta nda rd s c a n i nco r p o rate th i s I nter n atio n a l S ta nd a rd for the co ati n g b y quo ti ng its nu mb er, o r c a n i nco r p o rate it with mo d i fic atio n s p e c i fic to the p ro duc t Normative references T he fol lowi ng i nd i s p en s able c u ments , i n whole or i n p ar t, a re normatively re ference d i n th i s c u ment and are for its appl ic ation For date d re ference s , on ly the e d ition cite d appl ie s For u ndate d re ference s , the late s t e d ition o f the re ference d c u ment (i nclud i ng any amend ments) appl ie s ISO 14 , Metallic coatings — Hot dip galvanized coatings on ferrous materials — Gravimetric determination of the mass per unit area ISO 2064 , Metallic and other inorganic coatings — Definitions and conventions concerning the measurement of thickness ISO 2178, Non-magnetic coatings on magnetic substrates — Measurement of coating thickness — Magnetic method ISO 2808, Paints and varnishes — Determination of film thickness ISO Steel and steel products — Inspection documents 10 474, Terms and definitions For the pur p o s e s o f th i s c u ment, the term s a nd defi n ition s gi ven i n I S O and the fol lowi ng apply 3.1 sherardizing process zinc diffusion coating process in which articles are heated in close contact with a sherardizing mixture f (3.3 form sherardized layers (3.2) ) , com mon ly p er orme d i n a clo s e d s lowly ro tati ng conta i ner or a fi xe d (non-ro tati ng) conta i ner, to © ISO 2016 – All rights reserved ISO 7668: 01 6(E) 3.2 sherardized layer zinc diffusion layer z i nc d i ffu s ion co ati ng s i s ti ng o f z i nc-i ron a l loy layers ob ta i ne d b y s herard i z i ng N o te to entr y: T he “s hera rd i z e d l ayer ” i s re fer re d to i n th i s I nter n ation a l S ta nd a rd a s “co ati ng” N o te to entr y: T he s hera rd i z e d l ayer m ay s ub s e quentl y b e p o s t-tre ate d b y p ho s p h ati ng , ch rom ati n g or a no ther suitable passivation process (guidance for these post-treatments are given in Annex C and Annex D) 3.3 sherardizing mi xture m i x tu re s i s ti ng o f mai n ly z i nc du s t, with or without o ther pro ce s s- s upp or ti ng i ngre d ients N o te to entr y: Z i nc du s t i s a l s o known a s z i nc p owder mass of the zinc diffusion layer to ta l mas s o f z i nc/i ron a l loys p er u n it are a o f s u r face N o te to entr y: T he m a s s o f the z i nc d i ffu s ion l ayer i s e xp re s s e d i n gra m s p er s qu a re me tre ( g/m ) 3.5 thickness of the zinc diffusion layer to ta l z i nc d i ffu s ion layer th ickne s s s i s ti ng o f z i nc/i ron a l loys N o te to entr y: T he th ickne s s o f the z i nc d i ffu s ion l ayer i s e xp re s s e d i n m ic rome tre (µm) significant sur face part o f the article covered or to be covered by the es sential for s er viceability and/or zinc diffusion layer (3.2) and for which these coating is app earance and where the layer wi ll meet all the s peci fied requirements control sample article, or group of articles, from a lot which is selected for testing 3.8 reference area are a with i n wh ich a s p e c i fie d nu mb er o f s i ngle me a s u rements i s re qu i re d to b e made local thickness of the zinc diffusion layer me a n va lue o f z i nc d i ffu s ion layer th ickne s s ob tai ne d from the s p e c i fic nu mb er o f me a s u rements with i n a reference area (3.8) for a magnetic or electro-magnetic test or the single value of a gravimetric test N o te to e ntr y: Gu id a nc e 6.2.2 and Annex B fo r the me tho d s o f me a s u r e me n t o f the z i nc d i ffu s io n l aye r th ickne s s i s g i ve n i n 10 mean thickness of the zinc diffusion layer average va lue o f the lo c a l z i nc d i ffu s ion layer th ickne s s e s o f d i fferent N o te to entr y: I n c a s e there i s o n l y one reference area a s the lo c a l z i nc d i ffu s ion l ayer th ickne s s (3.8 reference areas (3.8) ) , the me a n z i nc d i ffu s ion l ayer th ickne s s i s the s a me 11 inspection lot one or more a r ticle s o f the s ame typ e a nd s i ze compri s i ng either a s i ngle order, a s i ngle del iver y lo ad or the nu mb er o f a r ticle s identi fie d a s a lo t b y the s hera rd i z er © ISO 2016 – All rights reserved ISO 7668: 01 6(E) acceptance inspection inspection of an inspection lot (3.11) at the sherardizer’s works (unless otherwise specified) General requirements 4.1 Surface condition base material The surface of the base material should be clean before sherardizing Sur face contamination that cannot be removed by grit blasting should be removed prior to the mechanical pre-treatment process The responsibility o f removing the contamination should be agreed upon between the sherardizer and purchaser The surface condition of the base material, the mass of the parts and the sherardizing conditions can a ffect the appearance, the thickness, sur face roughness and the physical and mechanical properties o f the coating This International Standard does not define any requirements regarding these properties NOTE 4.2 Guidance on these parameters can be found in ISO 14713-3:2009, Clause Information to be supplied by the purchaser In formation shall be supplied by the purchaser in accordance with A.1 and A.2 Acceptance inspection and sampling Acceptance inspection shall be undertaken be fore the products leave the sherardizer’s custody, unless otherwise specified at the time o f ordering by the purchaser Acceptance inspection involves the assessment of appearance of the coated article and testing the thickness The results o f other tests o f the coated article are normally not provided Other tests will only be carried out by agreement between purchaser and sherardizer made at the time o f ordering A control sample for thickness testing shall be taken randomly from each inspection lot selected for testing The minimum number of articles to form the control sample shall be taken in accordance with Table Alternatively, sampling procedures selected from ISO 2859-1, ISO 2859-2, and ISO 2859-3 can be used Table — Control sample size related to batch size Number of articles in the M inimum number of b atch articles in the control to to 500 501 to 200 201 to 200 201 to 10 000 Above 10 000 © ISO 2016 – All rights reserved s ample All 13 20 ISO 7668: 01 6(E) Coating properties 6.1 Appearance The sur face o f the coating has a grey (matt or lustrous) appearance and might show scratches resulting from normal contact with other articles, during processing or storage Such scratches are superficial and not detrimental to the corrosion resistance of the sherardized articles The coating shows a certain sur face roughness which is characteristic for the zinc-iron alloy type o f coating NOTE It is noted that “grey (matt or lustrous) appearance” and “sur face roughness” are relative terms These appearance properties can be influenced by the composition o f and sur face conditions o f the base material It is not possible to establish a definition o f appearance and finish o f the coating covering all requirements in practice Areas without a zinc di ffusion layer shall not be allowed, unless otherwise specified at the time o f ordering by the purchaser and agreed upon with the sherardizer Rejected articles, regarding these areas, shall be re-sherardized and resubmitted for inspection in agreement with the purchaser The development o f (grey-) white corrosion products (white staining), the formation o f mainly basic zinc oxide during storage in humid conditions a fter sherardizing, shall not be cause for rejection It is not permitted for the sherardized articles to have an orange-brown colouring at the time o f delivery, unless otherwise specified at the time o f ordering by the purchaser and agreed upon with the sherardizer Rejected articles, regarding this discolouring, shall be re-sherardized and resubmitted for inspection in agreement with the purchaser 6.2 6.2 Thickness General Coatings applied by sherardizing are designed to protect ferrous products against corrosion and wear The service li fe o f such coatings in a given environment is approximately proportional to the coating thickness (see ISO 14713-1) 6.2 Test methods Thickness measurements shall only be made on clean, washed sur faces In case cleaning and/or passivation is not part o f the coating process, samples for thickness testing shall be cleaned separately The local coating thickness shall be determined in accordance with the magnetic method given in ISO 2178 or the electro-magnetic method given in ISO 2808, also described in ISO 3882 These two methods are generally most appropriate for routine quality control within works The gravimetric method according to ISO 1460 can be used to determine the coating mass The coating thickness can be calculated by dividing the coating mass per unit area by the nominal coating density o f the coating 7,2 g/cm3 (see B.2) I f a su fficient number o f measurements is made within the re ference area (see 6.2.3 ), e ffectively the same coating thickness will be determined by the magnetic method, the electro-magnetic method as the gravimetric method For articles with a complex shape, in which no suitable reference area can be determined for measuring the thickness by the magnetic method, the electro-magnetic method, the gravimetric method shall be used © ISO 2016 – All rights reserved ISO 7668: 01 6(E) The microscopic cross-section method according to ISO 1463 is less appropriate for routine use because it is destructive and relates only to a single line However, this method can be used for special quality control operations for articles with a complex shape, agreed upon by the purchaser and the sherardizer at the time of ordering NOTE 6.2 Annex B provides additional information and guidance on the measurement of thickness Reference areas The number and position of reference areas and their sizes for the magnetic, electro-magnetic, micrometre method or gravimetric test shall be chosen with regard to the shape and size of the article(s) in order to obtain a result as representative as possible of mean coating thickness or mass per unit area as applicable The re ference areas shall be within the significant sur faces, unless otherwise agreed upon between purchaser and sherardizer For articles with a significant sur face area greater than or equal to 10 cm , there shall be at least one reference area on each article in the control sample Control sample sizes are given in Table For articles with a significant sur face area o f less than 10 cm , there shall be enough articles grouped together to provide at least 10 cm2 significant surface for one reference area Hence, the total number of articles tested shall equal the number of articles required to provide one reference area multiplied by the total number o f articles in a control sample, in accordance with Table (or the total number of articles sherardized, if that is less) Alternatively, sampling procedures selected from ISO 2859-1, ISO 2859-2, and ISO 2859-3 can be used 6.2 Magnetic method or electro-magnetic method A minimum o f five magnetic or electro-magnetic test readings shall be taken within each re ference area Because the area over which each measurement is made in this method is very small, individual figures can be lower (typically up to 15 %) than the values for the local thickness This is irrelevant as only the average value over the whole o f each re ference area is required to be equal to, or greater than, the local coating thickness minimum value When more than five articles have to be taken to make up a re ference area o f at least 10 cm , a single magnetic measurement shall be taken on each article i f a suitable area o f significant sur face exists I f such a suitable area does not exist, the gravimetric method according to ISO 1460 shall be used 6.2 Gravimetric method The mass of the coating per unit area, expressed in g/cm2 , shall be determined according to ISO 1460 This mass value can be converted to a thickness value in µm by dividing the nominal density o f the coating 7,2 g/cm3 NOTE 6.2 See B.2 Thickness requirements The thickness requirements for six classes are given in Table © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Table — Coating thickness C lass of coating 6.3 Local coating mass minimum 45 324 432 540 Class 10 Class 15 Class 30 µm 10 15 30 Class 60 Class 75 60 75 Class 45 NOTE Local coating thicknes s minimum g/m 72 108 216 For typical application fields o f the coating with di fferent coating thicknesses, see C.7 Acceptance criteria When tested in accordance with 6.2.2, for the appropriate number of reference areas given in 6.2.3, the local coating thickness shall be not less than the values given in Table If the local coating thickness on a control sample does not conform to Table 2, twice the original number of articles (or all the articles, if that is the lower number) shall be taken from the batch and tested If this larger control sample conforms to Table 2, the batch shall be deemed to conform Failure of the larger control sample to meet the thickness requirements shall constitute grounds for rejection o f the batch I f the batch is rejected, individual articles may be submitted for re-testing 6.4 Additional clearances for threaded components Although sherardizing gives a uni form coating without any significant changes in the profile o f threads, there shall be adequate clearance between external and internal threads before sherardizing The purchaser shall discuss the additional thread clearance requirement with the sherardizer, see A.2 e) Guidance on the adequate clearances can be found in ISO 14713-3 Ceritificate o f compliance When required by the purchaser, the sherardizer shall provide a certificate o f compliance with the requirements o f this International Standard according to ISO 10474 A request for such a certificate shall be made at the time of ordering, see A.2 i) © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Annex A (normative) Information to be supplied by the purchaser to the sherardizer A.1 Essential information The purchaser shall provide the following information on all relevant documents: a) numb er o f th i s I nternationa l Standa rd, i e I S O 176 ; b) cla s s o f co ati ng or, a lternatively, the m i n i mu m co ati ng th ickne s s re qu i re d (s e e A.2 Table 2) Additional information Where the fol lowi ng i n formation i s re qu i re d for p ar tic u la r pu rp o s e s b y the sherard i z er, it s l l b e s p e ci fie d b y the pu rcha s er at the ti me o f orderi ng: a) typ e o f materi a l to b e s herard i ze d; b) a ny l i kely e ffe c ts on the me ta l lu rgic a l prop er tie s of the basis materi a l c au s e d by he ati ng temp eratu re s o f up to 0 ° C , a nd i f the p a r ts are he at-tre ate d; c) surface area and weight of the articles to be sherardized, in case different articles are part of the same order the s ur face area and weight d) identi fic ation from o f s ign i fica nt s ur face s each group o f ar ticles o f the s ame kind shal l b e provided; and re ference are as , for exa mp le, by d rawi ngs or b y the provi s ion o f s u itably marke d s ample s; e) th re ad cle ara nce s , dep end i ng on the cla s s o f co ati ng th ickne s s s p e ci fie d, to b e s tate d on the pro duc t d rawi ng or on the order c u ment; f) g) a ny s p e ci a l pre -tre atment re qui rements; a ny p o s t-tre atments , a fter-tre atments or over- co ati ng to b e appl ie d to the s herard i z e d co ati ng (s e e C.7 ); for h) a ny re qui rements i n s p e c tion; i) whe ther a cer ti fic ate o f compl ia nce i s re qu i re d i n accorda nce with I S O 10 474 © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Annex B (informative) Determination of thickness B.1 General The most general non-destructive methods of determining coating thickness are the magnetic method according to ISO 2178 and the electro-magnetic method according to ISO 2808 These two methods are commonly used for in-works quality control procedures Destructive methods include the gravimetric method according to ISO 1460, determination of mass per area converted to thickness (see B.2) and microscopic cross-section method (see B.3) Careful consideration should be made of the relationship between local and average thickness where the magnetic method or the electro-magnetic method is used and the results are compared with those obtained by the gravimetric method I f a su fficient number o f measurements is made within a re ference area, e ffectively the same coating thickness will be determined by the magnetic method, electromagnetic method as by the gravimetric method Another non-destructive method, which can be used for routine quality control purposes, is measuring the thickness o f an article, be fore and a fter applying the coating, with a precision micrometre device (gauge) The local coating thickness is hal f o f the increase in thickness o f the article a fter applying the coating This method, however, is not standardized This method is less precise, but the accuracy increases with the coating thickness The method is useful in case the magnetic method or the electromagnetic method cannot be applied, such upon agreement between purchaser and sherardizer This method can success fully be used for small products such as rivets and washers B.2 Calculation of thickness from mass per unit area The gravimetric method according to ISO 1460 provides a result for the coating mass per unit area expressed in g/m2 This value can be converted to local thickness in micrometre by dividing by the nominal density o f the coating 7,2 g/cm The mean thickness L is given by Formula (B.1): L = ∆ M ( ρ ⋅ A) (B.1) where L is mean thickness o f the coating, in µm; ΔM is the weight di fference between a coated and an uncoated part, in g; A is the surface area of the part, in m ; ρ is the nominal density of the coating, 7,2 g/cm3 NOTE 7,2 g/cm is equal to 7,2 g/(àm ì m ) For equivalent values of coating thickness and coating mass, see Table © ISO 2016 – All rights reserved ISO 7668: 01 6(E) B.3 Microscopic cross section method T he m ic ro s copic cro s s s e c tion me tho d accord i ng to I S O 14 c an b e u s e d for s p e ci a l qua l ity control procedures for articles with a complex shape with recesses or with hollow parts, such as tubes, agreed upon between purchaser and sherardizer at the time of ordering Careful attention is required to prep are the cro s s- s e c tion and a s ki l le d p ers on nel i s re qu i re d to ana lys e the re s u lts o f the te s t pro ce du re T he me tho d c an a l s o b e u s e d for ana lys i ng the s truc tu re o f the z i nc-i ron a l loy layers T he m ic ro s copic cro s s - s e c tion me tho d i s le s s appropri ate for routi ne qua l ity control, b e c au s e it i s a de s tr uc tive me tho d and rel ate s on ly to a s i ngle l i ne © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Annex C (informative) General information C.1 Sherardizing process Sherardizing is a thermal di ffusion coating process which forms zinc-iron di ffusion layers on ferrous materials The articles are heated in the presence of a sherardizing mixture The coating closely follows the contours o f the base material, and uni form coatings are produced on articles, including those of complex shape The coating may subsequently be post-treated by phosphating, chromating or other suitable passivation processes, resulting in a clean passivated surface C.2 Base material Unalloyed carbon steels, low alloy steels, heat treated steels, high tensile steels, sintered material, malleable grey and cast iron are suitable for sherardizing The influence on mechanical properties o f heat treated steel articles is considerably low The process does not give rise to hydrogen embrittlement However, when pre-cleaning high tensile material (above 000 N/mm2 ), cathodic cleaning should not be employed Pre-treatment by anodic, mechanical or any other processes which not a ffect the base material are recommended The sur face should be free from any contamination prior to processing It is essential that parts having so ft-soldered or resin bonded joints are not sent for sherardizing as joints o f this nature are a ffected by the process Any such jointing should be carried out a fter sherardizing has taken place C.3 Post-treatments Post-treatments (passivation), after-treatments or organic over-coatings (Duplex) of sherardized articles are not in the scope o f this International Standard The service li fe can be significantly extended by these processes (see Annex D) The adhesion o f paints and organic overcoats can be improved by a post-treatment C.4 After-treatments A fter-treatments by oils, stains, sealants or special lubricants can subsequently be applied to the coating or to the passivated coating to enhance the corrosion resistance, to provide colour or to regulate the coe fficient o f friction o f the coating Overcoats such as paints or organic coatings can be supplied to the eventual passivated zinc diffusion coating to increase corrosion resistance and/or decorative appearance Guidance on these a fter-treatments can be found in ISO 14713-3 and ISO 12944-5 10 © ISO 2016 – All rights reserved ISO 7668: 01 6(E) C.5 Appearance T he s herard i z e d co ati ng h as a grey (matt or lu s trou s) app e a rance a nd may s how s cratche s re s u lti ng from norma l contac t with o ther a r ticle s , corrosion resistance s uch s cratche s T he o cc u rrence o f da rker and l ighter a re a s i s no re a s on Wh ite s tai n i ng , the formation shera rd i z i ng , i s no re a s on for for are s up er fici a l and no t de tri menta l to its rej e c tion o f ma i n ly b as ic z i nc oxide du ri ng s torage i n hu m id cond ition s a fter rej e c tion O nge -brown d i s colou ri ng at the ti me o f del iver y i s no t a l lowe d, u n le s s agre e d up on b e twe en the sherardizer and the purchaser C.6 Adhesion T he ad he s ion b e twe en the co ati ng and the b as i s me ta l genera l ly e s no t ne e d to b e te s te d a s go o d b ond i ng i s cha rac teri s tic o f the form i ng o f z i nc-i ron a l loy l ayers b y the s herard i z i ng pro ce s s co ati ng i s made b y therma l d i ffu s ion o f z i nc i nto/on the s u r face o f the i ron a nd s te el ar ticle s T he form i ng typic a l z i nc-i ron a l loy layers T he co ate d a r ticle s shou ld b e able to with s tand , without p e el i ng or fl a ki ng , nd l i ng s i s tent with the nature and thickness of the coating and the normal use of the article C.7 Coating thickness C la s s 10 co ati ngs c an b e s p e c i fie d co ati ng , C la s s 10 co ati ngs have for the appl ic ation as a b as e co at ( pri mer) on ly a l i m ite d c atho d ic corrosion protection without an additional after-treatment C la s s co ati ngs c an b e s p e c i fie d for C la s s co ati ngs shou ld b e s p e c i fie d pro te c tion and for a re a p nt or an organ ic no t re com mende d for norma l i ndo or and outdo or envi ron ments for outdo or appl ic ation s i n more s evere envi ron ments , or where there is a requirement for extended service life C la s s 45 co ati ngs s hou ld b e s p e ci fie d for u s e i n h igh ly corro s i ve and or abra s ive envi ron ments , e g industrial or marine Class 60 and Class 75 coatings can be required for special applications When thicker coatings are s p e ci fie d, there c an b e a re qu i rement Gu id ance for for a s p e ci fic pre -tre atment the co ati ng th ickne s s o f z i nc co ate d pro duc ts for u s e i n envi ronments with d i fferent corrosion categories is given in ISO Where doubt exists concerning the coating thickness required and/or the requirement of a post treatment, the purchaser should seek the advice of the sherardizer 1471 -1 : 0 9, Table C.8 Additional clearances for threaded components T h re ade d comp onents ne e d add itiona l cle arance dep end i ng on the co ati ng th ickne s s Gu ida nce on th i s s ubj e c t c an b e fou nd 12] i n I S O 1471 -3 : 0 9, Table a nd Re ference [ © ISO 2016 – All rights reserved 11 ISO 7668: 01 6(E) Annex D (informative) Corrosion resistance of sherardized layers D.1 General information C o ati ngs appl ie d by the sherard i z i ng pro ce s s a re de s igne d to pro te c t the base materi a l agai n s t corrosion and abrasion For most applications, the sherardized steel articles are exposed to atmospheric conditions Under these conditions, the service life of the coating is proportional to the thickness of the coating See ISO 1471 -1 Re ference shou ld b e made to I S O 2 for de term i nation o f corro s ion c ategor y for an ex terna l exp o s u re envi ron ment Gu ida nce on the l i kely p er formance o f the co ati ng i n a p ar tic u la r atmo s pheric exp o s u re environment can be found in ISO 1471 -1 S a lt s pray te s ts c an no t b e u s e d to acc u rately te s t z i nc d i ffu s ion co ate d (s herard i ze d) s te el b e c au s e they accelerate the wrong fa i lu re me cha n i s m Without a prop er we t/d r y c ycle, the z i nc co ati ng c an no t form p ati na layers T he ab s ence o f a p ati na l ayer a l lows s tant attack o f the z i nc me ta l and give s a ver y low pre d ic tion o f the co ati ng l i fe S e e I S O 1471 -1 : 0 9, C l au s e T h i s te s t pro ce du re ca n on ly b e u s e d for i n-hou s e pro duc tion control i n a comp a rative way D.2 Post treatment and passivation T he s er vice l i fe o f the co ati ng c a n b e e xtende d s ign i fic a ntly by a p o s t tre atment o f the co ati ng b y a passivation process, e.g phosphating, chromating or other suitable passivation process developed for tre ati ng z i nc-i ron a l loys (convers ion co ati ng) T he s igni fic a nce o f the p o s t-tre atment ( p as s ivation) c a n b e te s te d a nd comp a re d by an accelerate d corrosion test However, the result of this test cannot give an absolute indication of the total coating life of the surface D.3 Duplex systems T he s er vice l i fe o f p nte d or p owder co ate d sherard i ze d ar ticle s (D uplex s ys tem) can b e extremely long For the appl ic ation o f duple x s ys tem s i nvolvi ng the u s e o f p nts , i n formation ab out re qui rements on s u r face tre atment, p a i nti ng s ys tem s , co ati ng th ickne s s e tc , c an b e fou nd i n I S O 4 -5 For the appl ic ation o f duple x s ys tem s i nvolvi ng the u s e o f p owder co ati ngs , re com mendation s on co ati ng p owders , pre -tre atment, appl ic ation a nd s ys tem p er formance c a n b e 12 fou nd i n E N 43 a nd E N 7 © ISO 2016 – All rights reserved ISO 7668: 01 6(E) Bibliography [1] ISO [2] ISO [3] EN [4] 1471 -3 : 0 9, Zinc coatings — Guidelines and recommendations for the protection against corrosion of iron and steel in structures — Part 3: Sherardizing 1471 -1 : 0 9, Zinc coatings — Guidelines and recommendations for the protection against corrosion ofiron and steel in structures — Part 1: General principles ofdesign and corrosion resistance 43 , Paints and varnishes Powder organic coatings for hot dip galvanized or sherardized steel products for construction purposes EN 15773, Industrial application of powder organic coatings to hot dip galvanized or sherardized steel articles [duplex systems] — Specifications, recommendations and guidelines [5] ISO 4 -5 , Paints and varnishes — Corrosion protection of steel structures by protective paint systems — Part 5: Protective paint systems [6] ISO [7] [8] ISO 3882, Metallic and other inorganic coatings — Review of methods of measurement of thickness ISO Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed [9] ISO –2 , Sampling procedures for inspection by attributes — Part 2: Sampling plans indexed by limiting quality (LQ) for isolated lot inspection Corrosion o f metals and alloys — Corrosivity o f atmospheres — Classification, 59 –1 , by acceptance quality limit (AQL) for lot-by-lot inspection [10] ISO [11] ISO [1 ] 92 , determination and estimation 59 –3, Sampling procedures for inspection by attributes — Part 3: Skip-lot sampling procedures 14 63 , Metallic and oxide coatings — Measurement ofcoating thickness — Microscopical method Por ter, Fran k, Z i nc H andb o ok, C h ap ter 16 - S herard i z i ng , M arcel D ekker I nc pp −19 [13] M i ttemeijer E., & S omers M Thermochemical Surface Engineering of Steels: Improving Materials Performance f diffusion coatings, Woodhead Publishing pp , C hap ter 20 - S hera rd i z i ng: corro s ion pro te c tion o s te el s by z i nc 7 – 014 © ISO 2016 – All rights reserved 13 ISO 17668:2016(E) ICS 25.220.40 Price based on 13 pages © ISO 2016 – All rights reserved