Corrosion Analysis of Stainless and Plated Fasteners

COMPARATIVE ANALYSIS OF STAINLESS AND COATED CARBON STEEL FASTENERS

Corrosion of stainless and carbon steel are very different, and this guidance provides information regarding general attack or surface corrosion as this is often a primary consideration when selecting fastener material.

Carbon and stainless steel are the most common materials from which fasteners are manufactured and each is available in hundreds of alloys or grades with a wide variety of attributes.

Initial cost of a stainless steel component is generally higher than carbon steel though not always. For example – material is a smaller component of cost in a light duty Coiled Pin than it is in a Solid Pin of the same diameter and length. As a result the stainless steel Coiled Pin may be available at equal or lesser cost than the carbon steel Solid Pin. In addition, secondary processes, such as heat treating or plating, often add to the cost of carbon steel parts when they may not be incurred with stainless steel.

Cost is meaningless without consideration of value. For example, the manufacturer of an outdoor barbecue grill may select stainless steel fasteners capable of resisting corrosion much longer than the product’s expected life span. This selection would demonstrate commitment to product integrity, cosmetic appearance, and long life.

The ‘value’ of a fastener that provides maximum quality may offset any associated cost increase. Designers must weigh cost, benefit, and risk when choosing the appropriate fastener material.

Though carbon and stainless steel are both ferrous metals, their response to corrosive attack is much different. By definition stainless steel must contain at least 10.5% chromium. When exposed to oxygen this alloying element creates a layer of chromium oxide at the surface that quickly stops growing thus becoming ‘passive’. This passive layer is continuous, uniform in thickness, insoluble, and nonporous. The passive layer prevents contact between oxygen in the environment and base metal and will self heal if scratched or abraded as long as oxygen remains available.

The passive layer is only 10 to 100 atoms thick and as such has no dimensional impact on parts. Though stainless steel can corrode when exposed to some chemical agents under specific conditions, it will not rust by uniform or general attack as does carbon steel.

As an alloying element, the chromium is part of a homogenous blend and is deemed safe as it cannot be easily liberated from the alloy.

Stainless steel is 100% recyclable and industry analysts estimate 80-90% of discarded stainless steel is captured for recycling.

Rust occurs in iron and iron alloys such as steel. Rust is a layer of iron oxide created at the surface of a part when exposed to oxygen in the presence of moisture. This layer of iron oxide remains active and continues to convert iron to iron oxide as the outer layers lose integrity and fall away exposing new metal.

Iron oxide is also porous allowing it to absorb moisture and elements that may contribute to corrosion thus extending the period of active corrosion with each exposure. To prevent the formation of iron oxide or rust it is necessary to eliminate exposure to oxygen and moisture.

Carbon steel may be protected from corrosion by painting, plating, or coating. Paint is generally not appropriate for fasteners as it tends to be a rack rather than bulk process and is therefore not cost effective. Plating and coating are the predominant methods of preserving carbon steel though some of these processes have fallen out of favour due to their environmental impact. Examples of finishes considered hazardous are cadmium and hexavalent chromates. There are now many coatings capable of providing excellent corrosion resistance while satisfying current environmental regulations. Despite these advances, the base metal’s susceptibility to rust remains an Achilles heel. The vast majority of coatings are sacrificial, meaning they only provide protection until they are depleted. Once base metal is exposed, it will rust.

The most widely accepted method of corrosion testing is the salt spray test. The intent of salt spray testing is to provide a repeatable method by which a material or finish’s response to corrosive attack can be evaluated. In theory this also provides a means to compress life cycle testing into a practical period of time. It is important to understand that such comparisons may be grossly inaccurate though this method of test remains one of few choices available. No accurate correlation exists between real world conditions and time spent in a salt spray booth.

While many manufacturers now assign salt spray testing to stainless steel product; it is primarily intended for carbon steel parts. Stainless steel is generally tested in a humidity chamber without salt.

Salt spray test requirements generally consist of two simple criteria – the hours it takes for white corrosion to form and subsequently, the hours it takes for red rust (or base metal) attack to begin. Salt concentration, temperature, and time are carefully controlled.

The majority of coatings rely upon a layer of nonferrous metal applied over the base metal followed by a chromate dip and depending upon performance requirements, an organic or inorganic sealer. When these finishes fail, they do so opposite the order in which they were applied. Once the corrosive environment breaches the outer sealer and chromate layers it begins to attack the nonferrous metal beneath. At this point white corrosion begins.

Nonferrous metals such as zinc, nickel, aluminum, and magnesium do not contain iron and will not ‘red rust’.

 

Red rust only becomes visible after the sacrificial metal has been depleted.

An issue with both electro and mechanical plating is the difficulty associated with plating in deep holes and significant crevices. Unlike mechanical and electroplating, coatings applied as a liquid are capable of proper coverage in a part’s interior.

Hundreds of billions of plated fasteners have been utilized successfully over the years despite these issues. If a fastener is fully installed in a host material, it may be largely protected from attack.

In terms of galvanic corrosion, the finish need not be uniform to provide protection as it will be sacrificially depleted to protect the base metal as long as current can flow from one to the other. For example, a steel boat hull can be protected by bolting sacrificial zinc anodes in strategic locations – it is not necessary to galvanize or zinc plate the entire vessel.

The advantage of stainless steel remains its ability to resist corrosion without the aid of protective finishes. Corrosion resistance is provided by chromium and this is distributed throughout the alloy. Stainless steel can corrode and fail though it does not rust due to general attack or surface corrosion.

Pitting is the most common form of corrosion affecting stainless steel. Pitting may occur when environmental agents or mechanical abrasion/scratching defeat the passive layer under conditions where it cannot spontaneously reform. Local attack can then occur.

An alternate form of attack generally spread over a larger area is crevice corrosion. This form of attack may occur where there are sharp inside corners or where components touch in a manner that creates potential points of fluid deposition. Good design practices can minimize crevice corrosion, though in many instances it is inherent to the intended function of an application.

It is also possible to improve corrosion by moving to other stainless steel alloys.

Common methods used to increase resistance to pitting are the addition of molybdenum or higher concentrations of chromium and/or nickel.

Increased corrosion resistance typically elevates cost and should therefore only be pursued when necessary. Austenitic stainless steel provides the best corrosion resistance. The pairing of chrome and nickel in the proper ratio allows for the creation of an austenitic structure.

In summary, though great advances have been made in regard to coatings available for carbon steel they remain susceptible to corrosion. It is a matter of when, not if, the finish will eventually fail.

Stainless steel is often associated with higher cost, although the cost of failure can be much greater. The intended environment and life expectancy of a product must be given adequate consideration and the appropriate material and/or finish selected to ensure success.

It is also important to evaluate alternative solutions whenever possible. Reducing material volume by moving to smaller sizes can significantly reduce weight therefore lowering cost. Carbon steel does not always present the lowest installed cost solution and ‘value’ should always be considered. Stainless steel is not impervious to attack and the host materials and environment must always be evaluated carefully to ensure the correct grade/type is used.

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