JAMES 0. SCHRECK
filtrotes e residues
University of Northern Colorado Greeley, CO 80639
Investigating Protective Coatings for Steel Tom Runyan Monroe High School, Elm & Butler Streets, Monroe, OH 45050 Mary Herrmann University of Cincinnati-Raymond Walters College, 9555 Plainfield Road, Cincinnati, OH 45236
I t has been estimated that 25% of all the steel made in the United States is used to replace steel lost by rusting (I).Rusting or corrosion of steel is a common chemical reaction familiar to everyone. This familiarity and the fact that the reaction occurs easily and is safe make it a n excellent reaction for student investigation. antimrros. Most steel made is coated with a ive material (e.g., zinc metal, paint) before it is used. This experiment deals with methods to inhibit corrosion by applying a protective coating to bare steel and then allowing corrosion to occur. Students become actively involved as they make their own choice of protective material and
method of application. Students then learn a technique to evaluate quantitatively the effectiveness of their protective material and compare their results to others. Corrosion Chemistry Corrosion of iron (steel is mostly iron metal) occurs when iron is in an environment containing water and oxygen. Chemically, it is the result of two half-reactions. One is the oxidation of iron that occurs in areas of the metal surface referred to as the anodes (see diagram), The reaction at the anode is (2): Fe(d +FeS(aq) + 2 e(1)
This half-reaction is coupled to the reduction half-reaction (below)occurring at other regions ofthe metal surface referred to as the cathodes. Oz(g)+ 2Hz0 + 4e-+4OK(aq)
// '
t
Fe,O,. x H,O(s) Cathode:
'TI
/ ' I
Fe(s)-+FeE*(aq) + 2ee-
(2) The net result of the two half-reactions i v e n above is the formation of iron(I1) hydroxide [Fe(OH)d. 2Fe(s)+ Odd - + 2Hn0 2Fe(OH)ds) (3) The anodic reaction cannot occur without the cathodic reaction. Electrons produced a t the anode flow through the iron metal to the cathode where they are consumed. Electrolytes in the water complete the electric circuit. Rusting is faster where salt is present, because the ions from the salt serve as a catalyst by aiding the
Diagram of corrosion process in iron or steel.
-
Volume 70 Number 10 October 1993
843
movement of electrons and the ions that are involved directly. The iron(I1) hydroxide is then further oxidized by oxygen to form rust (Fe203.xH20)or iron oxide. 4Fe(OH)2(s)+ Oz(g)+ 2H20+ 2Fe2O3.3H2O(s)
(4)
The reactants for eqs 3 and 4 clearly show the three factors that must come together for corrosion to occur (iron metal, oxygen, and water). There are many types of localized corrosion (e.g., crevice, stress). The type of corrosion observed in this experiment is called pitting and is the most common (shown in the diagram above) (3).With pitting, the actual loss of iron metal is c o n h e d to small depressions (pits) that are the anodes of the exposed surface. Once a pit has been formed it continues to grow. Corrosion seldom progresses uniformly on a metal surface. Certain focal points usually serve a s initiating sites for corrosion, such a s soiled areas or areas of scale or rust on the surface of the metal. On a clean metal surface corrosion generally will take longer to occur but may begin a t areas of the metal called grain boundaries. (Agrain boundary is a break and reorientation of the grain lines that make up the metal crystalline structure.) Imperfections in the metal, such as areas of stress, also can serve as sites for corrosion onset.
Corrosion Prevention There are many ways of preventingor reducing corrosion of steel such a s alloying other metals (e.g., stainless steels) and cathodic protection (connecting iron to a more reactive metal). The most common method used is the application of protective coatings that block the access of atmospheric oxygen and water (needed for the reaction) from the iron surface. This is the method used in this experiment. Protective surface coatings can be metallic, inorganic, or organic. Coating metal surfaces with organic materials is by far the most important of all methods for corrosion prevention (4). Such coatings include paints, lacquers, enamel, grease, oil, wax, plastic, rubber, and others. Sometimes chemical inhibitors such a s ehromates, phosphates, or lead oxide are added to the organic coating in order to slow either the anodic or cathodic process. Perhaps the best anti-rust treatment is a primer coat containing a n inhibitor, followed by a topcoat. I n general, for a coating to be effective, it must block oxygen and water from the surface and also adhere well to the metal surface. Experimental Materials White vinegar full strength or 4% aqueous acetic acid Petri dishes (glass or plastic or similar container) Paper towels Medicine droppers Plastic disposable gloves (optional) Dishwashing detergent Coating materials (supplied by students) Straws, filmcanister caps, or other suitablenonmetallicsupports for metal pieces in Petri dishes (optional) Flat pieces ofcnld ndled, uncoated steel approximately 5 cm x 5 em or 2 in. A 2 i n . , Pusviblesources ofthesteel arc metal shops at schools, metal specinlty shops, steel pmcsmws or distributors, dealers in scrap steel, denlers rn sheet metal. 5 mm x 5 mm clear acetate grid* subdivided into 100 blocks made hy photocopying 3 squares-prr-inch ~tandardgrnph paper onto a sheet of acetate) l-in. roll of masking tape
. 844
Journal of Chemical Education
Gening Ready At some time before performing the experiment, the corrosion reaction-especially eqs 3 and Pshould be presented to the students. Teachers can choose their own level of depth for this, but students must be familiar with the factors necessary for corrosion (iron, oxygen, water). This experiment is well suited to follow other corrosion activities especially one that investigates the factors that cause corrosion.
A Suggested Simple Experiment Place two nails (size 4 or 6 uncoated) into each of five different clean test tubes. Label the test tubes A, B, C, D, and E. Add the materials below to the designated test tube. A. Add anhydrous calcium chloride to cover 314 of the nails and stopper the test tube. B. Add freshly boiled water (distilled,if available)to cover 314 of the nails and stopper. Note: The water should be allowed to boil for at least 5 mi". to remove dissolved gases, inrludmg oxygen. Cover uf the water aftcr h n h g to mmimize thr r~di~solution gases. C. Add water (distilled,if available)to cover about 314 ofthe nails. Do not stopper. D. Add a salt solution (5% NaC1) to cover about 314 of the nalls. Do not stopper. E. Leave this test tube empty except for the nails. Do not stopper. ~~~~
~
Allow the test tubes to remain for a few days and then examine them for the presence of corrosion. Students should find that iron, oxygen, and water all must be present for corrosion to occur, present in test tubes C and D. Now students better appreciate what is needed for corrosion to occur and they can formulate ideas about what might prevent it. Afew days prior to performing the experiment, students are informed of the procedure and are told that they need to supply a protective coating for steel. Students may want to do library research on the different types of corrosion inhibitors before they choose. Some possible resources for the students are references 5-8. The coatings chosen should be screened by the teacher prior to use. Some common coatings chosen are paints, glues, waxes, and fingernail polish. .Caution: Spray paint and epoxy glue application^ require well-ventilated work areas.
Procedure 1. Clean each steel sample with dishwashing detergent to remove the thin protective oil coat. Dry thoroughly. If available, use gloves to avoid leaving fingerprints on the clean metal surface. 2. Cwer one half of caeh metal plece with masking tape. 3. Apply the coating material m the top and edges of rhe ex~ o ~ metal e d surface. Allow the coating to cure overn~ghtif glue or a paint type material is used. 4. Remove the masking tape after application of the coating material. 5. Put approximately 5 mL of vinegar into each Petri dish. 6. Place each sample in a Petri dish on a 5 cm x 5 an piece of paper towel. The paper towel produces an even distribution of liquid and prevents splashing. Alternate Step 6. As an alternate to the paper towel, two straws bent in a triangular shape or film canister lids can be placed in each Petri dish and the sample placed on top. The su~oortsk e e ~the metal viece above the vineear of an acid atmomhe&. thereb;&monstritine the eff& , 7. Put the top on the ~ e Gdish i and set all uf the dishes in a similar enmronment to avwd mtroducing other variables. ~
~~
~~~~~~
~
~~~~~
8. Observe the pmgress of corrosion on the metal plates. To quantify the corrosion rate, an acetate grid is placed di-
rectlv on the Petri dish lid and held in dace with a small piece oftransparent tape. The number of grid squares in w h ~ corrosion h is seen are counted and mterrd onto data shwts at regular intervals, probably daily, though results may be seen within one hour. Additional observations of a qualitative nature (such as wlor or blistering)may be entered onto the data sheets. Replenish vinegar in measured amounts as needed to maintain a steadily acidic atmosphere. Comparisons in the rate of corrosion across the surface of different s a m ~ l emav s then be made on a continuine basis. (Differencesebuld be ibserved even when two stud&ts use the same coatine because of the im~ortanceof cleanine the surface prior to applying the mating, and the application itself.)
-
-
Variations 1. Any ofthe following could serve as the variable in the experiment instead of the protective material. A. Method of applying the antimrmsive material B. Temperature or sunlight
C. Type of corrosive environment (vinegar, salt water, degassed water, oil) D. Concentration of corrosive agent E. Atmosphere gases in a closed versus open-air system using Zip-loc bags as enclosures F. Pre-cleaning methods G. Heat treatment of metal 2. If balances are available, students can determine the mass of their dried samples over time. These data can be plotted as total mass change versus time on a daily basis. More advanced students can carry the experiment further by calculating the mass of iron oxide formed. Results and Discussion All of the factors necessary for corrosion (iron, oxygen, water) are present in the experiment. Acetic acid (vinegar) that speeds up the corrosion is used to provide acid (Hi) process. Vinegar also provides ions creating an even more corrosive environment, a similar effect to that of salt on a highway or salt water near the ocean. As a result, changes can be observed fairly rapidly. Indeed corrosion of the uncoated side of the metal, which serves as a control, should
begin within an hour! For the coated samples corrosion mav be observed within a dav and can be extensive after one week unless the sample"is well protected. Corrosion also occurs rapidlv when the metal is k e ~above t the liauid. Contact betiee'the vinegar and theAmetalare no; required for corrosion to occur. The vinegar evaporates in the Petri dish producing a corrosive atmosphere that attacks the metal surface. Results are quite variable and sometimes surprising (for example, lipstick works quite well a t preventing corrosion). Students gain a good appreciation of the factors important to corrosion protection. They should be challeneed to tr?. to explain why some coatings prevent corn~sinnbetter than others. A list ol'possible explanations include: (1) adhesion of coating to the metal (2) permeability of the coating to air and water (3) corrosive components contained in the coating (4) amount of time required for coatings ta cure (5) number of coats applied ( 6 ) thickness of mating
What is o e r h a ~ even s more beneficial to the students is that they iearn about the corrosion process itself. The emphasis on the prevention of corrosion leads them to pay particular attention to the components in the corrosion reaction. Students become familiar with this chemical reaction, because they observe it occurring and try to prevent it. Students enjoy the opportunity to provide input to the experiment and the chance to compare their results with others. The challenge is much like that faced by manufacturers of thousands of steel-containing products in this important industry. Literature Cited 1. J. Chem. Edue. Staff. J. Chem Educ. lS?g,56.673. 2. Walker, R. J.Chem. Educ. 1982.39.943. 3. Celdran, R.; C o n d o , P. J. Chem. Edue. 1988, 65, 730. 4. Wrangler, G . An htrodvcfion to Corrosion ond Pmktlon of Mefols: Chapman aod Hall:London. 1985. 5. Bormughs,Tom.Chsrnmoffers,April 1985. 6. Sci. Tech. Soc. SATIS 1987,No. 103. 7. Wonder Sci. ACS 1990,Vol. 4, No. 5. 8. Concise Ency~lopdioof the Scknes; 146. Van Nostrand Rinehold Co.: New Yorh 1918, p 146.
Volume 70 Number 10 October 1993
845
