QUANTIFICATION OF CORROSION ACTIVITY ON HDG STEEL SHEETS DURING CYCLIC DIP TESTS IN CLASSICAL AND ECOLOGICAL SALT SOLUTIONS

The paper is focused on the part of complex work dealing with verification of functionality of non-destructive technique of electrochemical noise applied on HDG steel sheets samples with different surface treatment, namely on samples to be treat with Cr 3+ , anticorrosion oil and nonoiled and non-passivated ones. Besides monitoring of quality of surface treated zinc sheets (activity/passivity) by ENA technique an extension of corrosion deterioration was quantified gravimetrically after 3, 15, 22 and 30 days of exposure in 3 % NaCl, 3% ecological salt mainly composed of MgCl2, CaCl2 and condensation chamber. Corrosion products were observed with support of SEM microscopy with EDX analysis.


Introduction
It is well known that increased caution in various kind of industry is focused on corrosion problems and corrosion protection of surface treated metallic materials.The consequences of corrosion in automotive industry thus should be revealed before long-term testing of car body components, eventually first visible failures of inadequate combination of engineering parts caused problems during final phase of big developer's projects, even before inspection.For that reason, it is justified the interest in new approaches in corrosion monitoring techniques [1,2,3,4].Application of modern corrosion monitoring methods served for shorter time needed for corrosion testing, for faster reaction on solution of occurred problems caused by corrosion aspects.The ability of evaluation of actual quality of coatings from the point of view of safe field application is very important.It is sufficient and more and more widespread also to forecast lifetime of materials.The current trend is mainly industrial inquiry for non-destructive monitoring of corrosion.Besides visual inspection, laboratory tests is still needed for verification.The added value of monitoring in terrain is that it helps for fast and relatively simple control in application environment with output in the form of detection of quality of surface treated material.At the same time it allows to determine defect free state, state with high passivity, low activity or high corrosion activity, which could lead finally to the failure of the protected system.Electrochemical noise methods (ENA) using no contact to the substrate arrangements (NOCS) [5,6] is one of suitable corrosion monitoring approaches and helps to detect a corrosion resistance of zinc coatings or final paints.The noise resistance' is defined as the ratio of the standard deviations of the potential noise and the current noise between two identical working electrodes which are linked by a zero resistance ammeter (ZRA).The noise resistance has been used considerably, recently, in the research of corrosion phenomena and it is often found in literature to be equivalent to the polarization resistance [7,8].It has been mathematically proved and empirically correlated to the polarization resistance in order to calculate the corrosion rate [9].Furthermore it has been suggested that certain parameters derived from the statistical analysis of electrochemical current noise data such as the localization index (LI) provide information about corrosion mechanism [10,11] or special statistical treatment of both measured potential and current noise cover state of passivity of metallic surface [12,13].During corrosion testing the original application environment is replaced by electrolytes with different agressivity simulated field application such as atmospheric acidic electrolyte, e.g.simulated acid rain solution (SARS), classical technical salt -NaCl, ecological salts on the basis of MgCl 2 , CaCl 2 or environments simulated cyclic temperature and humidity changes (condensation chamber, VDA tests etc.) [14].

Material and experimental methods
Testing was performed on hot dip galvanized steel sheets (HDG) with additional anticorrosion treatment, with passivation, passivation free with dimension of 100 x 120 mm.Three types of samples were used:  Alfa -HDG steel sheets with thickness of 120 g.m -2 of Zn coating, roughness 1.6 µm and 0% of Fe contain in Zn.Surface was non-passivated one with presence of anticorrosion oil. Gamma -HDG steel sheets with thickness of 120 g.m -2 of Zn coating, roughness 1.4 µm and 12.6% of Fe contain in Zn surface.The surface was non-passivated and nonoiled. GI -HDG steel sheets with thickness of 100 g.m -2 of Zn coating and passivated with Cr 3+ .Three types of simulated electrolytes were used in experiments:  3% NaClsimulation of solution served for technical service on roads in winter seasons. 3% solution of ecological salt solution on the basis of MgCl 2 and CaCl 2 . Condensation chamber for accelerated testing of corrosion resistance under cyclic conditions where temperature and humidity is changing.E.g.VDA tests are very specific and differ for each automotive company in temperature and humidity time interval.During this kind of test cyclic achievement of the dew point is simulated, which caused condensation of water with presence of salt.Thus create the most suitable environment with maximum rate of corrosion phenomenaunder thin layer of surface electrolyte.mentioned above -3% NaCl, 3% ecological salt solution with consecutive observation of corrosion resistance.According to the standard determined dip/drying was performed with 1000 cycles.The samples hang out during exposition on clamp holder made from isolated wires with assured clear distance ahead to be sure that no reaction between them occurred and no contact to bottom and wall taking place.At the same time the level of electrolyte do not decrease and was added continually during evaporation.

Electrochemical noise measurements
Electrochemical noise analysis with no contact to the substrate arrangement (ENA-NOCS) was chosen for non-destructive measurements (Fig. 2).The plastic sensor with two holes was located on the sample, filled with measurement solution.Reference electrodes were embedded within holder and attached to the sensor.The potential and current noise were measured using high sensitive digital multimeter KEITHLEY and data were evaluated by LabView [16].

Results and discussion
The output of non-destructive corrosion monitoring using ENA-NOCS setup is effective determination of activity/passivity of coatings with very high sensitivity.The results from measurements is shown in table outdoor non-polluted atmospheric conditions) R N values range between hundreds of kΩ.cm 2 and MΩ.cm 2 .Very low corrosion activity of zinc coatings under organic coatings, lacquer or multicomponent paint could reach more than MΩ.cm 2 , high quality of coatings has R N in GΩ.cm 2 .
From the results in Table 1 it is apparent that alfa HDG samples (non-passivated) were in active corrosion during whole exposition in 3% NaCl, which reveal by very low R N values.Nevertheless, after 22 days Rn values were unable to detect by non-destructive technique caused by high mass losses of zinc, destruction of zinc coatings and occurrence of iron substrate steel.This fact was observed by SEM microscopy with EDX analysis (Fig. 6).
On the other hand, alfa HDG samples exposed to ecological salt solution were passivated by suitable corrosion products with effective protection where relatively high R N of 215 015 Ω.cm 2 was obtained.R N values for HDG samples exposed in ecological salt solution and condensation chamber was very close to R N values as for as supply zinc coatings.R N values lower than 500 Ω.cm 2 were reached for zinc coatings exposed in 3% NaCl solution after 30 days of exposition and that is why zinc coatings completely lost passivation functionality.The gravimetric analysis of hot dip galvanized steel samples after exposure in 3% NaCl solution are shown on Fig. 3, 4, 5.The minimal mass losses of zinc coatings was reached after 30 days of exposition by gamma HDG sample, which contain 12.6% Fe in Zn coating.The maximum mass losses were detected on alfa HDG samples, non-passivated ones.The gravimetric analysis together with scanning electron microscopy with EDX analysis (Fig. 6, 7, 8) were supporting methods only, helping to verify functionality and results obtained from non-destructive corrosion monitoring technique based on electrochemical noise.The protection of zinc coatings on HDG samples exposed in 3% NaCl were completely lost, surface was locally deteriorated and the rest of zinc coatings were separated from steel substrate (Fig. 6).This phenomenon lead to effect that application of non-destructive technique of corrosion monitoring based on electrochemical noise was not able to detect any R N value.The reason is that technique is able to detect values higher than 1000 Ω.cm 2 and was below sensitivity of ENA NOCS setup.Zinc coating on HDG steel exposed to the ecological salt solution were completely covered by corrosion products with net structure and passivation character.The proof of this fact were

Conclusions
The paper demonstrates ability of electrochemical noise method in no contact to the substrate arrangement to be used as corrosion monitoring technique for fast detection of active/passive state of zinc coatings.The obtained R N values for active surface lie in few tens of Ω.cm 2 range, these of low activity, resp.passivity in hundreds of kΩ.cm 2 .It was also shown that ecological salt solution is much lower aggressive than 3% NaCl according to the norm EN ISO 11130:2011 for all alfa HDG steels.This results were supported by gravimetrical analysis where mass losses do not exceed 1.5 µg.m -2 .The passivity and protection character of zinc corrosion products after exposure in ecological salt solution and condensation chamber was proven also by scanning electron microscopy.The values of R N were not obtained for deteriorated zinc coatings after 30 days of exposition in 3% NaCl due to the pre-corroded zinc surface which was also visually observed.It was also shown that during exposition of alfa HDG samples in condensation chamber after 15 days and in ecological salt solution after 30 days initiation of passivation got started.For alfa HDG steels in 3% NaCl only active corrosion was observed.

Table 1 R
N values from corrosion monitoring measurements using ENA , samples do not have any corrosion mass losses after 30 days of exposure to eco salt.The same effect was observed on HDG steel samples exposed in condensation chamber, surface were covered by corrosion products with "needle" structure and passivation character mainly consisted of ZnO, ZnCO 3 and common hydrozincites.Corrosion monitoring technique with support of gravimetrical changes and scanning microscopy has shown that during exposition of alfa HDG samples, these start to passivate in condensation chamber after 15 days and in ecological salt solution after 30 days.For alfa HDGsteels in 3% NaCl only active state were detected.