NIST通过穿透的方法检测混凝土的早期腐蚀
这是一个位于加利福尼亚北部的麦克艾比海滩附近的废弃建筑，可以看出钢筋混凝土的腐蚀十分厉害。利用太赫兹波NIST评估新方法可以直接穿过覆盖的混凝土，检测钢筋在早期阶段的腐蚀。
An
abandoned building on Northern California's McAbee Beach shows the
destructive power of corrosion on a steel-reinforced concrete structure.
A new NIST evaluation method using terahertz waves can detect the early
stages of corrosion on steel rebars directly through their concrete
covering. Credit: With Permission by Per Loll, Denmark
Read more at: https://phys.org/news/2017-04-superman-nist-method-concrete-early-stage.html#jCp
当你遇到摔伤、撞伤或其他打击性创伤时，医生首先要做的是做X光、CT扫描或核磁共振检查，以确定内部是否有损伤。美国国家标准与技术研究所（NIST）利用与其相同的原理，在更大强度上来检测腐蚀。主要检测国家桥梁，道路和其他易老化的基础设施。
他们开发的是一种非侵入性的“光谱指纹”技术，揭示了混凝土包裹钢的结构可以引起任何明显的退化。该检测方法是在《磁共振应用》（Applied Magnetic Resonance）杂志中的一篇新论文中提出的。
当水和氧腐蚀铁，产生不同的氧化铁产品，其中最常见的是针铁矿和赤铁矿。当你敲开一块废弃面时，里面的褐色锈就是针铁矿，腐蚀钢筋混凝土桥面的主要是赤铁矿。”NIST物理化学家Dave Plusquellic说“我们已经展示了我们对针铁矿的新研究，我们以前的工作是研究赤铁矿。太赫兹辐射电磁波频率比用来做饭的微波炉电磁波频率高出10至100倍，因此可以检测到腐蚀产物早期的形成。”
目前，揭露腐蚀的成像方法是使用微波记录受影响的钢的物理状态的变化，如在桥梁或其他结构的混凝土的钢筋厚度的变化。
物理学家和NIST研究员Ed garboczi说“不幸的是，随着可检测时间的改变，腐蚀过程已经造成混凝土裂缝。”
此外,Garboczi说大部分的微波成像方法依赖于比较建筑时钢的基线测量,这种做法只追溯到大约25年。他解释说“这是一个真正的问题，因为在美国的40万个钢筋混凝土桥梁的平均年龄为50年，有许多他们没有基线数据。”
NIST的太赫兹波检测方法的工作基础，是针铁矿和赤铁矿为反铁磁。换句话说，在这些材料中的铁原子并排的电子对反方向旋转，不受外部磁场的影响。与此相反，一个家用磁铁就是铁磁，其铁原子的电子自旋在相同的方向上，由外部磁场吸引或排斥。
“太赫兹波将翻转一对电子的自旋排列，得到赤铁矿和针铁矿的吸收”，Plusquellic说到“我们发现，使用毫米波探测器，这种反铁磁吸收只发生在独特的针铁矿和赤铁矿身上，他们在狭窄的频率范围内产生“光谱指纹”，并且，铁腐蚀的电磁频谱在太赫兹区域。”
通过目前太赫兹源和探测器的进展，新的NIST测评技术能够从早期腐蚀钢材包裹的混凝土，聚合物复合材料 (如工厂的绝缘管)、颜料和其他防护材料中快速检测出微小的含铁氧化物。
“在实验室中，我们已经证明，一个2mW太赫兹源可以产生波通过25毫米的混凝土检测赤铁矿。”Plusquellic说“利用毫瓦太赫兹源和最先进的接收器，我们应该能够穿透50毫米的覆盖混凝土，这个厚度是大多采用钢筋混凝土结构的第一层厚度。”
接下来，NIST的研究小组将试图找到一种akageneite光谱指纹图谱，铁腐蚀是氯离子的产物，氯离子来自于海水和道路除冰盐。Garboczi说“akageneite光谱指纹图谱可以导致钢筋混凝土类似与针铁矿和赤铁矿。”
反铁磁腐蚀检测方法是由NIST磁性材料方面的研究员和开创者William Egelhoff在2009提出。
When
you suffer a fall, an on-the-field collision or some other traumatic
blow, the first thing the doctor will do is take an X-ray, CT scan or
MRI to determine if anything has been damaged internally. Researchers at
the National Institute of Standards and Technology (NIST) are using the
same principle, but in a more powerful form, to detect corrosion, the
primary danger threatening the health of the steel framework within the
nation's bridges, roads and other aging physical infrastructure.
What they have developed is a noninvasive "spectral fingerprint" technique that reveals the corrosion of concrete-encased steel before it can cause any significant degradation of the structure it supports. The detection method is described in a new paper in the journal Applied Magnetic Resonance.
When water and oxygen corrode iron, different iron oxide products are produced, with the two most common being goethite and hematite. "The brown rust that forms when you leave a hammer out in the rain is mostly goethite, and when a steel reinforcing bar [rebar] corrodes inside a concrete bridge deck, that is mostly hematite," said NIST physical chemist Dave Plusquellic. "We have shown in our new study with goethite, and our previous work with hematite, that terahertz radiation—electromagnetic waves with frequencies 10 to 100 times higher than the microwaves used to cook food—can detect both corrosion products in the early stages of formation."
Current imaging methods for uncovering corrosion use microwaves to record changes in the physical state of the affected steel, such as changes in the thickness of a rebar within the concrete of a bridge or other structure.
"Unfortunately, by the time such changes are detectable, the corrosive process is already well on its way toward causing cracks in the concrete," said physicist and NIST Fellow Ed Garboczi.
Additionally, Garboczi said most of the microwave imaging methods rely on comparisons with baseline measurements of the steel taken at the time of construction, a practice that only goes back about 25 years.
"That's a real problem since the average age of the 400,000 steel-reinforced concrete bridges in the United States is 50 years and there is no baseline data available for many of them," he explained.
The NIST terahertz wave detection method works because goethite and hematite are antiferromagnetic. In other words, the pairs of electrons sitting side-by-side within the iron atoms in these materials spin in opposite directions, leaving them unaffected by external magnetic fields. In contrast, the electrons in the iron atoms of a household magnet, which is ferromagnetic, spin in the same direction and are either attracted or repelled by external magnetic fields.
"Terahertz waves will flip the spin alignment of one of the electrons in a pair and get absorbed by hematite or goethite," Plusquellic said. "Using a millimeter wave detector, we discovered that this antiferromagnetic absorption only occurs within narrow frequency ranges in the terahertz region of the electromagnetic spectrum—yielding 'spectral fingerprints' unique to goethite and hematite, and in turn, iron corrosion."
With current advances in terahertz sources and detectors, the new NIST nondestructive evaluation technique has the potential to rapidly detect tiny amounts of iron-bearing oxides from early-stage corrosion of steel surrounded by concrete, polymer composites (such as pipe insulation in a factory), paints and other protective materials.
"In the laboratory, we have demonstrated that a 2-milliwatt terahertz source can produce waves that detect hematite through 25 millimeters of concrete," Plusquellic said. "Using terahertz sources with powers in the hundreds of milliwatts and state-of-the-art receivers with unprecedented signal-to-noise ratios, we should be able to penetrate 50 millimeters, the thickness of the concrete covering the first layer of rebar used in most steel-reinforced concrete structures."
Next up for the NIST team will be an attempt to find a spectral fingerprint for akageneite, an iron corrosion product formed in the presence of chloride ions, which come from sources such as seawater and road deicing salt.
"Akageneite can cause problems in steel-reinforced concrete similar to those seen with goethite and hematite," Garboczi said.
The antiferromagnetic corrosion detection method was first conceived in 2009 by the late William Egelhoff, a NIST fellow and pioneer in the field of magnetic materials.
Explore further: New technique for investigating the action of molybdate on carbon steel
More information: S. G. Chou et al, Using Terahertz Waves to Identify the Presence of Goethite via Antiferromagnetic Resonance, Applied Magnetic Resonance (2017). DOI: 10.1007/s00723-017-0884-y
