Fastener failures occur when the wrong fasteners are being used for a project, or the right fasteners are being treated incorrectly. Fastener corrosion is one of the most prevalent issues causing fastener failure. Let’s go over the what corrosion is, types of corrosion and how to avoid it in your project.
How Fastener Corrosion Occurs
Corrosion is the destructive alteration of metal by chemical or electrochemical reactions within its environment. Corroded parts fail for a variety of reasons including:
- Loss of mechanical integrity: corroded areas do not possess mechanical strength
- Fatigue: Corrosion site causes cracks in the fastener
- Loss of surrounding clamped materials: the material the fastener is clamping erodes around the head of the fastener
- Stress corrosion: exposure to harsh environment causing embrittlement
Types of Fastener Corrosion
There are two main categories of fastener corrosion: a direct chemical attack or electrochemical corrosion.
A direct chemical attack occurs when the material the fastener is made out of is soluble in its environment. This typically happens in harsh environments, like the oil and gas industry. To prevent this, make sure to choose a fastener material that is unaffected by solvent, or choose the proper coating to withstand the solvent.
Electrochemical corrosion encompasses many types of corrosion, and it results in many small electrical current flows through the fastener. This process results from the contact of two different types of metal or the presence of atmospheric oxygen within the presence of an electrolyte. Here are a few types of electrochemical corrosion you may encounter:
- Galvanic Corrosion - caused when two different metals are in contact, resulting in pitting or erosion damage.
- Stress Corrosion Cracking - caused when tensile stressed parts are in a corrosive environment. There is too much tension on the fastener, making it more susceptible to failure. Stress corrosion cracking can happen much more quickly than the other types of corrosion.
- Crevice corrosion - occurs in the crevices of a fastener that isn’t properly ventilated. This happens any place dirt or moisture accumulate. This is one of the more dangerous types of corrosion, as it often isn’t caught until after the damage has occurred.
- Uniform corrosion - occurs due to improper coating of the fastener, causing rust that makes the fastener difficult to replace.
- Pitting corrosion - caused when microscopic holes form on the fastener.
Avoiding Fastener Corrosion
The best way to avoid corrosion is to ensure you are using the right materials and coatings for the job. You should also make sure the fasteners are being stored properly on the jobsite, and are installed correctly. If you have any questions, always refer to your project engineer and follow the proper standards for the project.
This table can be used as reference to see potential substitutions for adhesives. Download a printable version here.
Anchor bolts are designed to attach structural elements to concrete. In our industry, anchor bolts are typically used to attach steel to concrete. One end is embedded into the concrete, while the opposite end is threaded to attach structural support. There are four main types of anchor bolts: L-shaped, double end rods with plate, headed, and swedge. Each type has their own uses, sizes, and advantages. Let’s go over each type of anchor bolt and how they are used in structural applications each day.
L-Shaped Anchor Bolts
L-Shaped anchor bolts are used in a variety of applications including light poles, sign structures, heavy equipment and tooling. At Birmingham Fastener, we distribute L-shaped bolts in 3/8”-2.5” diameter and any length required in the following standards: A307, F1554 -36, F1554-55, F1554-105, A193 B7, 304 Stainless, 316 stainless, and any grade the customer may require.
Double End Rods with Plate
These types of anchors are double end rods with a plate washer added to one end. The plate can either be welded to the anchor bolt itself, or it can be tack welded to a nut entrenched in concrete. Plate bolts are used primarily in constructing columns for buildings, highway signs, and other applications. At Birmingham Fastener, we distribute straight with plate bolts in 3/8”-4” diameter and any length required in the following standards: A307, F1554 -36, F1554-55, F1554-105, A193 B7, 304 Stainless, 316 stainless, and any grade the customer may require.
Headed Anchor Bolts
Unlike most anchor bolts, headed anchor bolts have a forged head on their unthreaded end. It is typically a hex, heavy hex, or square head. The head is embedded into the concrete to secure a variety of constructions including structural columns, bridge railing, and light poles. At Birmingham Fastener, we distribute straight with headed anchor bolts in 1/2”-2” diameter and any length required in the following standards: A307, F1554 -36, F1554-55, F1554-105, A193 B7, 304 Stainless, 316 stainless, and any grade the customer may require.
Swedge bolts consist of round bar steel that is threaded on one end and “swedged” on the other. “Swedged” means one end will have multiple indentions to allow concrete to flow into them. Swedge bolts are 100% domestic, as they are used to connect girders and piers. Most swedge bolts are hot dipped galvanized and come in diameters ranging from 5/8” to 4”. They are also available in other materials including stainless steel.
For an easy to read chart, view our Anchor Bolts Length Chart.
One of the great things about stainless steel fasteners is the intrinsic protection within the makeup of the steel. The fastener maintains a shiny, clean finish (when electropolished) while maintaining corrosion resistance. Let’s get into the meaning of stainless steel, its characteristics, and the differences in Type 304 and 316.
What Makes Stainless Steel Fasteners “Stainless?”
The term stainless steel is used for a family of iron-based alloys that contain at minimum 10.5% chromium. This amount of chromium allows for corrosion and heat resistance due to the creation of a surface layer that prevents oxygen from penetrating the steel. Rust cannot form, thus making it “stainless.” Often, their composition also includes other alloys such as nickel, manganese, sulfur, or titanium. These alloys add extra corrosion resistance.
In the fastener world, most stainless steel is austenitic stainless steel, commonly referred to as the 300 series. While there are many compositions that fall under the 300 series, we mostly use types 304 and 316. While they both provide excellent corrosion resistance and have similar compositions, their differences lie in the ratio of chromium to nickel and the environments they are used in.
Stainless Steel: Type 304
You will often hear the term “18-8” when researching the 304 series of stainless steel fasteners. 18-8 refers to the 18% chromium and 8% nickel in the steel. While many stainless materials in the 300 series have this 18-8 mixture, their chemistries and corrosion resistance do not necessarily meet that of type 304. Type 304 is the most popular of the 300 series of stainless steel, as it provides some of the best corrosion resistance in the series.
Stainless Steel: Type 316
Type 316 stainless steel fasteners fall right behind Type 304 in terms of popularity and provides excellent corrosion resistance. Unlike 304, Type 316 does not follow the 18-8 makeup. The chromium is lowered to 16% and the nickel is raised to 10%. Type 316 also contains 2% of molybdenum. Adding molybdenum provides superior resistance to chlorides which can cause pitting.
Because of its composition, Type 316 is used in more corrosive environments where the fasteners will be exposed to chemicals, solvents, or salt water. Type 316 stainless steel fasteners are best used in applications with abrasive chemicals or direct salt spray.
In other blogs, we discuss the ASTM International, what it is, and how we use them in the fastener industry. Our sales team will often refer our customers to the ASTM standards, and our team uses and adheres to them each day. For those unfamiliar, the ASTM standards may seem unapproachable, but the standards are actually accessible once you understand their organization. Let’s go over the standards, their designations, what they contain, and how you can find the right standard.
There are over 12,000 ASTM standards covering a variety of fields and industries. For our purposes, we’ll focus on the standards that relate to fasteners. The ASTM designation is a mandatory title of each standard (example: F3125 or A325). The first letter is the classification of the standard:
A - Ferrous metals and products (steel)
B - Nonferrous metals and products (coatings)
C - Cementitious, ceramic, concrete, and masonry materials
F - end-use materials and products (bolts)
G - Corrosion, deterioration, weathering, durability, and degradation of materials and products
After the letter, there is a reference number given to the standard. From there, a grade can be added. For example, F3125 is the standard for high strength bolts. A325 and A490 are grades that fall under that standard, so you’d call a bolt F3125 Grade A325. When reading an ASTM standard, you will usually see two numbers after the standard, like A449-14. The last two numbers signify the year the standard was updated.
Reading an ASTM Standard
ASTM standards are written in a specific, organized manner. Each standard begins with the scope, which informs the reader the purpose and limitations of the standard. It then has a bibliography of referenced documents within the standard and may also include a list of any terminology used that may be unfamiliar.
From there, in regards to fasteners, you will find details on everything needed to manufacture the fastener. This includes materials, mechanical properties, chemical compositions, dimensions, appearances, and testing methods. These details can also include fastener limitations, reference tables, supplementary requirements, and notes on packaging. Referring to an ASTM standard is a great way for manufacturers and customers alike to determine the right fastener for a particular job, as well recognize the advantages and limitations of a standard.
Finding an ASTM Standard
ASTM International provides a comprehensive search engine on their website for over 12,000 standards. Standards are available for purchase in print or electronically. For specific questions on a standard, contact your project engineer or the Birmingham Fastener sales team, who are always available to answer specific questions about the products we sell.