Methods for testing and evaluating the flash corrosion

Progress in Organic Coatings 44 (2002) 201–209

Methods for testing and evaluating the flash corrosion

Andréa KalendováFaculty of Chemical Technology, Department of Paints and Organic Coatings, University of Pardubice, nam. Cs. legii 565, 532 10 Pardubice, Czech Republic

Received 12 July 2001; received in revised form 22 October 2001; accepted 5 January 2002

Abstract

The paper deals with studying the factors leading to the appearance of flash corrosion in water-dilutable dispersion binders. Also thepossibilities of affecting the course and appearance of flash corrosion by inhibitor, binder, relative humidity, pH value and temperature arediscussed. Methods for evaluating pigmented and nonpigmented coatings are suggested. The tests for nonpigmented films can become a basefor a standard procedure of evaluation of dispersion binders and compounds showing an inhibitive effect on flash corrosion. © 2002 ElsevierScience B.V. All rights reserved.

Keywords:Flash corrosion; Polymer dispersion; Corrosion inhibitor; Color change; Corrosion loss

1. Introduction

Protection of metals from corrosion presents a heavy prob-lem of both economical and ecological nature [1]. Bothpoints of view are solved by the technical progress in theanticorrosion protection of metals [2]. At present the pro-tection against corrosion is under control by means of or-ganic coatings based on binders soluble in organic solventsand pigmented by rather effective anticorrosive pigments aszinc powder [3], zinc phosphate [4], or modified phosphates,calcium metaborate [5], and also iron mica [6] and a wholeseries of further anticorrosive pigments [7].

The coating compositions soluble in organic solvents,namely aliphatic or aromatic, meet the ecological measuresonly with problems [8], but the degree of their corrosion–protection efficiency is really high and their application ismostly very advantageous from the economy point of view.They are characterized by generally long lifetime and a highanticorrosion efficiency [9].

The coating compositions dilutable with water meet theecological requirements laid on the atmospheric protection.The organic solvents are present in a minimum amount, andwith respect to this fact their use is really progressive [10].Complications are encountered due to their water content, aswater is one of the primary conditions to the appearance ofcorrosion. Up to the complete evaporation of this diluent thesubstrate material—a metal—is in a direct contact with anaqueous medium more exactly with an electrolyte [11]. Bythe action on the substrate metal defects designated as flashcorrosion (flash rust) can appear [12], most frequently on

E-mail address:[email protected] (A. Kalendova).

using water polymer dispersions—lattices based on acrylatesor styrene-acrylate copolymers.

1.1. Flash corrosion

The term “flash corrosion” concerns a defect appearingexclusively on the application of water-dilutable coatings.Corrosion of this type does not appear every time, but onlyunder certain conditions, namely, as it was already observed,when the coating film is exposed to high relative humid-ity during the process of drying out. The flash corrosionis observed also then, when the substrate has been com-pletely blasted [12], on the contrary to metallic substratescovered with scale. The quality of substrate cleaning thusresults in the appearance of flash corrosion. Neverthelessthe problem cannot be reduced only to this fact, and thecoatings cannot be intentionally applied to badly cleanedmetal substrate [13]. This would deteriorate other coatingproperties.

The film formation from the water-dilutable coating com-positions runs under the evaporation of water, the coales-cence of polymer particles, the deformation thereof and, atthe end, by the diffusion of individual particles through thepolymer chains. If the evaporation of water from the coat-ing film is decelerated, e.g., due to the effect of high relativehumidity, then prior to coalescence washing out of solubleiron salts takes place. An effective solution of the problem offlash corrosion consists in using inhibitors decelerating thisprocess [14]. It is necessary to realize that the anticorrosiveinorganic pigments do not exhibit any inhibition capabilityfor flash corrosion, and their function in the coating film isdifferent [15].

0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0300-9440(02)00014-0

202 A. Kalendova / Progress in Organic Coatings 44 (2002) 201–209

The appearance and course of flash corrosion is affected,according to our experience, by a series of factors, of whichsome are discussed in this paper:

• effect of relative humidity on the drying out of the film;• effect of temperature in the process of drying out of the

film;• the type of polymer dispersion;• the type of corrosion inhibitor;• the surface pretreatment of substrate metal;• the pH value of the drying out coating composition.

The survey given shows evidently that the appearanceof corrosion can be affected by a series of factors, whichappear in the real practice and which can show a synergicaction. The study of flash corrosion is thus hindered by anumber of difficulties, and further problems connected withthe methods of testing appear:

• corrosion effects;• evaluation of the flash corrosion demonstrations.

1.2. Effect of external conditions on the film formation

The temperature is one of the most significant factors af-fecting the film formation of latex binders. The temperatureaffects the velocity of water evaporation and also the physi-cal properties of binder or polymer (minimum film-formingtemperature (MFT) and glass transition temperature (Tg))[11]. A lower temperature increases the polymer hardness,which creates difficulties with the deformation and coales-cence of particles. The coalescence of polymer particlesalone is explained not only by the action of surface tension,but also by the capillary forces or the self-adhesion of parti-cles [16]. This process can be represented as a diffusion ofpolymer chains from one particle to another. The velocityof water evaporation out of the film is reduced at a high rel-ative humidity, and this decelerates the process of polymerparticle coalescence. Even with the films in which completecoalescence of particles did not take part the additional coa-lescence in already dry film can become effective, providedthe film is exposed to elevated humidities.

2. Experimental details

2.1. Specimen characterization

The study of effects and methods for the description ofappearance of flash corrosion was performed using four

Table 1Characterization of the binders used

Binder Composition (type) MFT (◦C) pH Solid content (%)

A Anionactive aqueous styrene-acrylate copolymer dispersion 20 8.2 50B Aqueous versatic acid-acrylate ester-vinyl acetate copolymer dispersion 20 8.3 50C Aqueous versatic acid-acrylate ester copolymer dispersion 0 7.5 50D Aqueous styrene-acrylate copolymer dispersion 20 7.9 50

Table 2Characterization of the flash corrosion inhibitors used

Inhibitor designation Composition

1 Complex zinc compound in a mixture ofanionic ionogenic solvents

2 Aqueous 10% sodium benzoate andsodium nitrite solution at a ratio of 9:1

Table 3The salts used for creating an appropriate medium in the testing chamber

Saturated water solution Relative vapor tension at

20◦C 30◦C

NaOH 0.06 0.06CaCl2·6H2O 0.32 0.26Ca(NO3)2·4H2O 0.55 0.45NH4NO3 0.63 0.57NaCl 0.75 0.75KCl 0.86 0.85KNO3 0.95 0.94

aqueous polymer dispersions (Table 1). The effects offlash corrosion inhibitors were followed for two inhibitors,which is sufficient for modeling the corroding systems(Table 2).

2.2. Preparation of coatings

The coating compositions were prepared by using aque-ous polymer dispersions of the A–D types in combinationswith both corrosion inhibitors (of the types I and II) in a con-centration of 0–1.5 wt.%. Nonpigmented transparent coat-ings and pigmented coatings with a PVC value of 25% inrutile titanium dioxide were deposited directly on steel sub-strate. The steel panels were freed of scale by a mechanicalprocedure and degreased in chlorinated hydrocarbon vapors.The coatings were cast by means of a film applicator (witha 200�m gap).

2.3. Adjusting the atmospheric humidity

For the tests, which are characterized by various humidityvalues, testing chambers having a volume of 15 dm3, hermet-ically sealed from the surrounding medium, were designed.On the bottom of the testing cells, a saturated solution ofthe appropriate salt was poured, which guarantees a constantrelative humidity (Table 3). The relative vapor tensionP is a

A. Kalendova / Progress in Organic Coatings 44 (2002) 201–209 203

Table 4Iodine scale

Degree 1 2 3 4 5 6 7 8 9 10 11 12 13 14I2 (mg/100 cm3) 1 2 4 6 10 20 30 45 65 100 150 200 300 500

ratio of the water vapor tension above the solution (p1) to thetension of pure water vapor (p2) at the given temperature:

P = p1

p2, P × 100= A

whereA is the relative humidity (%).

2.4. Subjective evaluation of flash corrosion according tothe iodine scale (nonpigmented films)

After finding that the application of nonpigmented trans-parent films at elevated relative humidity directly to thesteel substrate is followed already in the stage of dry-ing out by an intense coloring of the whole film surface,methods for evaluating the changes in color shade wereselected. The most simple method used for this purposeconsists in comparing the standard iodine solutions withthe sample [17]. The iodine concentration in the standardscorresponds to arbitrarily accepted degrees of color shade(Table 4).

2.5. Objective determination of flash corrosion by colorchange measurement (nonpigmented films)

The color difference of the samples was measured bythe spectrophotometer Miniscan IEX of the firm Hunterlab(measuring range 400–700 nm, resolution 10 nm, spectralband with 12 nm, light source—xenon flash lamp) with thesoftware EasyMATCHTM. Each steel panel covered witha dispersion coating was scanned by the spectrocolori-meter, and the results were recorded asL∗a∗b∗ coordinatevalues of the color space CIE—L∗a∗b∗ (1976) in a graph-ical way. Further the color differences were expressed asfollows:

L∗ = 116

(Y

Y0

)1/3

− 16,

a∗ = 500

{(X

X0

)1/3

−(

Y

Y0

)1/3}

,

b∗ = 200

{(Y

Y0

)1/3

−(

Z

Z0

)1/3}

whereinx, y, z are the trichromatic sample coordinates;x0,y0, z0 are the trichromatic light coordinates.

The overall color difference�ECIE is then calculated us-ing the following formula:

�ECIE =√

(�L∗)2 + (�a∗)2 + (�b∗)2

where�ECIE is a measure of the color difference betweenthe sample and the standard

�L∗ = L∗sample− L∗

standard, �a∗ = a∗sample− a∗

standard,

�b∗ = b∗sample− b∗

standard

where�L∗ is a deviation of lightness,�a∗ and�b∗ repre-sent the differences of positions in thea∗, b∗ chart.

2.6. Determination of the corrosion losses

Clean degreased steel sheets freed from scale were ex-posed 24 h to aqueous mixtures of individual polymerdispersions and flash corrosion inhibitors. Weight lossmeasurements were performed and valuesKm andX weredetermined [18]:

Km = (m − m1)104

S

whereKm is the weight loss due to corrosion per surface unitin 24 h (g/m2), m the weight of clean sheet (g),m1 the weightof corroded sheet (g), andS the sheet surface area (cm2).

The weight loss values in 24 h can be expressed also inpercentages reduced to the losses of samples in clean water(blank test= 100%):

X = Km

Km(H2O)

× 100

where X is the weight loss reduced to the corrosion lossin a blank test in 24 h (%),Km(H2O) the corrosion weightloss based on a surface unit observed in clean water(g/m2).

2.7. Subjective evaluation of the flash corrosionaccording to ASTM D 610 (pigmented films)

Dispersions of the A–D types were pigmented withinert titanium dioxide, applied on steel plates and ex-posed at varying relative humidities. The defects causedby flash corrosion were observed. In this case the mostsuitable method of evaluation is ASTM D 610 standard[19], which relates the corrosion manifestations to sub-corroding under the coating due to atmospheric effects,but the scale can be applied with advantages also to theflash corrosion manifestations. The scale of corrosionmanifestations according to ASTM D 610 is presented inFig. 1.


Fig. 1. The scale of corrosion manifestations according to ASTM D 610-85 standard.

3. Results and discussion

Fig. 2 shows the difference between the appearance offlash corrosion for the nonpigmented and pigmented films(dispersion A) without any inhibitor added after drying in amedium of 95% relative humidity and at a temperature of23◦C.

As evident from the dependences presented in Fig. 3, theaqueous dispersions are variously resisting to flash corro-sion. Tables 5–8 give similar dependences of the changes inthe degree of color (by the iodine scale) on the relative hu-midity of surrounding atmosphere at the drying out of filmscontaining the corrosion inhibitors.

The results given in Tables 5–8 indicate that both flashcorrosion inhibitors can considerably affect the resistanceof films to flash corrosion. With the dispersions showing a

Table 5Degree of color according to iodine scale for nonpigmented films ofdispersion type A containing a flash corrosion inhibitor

Relativehumidity (%)

Color of the dried out film

Inhibitor no. 1 Inhibitor no. 2

0.5% 1.0% 1.5% 0.1% 0.3% 0.5%

32 1 1 1 7 5 155 1 1 1 9 8 963 1 1 1 10 10 875 11 13 1 10 11 1086 12 11 1 12 12 1295 14 14 14 14 14 14


Fig. 2. Manifestations of the flash corrosion at the (a) nonpigmented and(b) pigmented films (dispersion type A).

trend to the appearance of flash corrosion it is not possible,however, even at high inhibitor concentrations, to prevent thephenomenon, especially at high humidities. On the contrary,the dispersion with a low tendency to the appearance offlash corrosion shows outstanding results, and this dispersionexhibits a high stability even at a critical relative humidity.

The appearance of flash corrosion can be affected alsoby the pH dispersion value. The emulsion polymerization inan aqueous phase was used for the formation of copolymerdispersion showing a composition: acrylic acid, methacrylic

Fig. 3. Change in film color (according to the iodine scale) for nonpig-mented dispersions in function of relative humidity during film dryingout (T = 23◦C).

Table 6Degree of color according to iodine scale for nonpigmented films ofdispersion type B containing a flash corrosion inhibitor




0.5% 1.0% 1.5% 0.1% 0.3% 0.5%

32 9 9 8 8 9 855 10 9 10 10 10 1063 11 10 11 11 11 1075 12 11 11 11 12 1186 13 13 13 13 13 1395 14 14 14 14 14 14

Table 7Degree of color according to iodine scale for nonpigmented films ofdispersion type C containing a flash corrosion inhibitor




0.5% 1.0% 1.5% 0.1% 0.3% 0.5%

32 1 1 1 1 1 155 1 1 1 1 1 163 1 1 1 1 1 175 1 1 1 1 1 186 1 1 1 1 1 195 12 1 1 14 1 1

acid, 2-ethylhexyl acrylate, and styrene. The prepared dis-persion exhibits a pH value of 5.8. Using the partial and suc-cessive method of neutralizing with ammonia gave a seriesof dispersions showing a pH value equal to 5.7, 6.5, 8.2 and10.5. These dispersions were used for evaluating the flashcorrosion effects at different relative humidities (Fig. 4).

The dependence of flash corrosion manifestations on thepH dispersion value cannot be demonstrated and appearsmore significant at low relative humidities. At elevated hu-midity values the pH effect does not manifest itself, and therelative humidity alone is a driving force for the appearanceof defects induced by the flash corrosion.

Spectrophotometric procedures gave the color differencesof the dispersions applied to steel panels and drying out at

Table 8Degree of color according to iodine scale for nonpigmented films ofdispersion type D containing a flash corrosion inhibitor




0.5% 1.0% 1.5% 0.1% 0.3% 0.5%

32 1 1 1 1 1 155 1 1 1 1 1 163 1 1 1 8 1 175 1 1 1 10 1 186 1 1 1 12 1 195 13 1 1 14 1 1


Fig. 4. Effect of the pH dispersion value on the manifestation of flash corrosion in function of relative humidity.

various relative humidities. Color measurements were per-formed on dispersions with and without flash corrosion in-hibitors. An example of the results is given in Fig. 5 andTable 9 for the aqueous styrene-acrylate dispersion of typeA without any flash corrosion inhibitor.

As evidenced in Fig. 5 and Table 9, the increase of relativehumidity gives rise at first to the shift into the yellow regionand subsequently to the red one, i.e. to the typical colorof the flash corrosion products. At a standard humidity of50%, no difference is observed in the manifestations of flashcorrosion compared to a humidity of 75%. Only above 80%RH red or, occasionally, red-brown color appears.

Starting with the observedL∗, a∗, b∗ values of an aqueousdispersion of type D without inhibitor and adding 0.5% ofthe inhibitor type II to the system, shifts of colors in thearrow directions appear (Fig. 6).

It is evident that a styrene-acrylic type D dispersion ex-hibits a reduced tendency to the flash corrosion formation

Fig. 5. Color shades of coatings drying out at different relative humidities in anL∗a∗b∗ system for a type A dispersion.

than a type A dispersion. Adding a low amount of inhibitoreliminates the flash corrosion almost completely (in the di-rections of arrows in Fig. 6). These results are in an agree-ment with the subjective measurements of color by meansof the iodine scale.

Fig. 7 gives the weight losses of steel sheetsX (%) relatedto a blank experiment in pure water. A substantial differenceexists between the corrosion of steel sheets immersed in theliquid dispersions A–D. The dependence given in Fig. 8 re-ports the possible reduction of sample corrosion by meansof the flash corrosion inhibitors (type II) for the C-type dis-persion.

The ASTM D 610 based method gives a subjectiveevaluation, but is rather applied in practice. Applicationof it to the flash corrosion manifestations would make theevaluation of coatings defects easy. To describe preciselythe appearance of the surface of pigmented films struckby flash corrosion, the coating surface was recorded by


Table 9Characteristic values of color changes during drying out at different relative humidities (dispersion type A)

Sample Relative humidity (%) L∗ a∗ b∗ l:c = 2 �L∗ �a∗ �b∗ �ECIE

Standard 64.24 0.06 1.49 a 64.24 0.06 1.49Tolerance+ 2.47Tolerance− 2.47

Sample 1 6 55.02 1.06 10.23 −9.21 1.00 8.73 12.73Sample 2 32 47.35 6.05 27.04 −16.88 5.99 25.55 31.20Sample 3 55 37.66 14.12 21.81 −26.58 14.06 20.32 36.29Sample 4 63 38.18 15.66 22.88 −26.06 15.60 21.39 37.14Sample 5 75 38.15 14.35 22.22 −26.09 14.29 20.73 36.25Sample 6 86 30.50 13.97 10.21 −33.73 13.91 8.71 37.52Sample 7 95 28.74 11.96 7.09 −35.50 11.90 5.60 37.60

a l and c are the weight factors for the color difference.

Fig. 6. Effect of an inhibitor on color shades of coatings of type D dispersion drying out at different relative humidities.

means of a videocamera Panasonic. The record was sub-sequently transformed to graphical PC format and adaptedby means of the Screen Measurement software to enablethe quantification of the area attacked by flash corrosion.Fig. 9A–C represent this procedure on a pigmented coating.

Fig. 7. Dependence of the weight loss of steel sheet (X) reduced to theblank test in pure water for individual dispersion types.

Table 10 presents several examples comparing the ASTMD 610 method to the scanning method. As evident, theiodometric method and the scanning method give resultsclose to each other but the computer-aided evaluationshould be more precise than the subjective evaluation.

Fig. 8. Dependence of the weight loss of steel sheet (X) for a type Cdispersion in function of flash corrosion inhibitor concentration.


Table 10Comparison of the objective and subjective evaluations of the surface extent of flash corrosion on the pigmented coating

Relative humidity at the dryingout of coating (%)

Surface with the flash corrosion manifestations (%)

ASTM D 610 method (subjective procedure) Screen Measurement (objective procedure)

32 0.0 0.0055 0.3 0.3063 1.0 0.5475 1.0 0.6186 16.0 15.5495 33.0 24.22

Fig. 9. Videocamera-aided procedure for determining the surface areaattacked: (A) steel plate showing corrosion manifestations in a graphicalformat; (B) the image after transformation; (C) transformation of theimage to a binary color system.

4. Conclusion

The appearance of flash corrosion, which manifests itselfin water-dilutable dispersion coatings, depends on a seriesof factors. The primary factor of flash corrosion formation isthe dispersion itself. Aqueous copolymer dispersions exhibitvarying tendency to flash corrosion initiation, to such an ex-tent, that a difference exists for very similar dispersions ofthe styrene-acrylate type (dispersions A and D). A consid-erable effect on the flash corrosion appearance is also dueto the conditions occurring during film formation. At a rel-ative humidity between 50 and 70% a considerable increaseof flash corrosion is observed. The pH value of the aqueousdispersion does not show a considerable effect on flash cor-rosion formation. The results obtained allow us to concludethat the neutralized binders or alkaline-nature binders aremore appropriate than those of acidic nature. The flash cor-rosion can be hindered by means of inhibitors, the efficiencythereof being significantly dependent on the concentrationof inhibitors in the coating composition. In this study it wasfound that the temperature at a constant relative humiditydid not affect the formation of flash corrosion.

The factors affecting the formation of flash corrosion canbe summarized in several points:

• binder (aqueous polymer dispersion) type—a significantfactor;

• relative humidity during film formation—an extraordinar-ily significant factor;

• application of the flash corrosion inhibitor—a significantfactor;

• flash corrosion inhibitor concentration—a significant fac-tor;

• pH value of the binder—a factor of low significance;• MFT—an insignificant factor.

It was also found that the aqueous copolymer dispersionalone applied onto a steel substrate was accompanied byflash corrosion formation on the whole surface and that thesubstrate was typically colored to a red-brown shade. Theapplication of pigmented coating based on the same binderis followed only by the formation of local sharply limiteddefects, the size and frequency of which can depend on thethickness and porosity of the coating film (and also on thesize and form of the pigment particles).


Testing of flash corrosion development on samples wasperformed successfully by using the following methods:

• iodometric determination of color (in case of nonpig-mented films);

• spectrophotometric color determination (in case of non-pigmented films);

• the ASTM D 610 based method (in case of pigmentedfilms);

• method of scanning the surface by a videocamera (in caseof pigmented films);

• determination of corrosion losses (for liquid dispersions).

Except in the case of the last method, subjective and ob-jective methods were applied. These methods were used withnonpigmented films and with pigmented films. A change incolor shade under a nonpigmented film was rather preciselyreflected by the comparative method using iodine solutions,as evidenced by the results of the really precise spectropho-tometric method.

For pigmented films the comparison with the standardsgiven in the ASTM D 610 procedure was used. This methodis based on a subjective evaluation of the area attacked bythe flash corrosion. A precise method for pigmented coat-ings consists, of course, in the scanning procedure with acomputer-aided evaluation of the area where a color changetook place. It is to be noted that the flash corrosion mani-festations are best observed on white coatings.

The method reflecting the corrosion losses of steel givesevidence of the action of liquid dispersions on steel duringan immersion time long compared to the drying time of thecoating. This method confirmed that on the exposure of steelsample to an aqueous dispersion solution dissolution of steeltook place and steel was covered with corrosion products,the quantity of which can be evaluated with a rather highaccuracy. Also in this case differences in binder aggressivityare observed in terms of flash corrosion formation.

This work outlines some factors leading to the appear-ance of this undesirable phenomenon, namely the flashcorrosion of steel on the application of water-dilutable dis-persion coatings. It also describes the methods of evaluationof flash corrosion on pigmented and nonpigmented coat-ings. Some methods are really simple other rather complexand demanding from the instrumentation point of view. Re-sults obtained by both types of methods, simple and morecomplex, are compared and definite intercorrelations aresought.

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Methods for testing and evaluating the flash corrosion