Alternative Corrosion Coating- Their Effect and Influence on Adhesive Performance

The corrosion behavior of Cr203 and Ti02 based surface treatments in 0.5N NaCl solution was investigated. These surface treatments are used for adhesive bonding of SS3 l 6L both to itself, other metals and non-metals. In order to quantify corrosion behavior and determine their ability to passivate in a chloride environment, a poteniodynamic test was employed. To measure the adhesive strength of bonds using the different surface treatments, a standard test method, ASTM D 1002, was used. This measured the apparent shear strength of single lap joints made by adhesively bonding metals specimens together. A general-purpose epoxy adhesive was used in the experiments. To investigate the effect of marine exposure, lap joints samples were placed in salt spray apparatus for different exposure periods and the residual shear strength measured. Results indicated that the shear strength of adhesive joints coated with Cr203 decreased in strength to a value less than joints treated with Ti02, even though the initial strength was higher. Adhesive and cohesive failure of joints was noticed. In order to examine the surface conditions of the samples after failure, a scanning electron microscopy (SEM) was employed. The surface treatments did not change the surface features markedly. It is suggested that the decrease in bond strength for the Cr20 3 treatment was due to crevice corrosion between the SS316 and the adhesive. The Ti02 treatment did not show the same degree of crevice corrosion. The potentiodynamic data supported


Abstract
The corrosion behavior of Cr203 and Ti02 based surface treatments in 0.5N NaCl solution was investigated. These surface treatments are used for adhesive bonding of SS3 l 6L both to itself, other metals and non-metals. In order to quantify corrosion behavior and determine their ability to passivate in a chloride environment, a poteniodynamic test was employed.
To measure the adhesive strength of bonds using the different surface treatments, a standard test method, ASTM D 1002, was used. This measured the apparent shear strength of single lap joints made by adhesively bonding metals specimens together. A general-purpose epoxy adhesive was used in the experiments.
To investigate the effect of marine exposure, lap joints samples were placed in salt spray apparatus for different exposure periods and the residual shear strength measured. Results indicated that the shear strength of adhesive joints coated with Cr203 decreased in strength to a value less than joints treated with Ti0 2 , even though the initial strength was higher.
Adhesive and cohesive failure of joints was noticed. In order to examine the surface conditions of the samples after failure, a scanning electron microscopy (SEM) was employed. The surface treatments did not change the surface features markedly.
It is suggested that the decrease in bond strength for the Cr 2 0 3 treatment was due to crevice corrosion between the SS316 and the adhesive. The Ti0 2 treatment did not show the same degree of crevice corrosion. The potentiodynamic data supported 11 this theory as the Cr203 treatment showed a tendency to localized corrosion while the Ti0 2 treatment did not. 111 Brown. whose continuous guidance and constant encouragement made this work  x Figure 9 Comparison of failure load for the two pairs of chromium trioxide coated samples and another two pairs of titanium dioxide coated samples that were Corrosion of stainless steel can be initiated by exposure to seawater . Two processes occur upon immersion of a stainless steel in natural seawater, both of which displace the corrosion potential in the noble direction. Firstly, during the readjustment process, the air-formed passive film on the stainless steel adjusts its chemical composition and becomes thicker. Secondly, a biofilm develops on top of the passive film. This biofilm formation is also known as a " natural population biofilm" because it comprises of a variety of bacteria and algae. These films also raise the corrosion potential above the pitting potential for type 304 and type 316 stainless steel . 3 Crevice corrosion is a localized type of corrosive attack. It occurs when a small crevice is formed between a metal and a non-metal in the presence of an aggressive environment. If the metal is susceptible under these conditions, then crevice corrosion will occur. This is highly probable in a lap joint under marine exposure. Analogous to Pitting, crevice corrosion will occur when the metal potential equals or exceeds the crevice corrosion potential . Since this value is usually less noble than pitting potentials, crevice corrosion will usually occur before pitting, and the corroding area may in fact prevent pitting by acting as a sacrificial anode. For stainless steel in seawater, crevice corrosion is considered to be a more serious corrosion problem than pitting . can be determined . 3 Some materials, under certain environmental conditions, experience a significant reduction of the corrosion rate caused by the formation of an adherent protective film on the surface. This phenomenon is called passivity. A materials passive region can be determined by continuing to apply increasing anodic potentials from Eoc until the current density reaches a peak and then decrease to a considerably lower value . This peak is called the Primary Passivating Potential (Epp) . This is the beginning of the passive region . As the potential increases in the anodic direction, current density will remain low until a rapid increase occurs . This is called the Transpassive region and is due to the dissolution of the passive film layer . The potential at this point is called the iranspassive ( ETP) . If the potential is increased well into the transpassive region and allowed to reverse to a lower potential, the ability of the passivating material to resist localized, pitting or crevice, corrosion may be determined. If upon reversing the potential, moving in a cathodic direction, the current either follows or decreases faster than the forward scan , then any damage to the passive film will be repaired 3 tuunediately. This material would be considered to have a good localized corrosion resistance . 4 Alternatively, upon reversing potential the current is greater than the forward scan , then the material has a poor resistance to localized corrosion.

t.3 DC CORROSION TESTING TECHNIQUES:
The basic equipment and experimental procedures used for potetiodynamic anodic polarization scans is described in ASTM G5. The counter electrode was a high density high purity, graphite rod which is a commonly used alternate to the ASTM suggested platinum electrode. ASTM maximum recommended scan rate of 0.6 V /hr was reduced to a more conservative 0.1 mV/sec. Scans began at 125mV below, or more cathodic, than Eoc and moved in the anodic direction. The scan direction was reversed to cathodic when the potential reached 200m V above Eoc as long as the current density was at least 200 uA/cm2. The ASTM procedure specifies the scan to reverse at SOOOuA/cm2. The ASTM required the scan to continue until the hysteresis loop close or until corrosion potential is reached 1 OOm V above Ecorr • This allowed automatic testing that identified all the pertinent data points .

CHROMATE CONVERSION COATING:
Conversion coating is any coating that chemically changes the surface of a metallic part. Conversion coatings serve two purposes, firstly corrosion protection and llCCOndly assuring good adhesion of the coated surface to other materials such as paint and adhesives. There are two main conversion treatments that have been used in this project , coating by chromium trioxide or by titanium dioxide . Before applying the conversion coating the surface must be degreased in either acid or alkali degreasers and, after coating application, rinsed and dried . Conversion coatings can be applied either 4 by immersion or spraying. Spraying equipment will result in higher investments and some parts of a complicated structure may be difficult to treat properly. 5 On the other band, the immersion treating of certain geometry's can cause drag out or carryover of the chemicals which can cause cross contamination of the process chemicals thereby reducing their active life. The choice between spray and immersion must be done after a total evaluation of the economy of the process and the quality of the coating . Chromate conversion coating has two main benefits ; one of them is that the mixed 91'omium/substrate metal oxide coating provides better corrosion resistance than the substrate metal oxide alone. Additional corrosion protection is provided by chromate ions entrapped in the coating. These ions are readily leached from the coating and act as corrosion inhibitors. Another property of the chromate coating is its ability to improve the adhesion ofpaint 6 • This is probably related to the cellular structure of the mixed oxide film, which provides a base with more attachment points . The composition of the film is rather indefinite, since it contains varying quantities of the reactants, reaction products and water of hydration, as well as the associated ions of the particular systems . There are many important factors that control the formation of the chromate film. One of those factors is the pH of the treatment solution. For any given metal/chromate solution system there will exit a pH at which the rate of coating formation is maximum. As the pH is lowered from this point, the reaction products become more soluble, tending to remain in solution rather than deposit as a coating on the metal surface. Even though the rate of metal dissolution increases, the coating thickness will remain low . Increasing the pH beyond the maximum gradually lower the rate of metal dissolution and coating formation to the point at which the reaction, for all practical purposes, ceases. Another factor that controls the film formation is the activator . Chromate films will not form without the present of certain anions in regulated amount. They are commonly referred to as " activators" and include acetate, anions such as acetate, format, chloride, fluoride, nitrate phosphate and sulfamate . The character, rate of formation and properties of chromate films vary with the particular activator and its concentration. In addition to the chemical makeup of the chromating solutions three more parameters that should be considered during film formation. One of the factors is treatment time, immersion time or contact time of the metal surface and the solution. This can vary from one second to one hour, depending on the solution being used and metal being treated. Another factor is the solution temperature. 6 Cbromating temperature varies from ambient to boiling, depending on the particular solution and the metal being processed. For a given system, an increase in the solution temperature will accelerate both the film forming rate and the rate of attack on the metal surface. This can result in a change in the character of the chromate film. Thus, temperature should be adequately maintained to insure consistent results. Solution agitation is another factor that affect the chromate film formation. Agitation of the working solution, or movement of the work in the solution, generally speeds the reaction and provides for more uniform film formation. Air agitation and spray installation have been used for this purpose .

HEAL TH HAZARDS OF CHROMIUM:
Chromium can enter the body when people breath air, eat food, or drink water containing it. Chromium is also found in house dust and soil, which can be ingested or inhaled. Of the various forms of chromium, hexavalent chromium is the most toxic.
Certain hexavalent chromium compounds have been found to be carcinogenic in humans, but the evidence to date indicates that the carcinogenicity is site-specific-- The high corrosion resistance and good biocompatibility of titanium and its alloys are dure to a thin passive film that consists essentially of titanium dioxide. There is increasing evidence, however, that under certain conditions extensive titanium release may occur in vivo. An ion beam assisted sputtering deposition technique deposited thick and dense Ti0 2 film on titanium and stainless steel surfaces. Titanium films have been investigated in phosphate buffered saline solution using the following measurements: (1) open circuit potential versus time of exposure, (2) electrochemical impedance &>eCtroscopy, (3) potentiodynamic polarization, and (4) Mott-Schottky plot. 8 A higher electrical film resistance, lower passive current density, and lower donor density have been measured for sputter-deposited oxide film on titanium. The improved corrosion protection of the sputter-deposited oxide film can be explained by a low defect concentration and, consequently, by a slow mass transport process across the film. As opposed to Ti0 2 on titanium, a deviation from normal n-type semiconducting Mott-Schottky behavior was observed for Ti0 2 on stainless steel. 9 In 1994 Imokawa, Fujisawa, Suda and Tsuikawa studied the protection of 304 stainless steel by titanium dioxide.
The photo electrochemical behavior of 304 stainless steel sputter coated with TiOi in NaCl solutions at ambient temperature was studied. 15 It was found that coating the 304 with a Ti0 2 thickness greater than 3 run initiated cathodic protection under light irradiation condition through the Ti0 2 coating layer acting as a non-sacrificial anode.
Up As results of these results, it was confirmed that coating from 30 to 100 nm thick could protect 304 stainless steel cathodically under illumination. The coating defects don't hinder the protection performance when its area ratio is less than 1/10. As shown by Honda and Fujishima, the anodic reaction on Ti0 2 is neither dissolution nor "'9t00ecomposition but oxygen evolution. Therefore, the Ti0 2 coating is expected to work as a non-sacrificed anode. This is a highly contrasted feature of the coating as compared with zinc coating for steels, which is destined to be consumed. TiOi thick films were prepared on stainless steel by plasma-spray coating and the electrode potential of the films were reduced by about 250mV under ultra-violet irradiation. This potential drop value is sufficient for protection from corrosion . 16 Cathodic protection for stainless steel 304L was studied by sol-gel-derived Ti0 2 coating under illumination of light and they found out that Fe was much more intense than other elements, which decrease the photo effect ofTi0 2 coating. In order to avoid

OBJECTIVE:
The objective of this study is to measure the residual lap shear strength of adhesively bonded 316L stainless steel after marine exposure. Different surface treatments will be used prior to adhesive bonding, the normal chromate based and an alternate to chromate. The data will determine the effectiveness of an alternate to chromate. 12 CHAPTER II

EXPERIMENT AL METHODS
The material used in this study is stainless steel 316L. 316 L is known for its low carbon content, the composition of this type of stainless steel is 16-18% chromium, t0-14% nickel, 0.03% carbon, 2.0% molybdenum, 1.0% silicon, 0.045 phosphate and 0.03% sulfur.316 type of stainless steel contains molybdenum and has greater resistance to pitting in marine and chemical industry environment. The low carbon content is for "Weldability to avoid weld decay .

SPECIMEN SIZE:
The measurements of the specimen used in this work were as follows: the length was 75 mm, the width was 25 mm, and the thickness was 1.6 mm.

COATING OF 316L BY CHROMIUM TRIOXIDE:
To After the cooling, adhesive was applied to some of the specimens using the standard method while some of the cooled specimens were tested electrochemically.

COATING OF 316L BY TITANIUM DIOXIDE:
For the alternate to chromate, the only change was to replace chromium dioxide with titanium dioxide so the bath consisted of 50 grams of titanium dioxide dissolved in 500 cc DI for 10 minutes at 140-190 F. The remainder of the coating process was identical to that used for chromium dioxide.

POTENTIODYNAMIC POLARIZATION METHODS:
Polarization methods such as potentiodynamic polarization, potentiostaircase, and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environment. Polarization methods involve changing the potential of the working electrode and monitoring the current, which is produced as a function of time or potential.

2.S CYCLIC POLARIZATION:
The advantage of cyclic potentiodynamic anodic polarization scans is quick results and easily interpreted date. The disadvantages of this test are the data cannot predict long-term behavior; it is a destructive test; and it cannot be used in certain high resistance applications.
14 The localized corrosion behavior of the specimens was investigated by cyclic reference electrode. The sample whose anodic behavior was investigated serve as the working electrode. To ensure a maximum cathodic reaction, oxygen was purged into the 0.5 normal sodium chloride electrolyte.

ADHESIVE BONDING TECHNIQUES:
Adhesion is one of the most complex and important parameters that determine the quality of coating systems. The theoretical adhesion strength is a result of all interfacial and intennolecular forces. However, the practical adhesion strength, which is the force or energy needed for detachment of the coating, never reaches this theoretical value.
The difference is caused by the hollow spaces and defects at the interface of substrate and coating. The environment (temperature, diffusion of water, oxygen) contributes to 15 the adhesive strength of a system, therefore, dry as well as wet adhesion are important parameters for characterization .  in contact with the jaws, and so that the long axis of the tested specimens coincided with the direction of the applied pull through the centerline of the grip assembly. The loading was applied immediately to the specimen at rate of 1200-1400 psi of the shear area per minute. The loading was continued to failure. The rate of the loading was _,roximated by free crosshead speed of0.05 inch/min. The loading at failure and the nature and the amount of this failure for each specimen was recorded.

2.8SALT SPRAY TESTING:
The oldest and the most widely used test is ASTM B 117, method for salt spray testing, a test that introduces a spray into a close chamber where some specimens are exposed at specific location and angles. The concentration of the NaCl solution was 5% by weight in deionized water. There is a wide range of chambers designs and sizes including walk-in rooms that are capable of performing this test.
Hot, humid air is created by bubbling compressed air through a bubble tower containing hot deionized water. Salt solution is typically moved from a reservoir through a filter to the nozzle by a gravity-feed system. When the hot, humid air and the salt solution mix at the nozzle, the solution is atomized into a corrosive atmosphere.
This created a 100 percent relative humidity condition in the exposure zone. For a lowhumidity state the exposure zone of the chamber, air is forced into the exposure zone via a blower motor that directs air over the energized chamber heaters. The test should be continuous for the duration of the entire test period.
Continuous operation implied that the chamber be closed and the spray operating continuously except for the short daily interruption necessary to inspect, rearrange, or remove test specimens to check and replenish the solution in the reservoir, and to make necessary recording. In this work 21 pairs coated with chromium trioxide were exposed to the salt spray for one week, two weeks, three weeks, four weeks and five weeks.
Another 21 pairs coated with titanium dioxide were exposed to the salt spay test for one week, two weeks, three weeks, four weeks and five weeks. After the exposure period, the samples were tested to failure.

SCANNING ELECTRON MICRSOCOPY:
Scanning electron microscopy was used to detail the surface appearance of the coating prior to and after failure. It provides direct image of the topographical nature of the surface from all the emitted secondary electrons. It helps investigate the mode of failure such as fatigue, creep, shear overload, tensile over load or other complex failure 18 JDodes. The samples were scanned with a high energy electron beam in a raster pattern which causes the ejection of numbers of particles, including secondary electrons which fonn an image of the surface ejecting them. One disadvantage of the scanning electron JDicroscope is that it is normally not possible to examine samples that produce any significant amount of vapor when placed in vacuum and many samples like grease and adhesive liquids, foods, gels cannot be examined. 19 CHAPTER III RESULTS AND DISCUSSION

ELECTROCHEMICAL BEHAVIOR:
A study of the electrochemical behavior ofTi0 2 and Cr203 coatings on 316L stainless steel was conducted. The aggressive environment was 0.5 N NaCL Cyclic polarization scans plots for bare 316L is shown in figure ( 1 ). Upon reversal of the potential in a cathodic direction, the current density is higher than the forward anodic direction indicating that uncoated 316L is susceptible to localized corrosion.  It is expected that localized corrosion due to the geometry from adhesive bonding occurred on the SS316L. Moisture adsorption might play a significant part in order to decrease the bonding strength, however this decrease should be the same for both coatings as the same adhesive and exposure conditions were used. Figure  The localized corrosion resistance of the titanate compared to the poor resistance chromate in retarding crevice corrosion was thought to be the important factor in increasing bond strength after marine exposure. 25

FUTURE WORK
Applying an interlayer between the substrate and the coating .
Use of the TiOi instead of Cr203 shows some promise of use in corrosion resistance applications and should be investigated .
Conduct more adhesive bonding testing by examining the effect of adhesive thickness and continue salt spry testing .
Examine the effect of the alternative coating process .
Use more samples of SS3 l 6L to identify corrosion effect and testing procedure errors .        Figure 5 Comparison of failure load for the two pairs of chromium trioxide coated samples and another two pairs of titanium dioxide coated samples that were exposed to salt spray test for three weeks with adhesive thickness of 0.005 inch.  Figure 7 Comparison of failure load for the two pairs of chromium trioxide coated samples and another two pairs of titanium dioxide coated samples that were exposed to salt spray test for five weeks with adhesive thickness of 0.005 inch.

Tensile Test of Chromium
Tensll Test Of Chromium Trioxide and Titanium dioxide with adhesive thickness of ( 0.01 inch)  l•series1 I Figure 10 Comparison of failure load for the two pairs of chromium trioxide coated samples and another two pairs of titanium dioxide coated samples that were exposed to salt spray test for three weeks with adhesive thickness of0.01 inch.
Tensile Test of coated SS316L with Cr 2 0 3 & Ti0 2 with adhesive thickness of ( 0.01 ) l•series1 I Figure 12. Comparison of failure load for the two pairs of chromium trioxide coated samples and another two pairs of titanium dioxide coated samples that were exposed to salt spray test for five weeks with adhesive thickness of 0.01 inch.

5
• 'Chromium DTitanium Figure 13 Average failure load for all coated samples either by trioxide chromium or titanium dioxide exposed to salt spray test on weekly bases with adhesive thickness of 0.005 inch.