Organic coatings applied by "dip-spin" are competing with electroplates for protection and decoration of fasteners. Here's a look at why and how...
Fasteners--nuts, bolts, screws, washers, etc.--hold together a myriad of manufactured products--from toys to spacecraft and everything in between. Most fasteners are coated to protect from corrosion, to improve appearance, or to achieve some special property such as controlling the amount of torque required to tighten a threaded fastener.
It's big business for some finishers. An East-Coast job shop finishes over 100 million fasteners annually. A Detroit shop processes one million pounds daily.
The trends in fastener finishing differ from industry to industry. But ask about trends and you'll learn quickly that the trend setter is the automotive industry. It requires hundreds of millions of fasteners annually. World-wide competition has mandated vehicles that last longer in environments from sub-zero northern winters to tropical rain forests. Cars and their fasteners are subjected to deicing salts in slush on city streets; high temperatures in the vicinity of the engine; abrasion from sand and gravel thrown by the wheels; UV exposure, and wet-and-dry detergent-wash cycles associated with rain and car washing.
Fasteners are an important part of avoiding failures and expensive recalls. Quality has become much more important as fewer fasteners are used, to save weight.
So most of what we report here as trends in fastener finishing is really about trends in automotive-fastener finishing. But in many cases manufacturers of other products can learn from what automotive manufacturers are doing to improve the durability of their fasteners.
Before we look at trends in finishes and finishing methods, let's consider the forces that are driving change.
Cadmium plating is still in use for some automotive, military and aerospace applications, it should be noted, despite the long-ago conference to seek replacements. But even in these applications its use is declining. Cadmium plating on automotive fasteners has been replaced by other finishes. More about that later.
Now we are hearing about moves to eliminate chromium, another metal on environmentalists' hit list. Opel, part of GM in Germany, reportedly has plans to eliminate use of hexavalent chromium from its autos. This may spread throughout GM world-wide. Volvo has a test involving perspiration and how much chromium is leached in a given time-a worker-safety-related concern.
Elimination of chromium would be a problem for fastener manufacturers who depend upon chromate conversion coatings applied after zinc plating to prevent the early onset of white corrosion. There are substitutes, but none thus far is as versatile and as protective as the existing families of chromates.
Since very little electroplating is done inside automotive plants these days (most is out-sourced to fastener manufacturers and the contract electroplaters who do the finishing for them), why should auto manufacturers be concerned? Platers point out that the technology for dealing with pollutants in their plants is well understood, and regulated.
The answer, apparently, is that the producers want to be viewed as part of the "green" movement, actively involved in complete elimination of materials that could be toxic if they enter streams. They want to be ahead of the "green" movement.
The concept of the "world car" is a driving force encouraging use of identical materials and avoidance of certain materials wherever a multi-national company produces automobiles.
The auto companies understand this and are encouraging movement toward consensus specifications developed by SAE (Society of Automotive Engineers), ASTM (American Society for Testing and Materials), etc. Groups such as USCAR (United States Council for Automotive Research) and AIFG (Automotive Industry Fastener Group) are using the expertise of the automotive companies as well as their suppliers and finishers of fasteners to develop common specs, and standards for testing. Internationally, there is cooperation between European and North American suppliers to develop commonality.
For similar reasons auto producers are encouraging suppliers to use fewer different types of finishes, encouraging world-wide standardization and the economies of scale that would result.
Auto companies are reportedly moving away from dependence on this test, however, increasing their reliance on cyclic tests (alternating cold-hot-dry-moist-salt-water-immersion cycles). Part of the criticism of this test has been that some specs are calling for thousands of hours of exposure before failure. At the same time manufacturers want "just-in-time" delivery. So to meet these two needs a plater might have to keep a large inventory while he awaits test results. Cyclic testing meets this need, at least half-way.
Accelerated tests such as salt spray and CASS have much more validity in predicting what happens in short field exposures. Correlation of longer tests with longer field exposure is not good.
Test-track results in which vehicles are run through simulated conditions of use for extended periods of time are considered still more reliable, but they take far longer to get results. Some coatings perform better in salt spray than in cyclic testing or mobile exposure.
Bottom line, the auto companies want to tell their suppliers what they need in terms of performance and let the suppliers find the best and most economical way to apply it. But we're not there yet.
With the increasing use of aluminum and magnesium, the fastener finisher has to deal with avoiding bimetallic corrosion on a scale not seen previously. Wherever a steel fastener is in contact with a dissimilar metal, there is a potential problem of one surface corroding more rapidly than the mating surface, because of the formation of galvanic cells. Aluminum and magnesium in contact with steel will sacrificially corrode if not protected.
The car makers want to get away from on-site surveys. Instead they want their suppliers to meet requirements of QS 9000, an automotive-industry version of ISO 9000. To qualify, a supplier must pass a quality audit from an outside auditor, proving that he is producing and continuously improving quality. Many of the finishers who plate or coat fasteners for automotive Tier 1 and 2 suppliers will have to meet these requirements.
The Act requires that testing also be done by laboratories certified by the National Institute of Standards and Technology (NIST). Because there are not enough laboratories certified as yet, implementation of the act has been delayed, at least until May 27, 1998. When applied, the act may impose additional paperwork costs on finishers.
It wasn't that many years ago that when you spoke of finishing fasteners for automotive, you really were talking primarily about electroplating. Most of the electroplating was zinc, followed by chromate conversion coating to delay the onset of white corrosion of the zinc. Most of the remaining electroplating was cadmium, also chromated. For interior fasteners and under-hood parts, manufacturers specified a lot of phosphate-and-oil. And some under-hood and under-body fasteners were simply painted black, producing enough corrosion resistance hopefully to get out of the show room with not too much rust showing. Indeed there were fasteners that were not coated at all-they were heavy enough to corrode for a long time before failure, and red rust locked the fastener in place.
Zinc plating is still the most common finish for automotive fasteners. But to meet today's salt-spray test requirements (see Table of Specifications) thickness of some electroplates has been increased by 50 to 100 pct-from 0.2 mil to 0.3 or 0.45 mil; and organic coatings may be applied over the zinc plate to further extend corrosion resistance.
You will note as you examine the specifications for plated finishes that 0.2 mil is still in a number of the specs for zinc plate. But platers point out that they may have to apply 0.35 mil to assure application of 0.2 mil minimum at the point where the measurement must be made-which may be a low-current-density area. Suppliers point out that alkaline zinc plating baths plate more uniformly, and in some cases this may help to provide the minimum thickness required.
Phosphate/oil. There is still some phosphate and oil, used alone or as an undercoat on fasteners that will be painted. But some of it has been replaced. One reason is that the oil used was transferred to hands and gloves of assembly-line workers, which in turn soiled upholstery and other surfaces inside the car. This was exacerbated by just-in-time delivery, which meant that the oil on these fasteners was fresher and there was more of it. The oil also damaged some plastic parts, and caused problems in automated-feed bowls. There is now a specification requiring that phosphate-and-oil finishes be dry to the touch.
Zinc Alloys. Electrodeposition of zinc alloys has been used more extensively in Europe and Asia than in North America, but its use is increasing here, mostly for automotive applications. It is estimated that 10 pct of zinc platers now apply alloy zinc as well.
Some alloys are specified by U.S. manufacturers seeking a replacement for cadmium. Some are specified because corrosion resistance of the alloys is much better than that of non-alloy zinc-three to six times as much salt-spray resistance as straight zinc. These results have been confirmed by four-year mobile environmental tests in Europe. It may be possible to apply thinner zinc alloys while meeting the corrosion-resistance specs of all-zinc plates, thus improving productivity. There is also interest in the aerospace industry, for the same reasons.
Electroplaters are using zinc alloyed with nickel, cobalt and iron as fastener finishes. The alloys provide more corrosion resistance than all-zinc plates of the same thickness. As with conventional zinc plating, commercially available solutions can be acidic or alkaline in the case of the cobalt and nickel alloys, while the iron alloy is available only as an alkaline solution. And as with zinc plating, there are advantages and disadvantages for each. The acidic solution is better for plating of hardened or heat-treated steel fasteners, for example, while the alkaline bath provides more even deposit-thickness distribution, but plates at only 60 pct efficiency. There are applications for which each type of bath is better suited.
The zinc-cobalt alloys provide optimum corrosion resistance at 0.4 to 1.0 pct by weight cobalt in the deposit. Cobalt content lower or higher than that does not improve corrosion resistance.
Zinc-nickel alloys are similar in that they can be plated from acid or alkaline electrolytes, but different in that the percentage of alloying element is considerably higher. The range is six to 20 pct for acid solutions. There are alkaline baths that plate in the range of five to 10 pct, and others that work in the range of 10-15 pct. The higher percentages of nickel reportedly improve corrosion resistance over that provided by the cobalt alloy of the same thickness.
Zinc-iron alloys are normally plated from alkaline solutions. Iron content is in the range of 0.4 to one pct. These deposits readily accept yellow and non-silver black chromates. The mechanism of corrosion protection is somewhat different from that of the other chromated zinc alloys. The iron in the deposit migrates into the chromate film and reacts with it to produce a corrosion resistant film.
There is also a tin-zinc alloy for fasteners used in electrical applications. It accepts a chromate and can be bent or crushed while maintaining corrosion resistance.
One plater applies two layers of electroplate--a proprietary nickel plate followed by a zinc-nickel alloy. These fasteners then receive a black electrocoat paint. In another version of this he applies the nickel plate, then a zinc plate and a black chromate. Both finishes are applied to meet certain GM specifications.
Electrodepositing alloys requires more careful control, regardless of the metal being plated. The electrolyte composition and the operating conditions--temperature, anode placement, agitation, etc.--must be monitored more carefully to assure that the percentage of alloying element is what is specified. It is important to control deposit thickness as well. And while some baths offer easier control of alloy constituents than others, plating shops applying the alloys should have analytical equipment such as atomic absorption spectrophotometers (AA) as well as reliable thickness-testing equipment. If the alloy-metal content and the thickness are not within the ranges required, corrosion resistance will suffer. Inconsistency can be a problem.
Because of the additional investment required, not every zinc plater has rushed in to provide alloys as well. It's a special niche that some platers have carved out for themselves in hope of gaining competitive advantage.
The situation is further complicated by the fact that specifiers have not come to consensus on which of the alloys should be specified. One European auto maker has decided that zinc/iron best meets his needs; another will accept only zinc/nickel. Thus a plater who wants to meet demands of more than one auto manufacturer may have to install more than one type of solution and train his workers to operate the different solutions-an expense that may be difficult to recover in the competitive automotive-parts marketplace. Nevertheless, use of alloy plates worldwide has expanded through the 1980s, and this is expected to continue.
Chromates. Both zinc and zinc-alloy electroplates require application of chromate conversion coatings, to improve corrosion resistance. These post-plate chemical treatments delay the formation of white corrosion products that form as zinc begins to oxidize. Red rust does not form until the zinc has been penetrated to the basis metal, and this is a function of the thickness of the plate and of its ability to provide sacrificial protection of the steel.
Zinc-nickel alloys are somewhat more difficult to chromate than all-zinc plates. They require special chromate treatments.
The colors of chromate coatings and their protective qualities vary with the type of chromate and with the deposit being treated-whether zinc or zinc alloy. Many of the fasteners one sees as a consumer are treated with a "blue-bright" chromate that approaches the appearance of chromium plating when applied on a very bright zinc plate. But chromates also can produce yellow, olive drab, black and green finishes, some of which are more protective than the common "blue bright." In applying zinc electroplates, finishers say the yellow chromate is the most commonly applied, followed by black, green and clear coatings.
While alloy plates can be chromated, the appearance may be different from what would be expected in chromating all-zinc plates. The traditional yellow chromate, for example, may be bronze, purple and iridescent when applied on alloy deposits. This does not mean it is less protective. It is simply different in color.
Some manufacturers specify the use of special post-plate treatments when a zinc plated steel fastener is to be in contact with magnesium. A yellow chromate coating is applied, then a proprietary alkaline silicate solution is used as a leachant. The latter improves corrosion resistance and minimizes the damage resulting from galvanic corrosion between a zinc plated steel fastener and a magnesium part.
In automotive applications there is strong demand for black fasteners. In the case of all-zinc deposits this is achieved by the use of silver additives, which produce black in the chromating process. General Motors is seriously considering elimination of the use of silver additives, however.
Cobalt, nickel and iron alloys can be chromated to produce a black color without the addition of silver, and the corrosion protection offered is better than that of the silver-containing chromates used on all-zinc deposits.
Blue-bright chromates are generally not recommended for use on zinc alloys. Only minimal improvement in corrosion resistance could be expected.
Mechanical plating. Fasteners for automotive applications are mechanically plated with zinc and zinc alloys (see specifications). Mechanical plating is a process for applying one metal over another as a coating, without the use of electrical current. It is done by tumbling the parts--fasteners in this case--in a mixture of glass beads, "promoter chemicals," and metallic particles. The glass beads impact the metallic particles and the fasteners, pounding the particles against the surface being plated. The result is "cold welding" of small metallic particles onto the fasteners. The process also has been referred to as "peen plating" and "mechanical galvanizing."
An advantage of mechanical plating is that the process itself does not cause hydrogen embrittlement. In electroplating, hydrogen embrittlement can cause fasteners to fracture under load, leading to catastrophic failure. Risk of embrittlement increases in electroplating of high-strength steels. Thus a process that does not in itself engender embrittlement is helpful. Indeed, certain high-strength steel used for aircraft applications are typically mechanically plated rather than electroplated. It should be noted, however, that while mechanical plating does not cause embrittlement, the acids used in pickling and pretreatment can. Thus, baking to provide hydrogen-embrittlement relief still may be necessary, although there is less tendency for hydrogen to be entrapped than if the metal coating is applied by electrodeposition. It is easier for hydrogen to escape through a mechanical plate.
In passing we should note that hot-dipped galvanized fasteners have been widely used outside of the auto industry, particularly in construction. Zinc thicknesses exceeding two mils provide excellent corrosion resistance in these applications, although re-cutting of threads may be necessary when such heavy coatings are applied, and this may leave the threads bare.
Dip-Spin Coatings. The greatest change in fastener finishing for automotive applications is the increasing acceptance of so-called dip-spin finishes. This is not new technology, but dip-spin coatings have been improving and increasing their share of the market for fastener finishing.
A typical dip-spin procedure begins by pretreating the fasteners. Abrasive blasting followed by alkaline cleaning may be the procedure. Or a multi-stage clean/phosphate cycle may precede paint application. Sometimes an electroplate is applied before dip-spin processing. It depends upon the intended use of the fastener.
The pretreated fasteners feed into a basket, which is securely attached to a spin platform. A coating container rises to submerge the basket of fasteners. After a preset immersion time, the basket lowers half way, so that the fasteners are no longer submerged, but the paint container above the liquid level still surrounds the basket. The basket drains and the equipment spins the basket to centrifuge excess coating material from the fasteners. The excess coating material flows back into the paint container.
The equipment may incline the basket or otherwise change the position of fasteners, so as to move fasteners from the center of the load, where there is no centrifugal force, to the outside. This helps to assure better draining of all parts.
The basket empties onto a wire-mesh-belt conveyor. The latter carries the fasteners through an oven where relatively low temperatures--perhaps 275F--evaporate solvent, water or other volatiles. Then fasteners enter a second zone where the coating cures at higher temperatures--perhaps 350-425 for 10-15 min.
In most cases a second coat will be applied by the same procedure, and in some cases, a third coat. The undercoats may provide the basic corrosion resistance while topcoats may produce lubricity, color, appearance or other properties specified. The multiple coatings also aid in covering voids formed by the parts touching one another on the belt conveyor.
Some of the coatings used are simply specialized organic coatings, offering the properties of epoxies, acrylics or whatever the resin system. One supplier claims much improved corrosion resistance as a result of applying an amorphous silica coating over chromated zinc and zinc alloy electroplates. Other coatings are packed with zinc or aluminum flakes and in some cases, chromates as well. The metal-containing coatings offer higher electrical conductivity and perhaps more importantly, sacrificial protection of steel, to increase corrosion resistance.
Other dip-spin coatings are water-borne dispersions, also containing metallic zinc and aluminum platelets and/or chromium. Some of these cure at over 600F, and their manufacturer characterizes the cured coatings as "inorganic."
Silicone topcoats applied over basecoats containing ceramics in a binder also are applied, for high-temperature applications--fasteners in engine compartments or close to exhaust systems, for examples.
Electrocoat. Application of organic coatings by electrodeposition or electrophoresis, from waterborne coating materials, is also a part of today's fastener finishing. This method of applying organic coatings has the advantage of very even deposit-thickness distribution on complex shapes. Anodic electrocoats are applied on bulk fasteners in barrels. Cathodic e-coats may be applied in baskets or barrels. In both cases fasteners may have been electroplated or pretreated by phosphate conversion coating. Electrocoating is one way to apply the black coating so widely requested by auto manufacturers.
In the roofing industry finishers electrocoat large quantities of fasteners cathodically after pretreatment in an eight-stage washer. These fasteners are one to 20 inches long. The coatings are 1 to 1.2 mils thick. Application of multiple colors aids users in identification of specific types of fasteners.
"If someone could develop a thin, dry, highly corrosion-resistant, cosmetically attractive, durable, metallic, chemical-resistant conductive finish that would provide accurate torque/tension performance and withstand temperatures over 1000F, they could corner the automotive finishing market,"1 says Brian Lowry of Curtis Metal Finishing Co.
But there is no such finish. As is obvious, there are many combinations of finishes applied on today's fasteners. Some of these combinations offer tenfold improvement of corrosion resistance as measured by salt-spray tests, but they may not provide one of the other qualities required by automotive customers.
If cost were no concern it would be relatively easy to improve the quality of coatings. Consider application of an alloy electroplate followed by a heavy chromate and then a special organic topcoat applied by dip-spin. Corrosion resistance should be outstanding, as should resistance to abrasion, heat, automotive fluids, etc.--assuming that the inorganic and organic topcoats were selected carefully for the intended application.
But this is not in the real world. Reality is that automotive companies ask their suppliers to reduce their prices by five pct annually. Some cynics will tell you that price is still more of a determinant than quality.
So finishers and their suppliers labor to find combinations of finishes that perform better but cost less. That's not easy if we remember that auto companies want continuous improvement in quality as well as lower cost.
The cost of applying fastener coatings varies from one type to another, but in general the least costly is a zinc electroplate. And while finishers have to apply as much as twice the thickness of zinc to meet today's specs, their cost for doing this has not doubled. But it has increased by perhaps 25 pct in those cases where they've had to double the thickness to assure a specified minimum thickness.
The most expensive coating is an aluminum-rich organic, and the alloy zincs are on the high side of the middle ground.
There is increasing acceptance of alloy electroplates and of dip-spin coatings for fasteners. Corrosion resistance is much better, and the alloy plates with the proper topcoat provide lubricity similar to that of cadmium plate. Thus the tools used to assemble fasteners that used to be cadmium plated can be used with minor modifications.
The increase in use of organic coatings is occurring more in exterior applications, particularly underbody fasteners, than in finishing fasteners for interior applications. And these changes are more likely when design changes occur. Parts that have been plated usually continue to be plated.
There is also increasing use of dip-spin coatings over electroplates. One manufacturer of dip-spin coatings estimates that 40 pct of his coatings for fasteners are applied over electroplates and 80 pct of the remainder is applied over phosphate-coated steel.
But zinc electroplating is still by far the most-used coating. Platers who offer all three coatings typically still have 50-60 pct of their production in barrel zinc electroplate; 20-30 pct is alloy zinc; and 10-20 pct is dip-spin. This is not an industry-wide average, but it does give some indication of what customers choose when all three finishes are offered.
Beyond cost considerations, there are choices to be made in terms of coating properties. Aside from considering the obvious differences between electroplated metals and organic coatings, the buyer has to deal with the fact that organic coatings tend to fill recessed drives - Phillips head and Torx fasteners--thus making it difficult or impossible to use the tools meant to drive them.
Screws have roots (the valleys) and crests (tops of the threads). Electroplates build up more rapidly on the crests, and thus have more tendency to interfere with torque-tension relationships if applied too heavy. Not so for organic coatings.
The organic coatings do not offer the galvanic protection of zinc electroplates unless heavily loaded with zinc or aluminum flakes, which in turn demands more expensive materials and more expensive processing.
Electroplating contributes to the danger of hydrogen embrittlement. Yes, the fasteners can be and are baked to relieve hydrogen, but that is an expense. Organic and inorganic coatings applied by dip-spin eliminate this hazard. Mechanical plating also minimizes this hazard, although regardless of the coating applied the danger of embrittlement remains if fasteners have to be acid pickled before application of the coating.
Chromate coatings applied on electroplates are not very abrasion resistant. If gravel impingement or abrasion from handling is a consideration, it may be necessary to apply an organic coating just to increase abrasion resistance; or to apply a metal-flake-containing dip-spin coating over a properly prepared surface. If color is important for appearance or part identification, chromate coatings alone may not be able to provide the color desired.
All these coatings have ideal applications. Some combinations of coatings are synergistic. The specifier's task is to sort out what is really important for a given application in terms of cost and performance.