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Understanding Fasteners

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Understanding Fasteners MABUILDINterialsG MAselectionterials UNDERSTANDING faSTENERS Fasteners are an essential building material for the industry, but do you know the difference between the various types of steel and stainless steel products? Used in the wrong place, they are not going to stand the test of time. By Zhengwei Li, BRANZ Corrosion Scientist asteners, such as bolts, nails and a homogeneous structure, and the interface Coating screws, are used in timber-framed between the coating and the steel substrate is construction to hold structural compon- sharp. The adhesion is governed by the surface Steel ents together and attach claddings cleaning process before plating. Coating Micro-structure of electroplated zinc coating on steel. The F interface between the coating and the steel substrate is sharp. to the frame. Holding power (withdrawal thickness typically ranges from 7–15 µm. resistance) and corrosion perform ance are Mechanical plating probably the most important concerns when Mechanical plating is a batch process carried out choosing fasteners. in a rotating drum. The steel part is tumbled in Coating Improperly specified fasteners can loosen a mixture of zinc dust, chemicals (promoter or Steel when timber shrinks and swells with moisture accelerator), glass beads and water. The coating cycling. Deterioration of fasteners from corrosion is impacted onto the steel surface by the cold Micro-structure of mechanically plated zinc coating on steel. not only weakens the fastener, but the chemicals welding of fine-powdered zinc particles through the generated by corrosion can also attack timber tumbling action. As a result, the coating consists η - Zn phase surrounding the fastener. This signif­icantly of flattened particles of zinc loosely bonded reduces the holding ability of the fastener-timber together. The adhesion of the zinc to the steel assembly. Corrosion resistance is particularly and zinc to zinc is weaker than the metallurgical ζ - FeZn13 phase important for fasteners in joints that are taken bond formed during hot-dip galvanis ing but is δ – FeZn10 phase apart and reassembled for maintenance. better than an electroplated coating. Coating γ – Fe3Zn10 phase thickness typically ranges from 5–70 µm. Steel 25 μm Range of metal fasteners Hot-DIP galvanising Fasteners used in building and construction are In hot-dip galvanising (HDG), items are Micro-structure of hot-dip galvanised zinc coating on steel. The made from metals including aluminium, brass, immersed into a molten zinc bath of 445–455°C zinc coating is integral with the underlying steel. copper, mild steel and stainless steel. for 3–6 minutes. The zinc coating produced Fasteners with zinc coatings are usually is integral with the underlying steel through of the electrochemical and mechanical processes. chosen when corrosion and staining resistance several hard iron-zinc alloy layers that are Localised failures are more likely, especially if are required. These coatings are applied onto overcoated with pure zinc. There is no real line of there are physical defects in the coating. fasteners by several processing techniques demarcation between the steel and the coating. Thickness alone doesn’t tell the whole story – electroplating, hot-dip galvanising and This gradual transition provides a metallurgical when comparing coatings – consider the mechanical plating. The structural properties bond. HDG coatings have excellent continuity amount of available zinc per unit volume. Hot- are different for each of these coatings, hence and uniformity, and any discontinuities are dip galvanised coatings have a higher density they react differently to their environment. readily visible as black spots. The coating has a with over 30% more zinc per unit volume than MILD STEEL dull and less smooth surface. Thick coatings of mechanically plated coatings and so should Mild steel, mainly made of iron, is the most 50–125 µm are attainable. offer about 30% better corrosion resistance widely used material. It has high formability, under identical service conditions. good tensile strength and a low cost. However, Hot-dip galvanised better Hot-dip galvanised coatings have a series mild steel fasteners are not durable outdoors. Corrosion of zinc is approximately linear between of iron-zinc alloy layers, while electroplated The inescapable fact is that iron in the presence mass loss and time. In other words, the service and mechanically plated coatings have a of oxygen and water will rust. life of a zinc coating usually increases in simpler homogeneous structure composed of Electroplating proportion to its thickness – a thicker coating pure zinc. The iron-zinc phases have a higher In electroplating, zinc coatings are applied by will last longer. Generally, electroplated and corrosion resistance than zinc in neutral and electrolysis in an electrolytic cell containing a mechanically plated zinc coatings have lower acidic environments. Also, without these hard solution of zinc salt and an electric current. thicknesses than hot-dip galvanised coatings. In abrasion-resistant iron-zinc alloy layers, more They have a bright and silvery appearance with addition, coatings on sharp edges, corners and structural defects (scratches or debonding) are a smooth surface. These smooth coatings have threads have lower thicknesses due to the nature likely to be introduced into the relatively soft 34 BUILD 117 April/May 2010 pure zinc layer when the fasteners are driven plating are significantly different in their structural in most atmospheres. AISI 316 is better still, into the timber. characteristics and corrosion performance. especially in marine environments due to its BRANZ tests have shown that mechanically Take care when buying fasteners – examine higher molybdenum content. plated nails are commonly inferior to hot-dip them to check they are the type needed. Common ferritic stainless steels for fasteners galvanised nails under identical conditions. are AISI 410, 416 and 431. AISI 430 and 430F Corrosion typically starts on edges and corners Stainless steel best choice are the best martensitic alloys for fasteners. where the coating is thinner. Although some Stainless steel is well known in industry for Ferritic and martensitic steels are mainly used accelerated tests are claimed to show that its excellent environmental stability. This for economic reasons and where corrosion electroplated zinc coatings have a comparable extraordinary characteristic is related to the resistance requirements are not too severe. corrosion resistance to hot-dip galvanised formation of a thin chromium-rich passive film Stainless steel fasteners are more durable coat ings, in use, it has been found that most on its surface when exposed to oxygen and than zinc-coated steel fasteners. Field testing fasteners with zinc plating will not provide water. Once formed, this film can largely inhibit carried out at BRANZ’s Judgeford site confirmed adequate corrosion protection in long-term reactions between the metallic substrate and that bolts, nails and screws made of AISI 304 external exposures. This is supported by the environment, resulting in a reaction rate that and 316 stainless steel showed very little degrad- AS 3566: Self-drilling screws for the building is several orders of magnitude lower. ation even in timbers treated with ACQ and CuAz and construction industries, that states screws DIFFERENT stainless steels preservatives, known to be more corrosive than with electroplated zinc coatings (passivated) of Stainless steels for fasteners can be divided CCA (see page 37). Stainless steel fasteners <12 µm thick should only be used in internal into three main categories: austenitic, ferritic also reduce the possibility of staining. and dry areas. and martensitic. These steels have different Lower cost over LIFETIME phase structures and compositions and so have The material cost for stainless steel fasteners Check what you are buying differing abilities to form the passive film. might be high compared with hot-dip galvanised Fasteners with a zinc coating are often marked Austenitic stainless steels are non- fasteners. However, the overall cost of ownership ‘galvanised’. This is misleading, as galvanised im- hardenable, non-magnetic and offer the will generally be lower as a result of the reduced plies items coated by hot-dipping. Zinc coatings greatest degree of corrosion resistance. Typical inspection and maintenance needed to ensure produced by electroplating and mechan­­ical alloys are the 300 series. AISI 304 works well the fasteners are still sound. BUILD 117 April/May 2010 35.
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